Codify capture-pipe hang lesson in skills

Encode the hard-won lesson from the forkserver
cancel-cascade investigation into two skill docs
so future sessions grep-find it before spelunking
into trio internals.

Deats,
- `.claude/skills/conc-anal/SKILL.md`:
  - new "Unbounded waits in cleanup paths"
    section — rule: bound every `await X.wait()`
    in cleanup paths with `trio.move_on_after()`
    unless the setter is unconditionally
    reachable. Recent example:
    `ipc_server.wait_for_no_more_peers()` in
    `async_main`'s finally (was unbounded,
    deadlocked when any peer handler stuck)
  - new "The capture-pipe-fill hang pattern"
    section — mechanism, grep-pointers to the
    existing `conftest.py` guards (`tests/conftest
    .py:258`, `:316`), cross-ref to the full
    post-mortem doc, and the grep-note: "if a
    multi-subproc tractor test hangs, `pytest -s`
    first, conc-anal second"
- `.claude/skills/run-tests/SKILL.md`: new
  "Section 9: The pytest-capture hang pattern
  (CHECK THIS FIRST)" with symptom / cause /
  pre-existing guards to grep / three-step debug
  recipe (try `-s`, lower loglevel, redirect
  stdout/stderr) / signature of this bug vs. a
  real code hang / historical reference

Cost several investigation sessions before the
capture-pipe issue surfaced — it was masked by
deeper cascade deadlocks. Once the cascades were
fixed, the tree tore down enough to generate
pipe-filling log volume. Lesson: **grep this
pattern first when any multi-subproc tractor test
hangs under default pytest but passes with `-s`.**

(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

.claude/settings.local.json

@@ -1,8 +1,16 @@
 {
   "permissions": {
     "allow": [
-      "Bash(date *)",
       "Bash(cp .claude/*)",
+      "Read(.claude/**)",
+      "Read(.claude/skills/run-tests/**)",
+      "Write(.claude/**/*commit_msg*)",
+      "Write(.claude/git_commit_msg_LATEST.md)",
+      "Skill(run-tests)",
+      "Skill(close-wkt)",
+      "Skill(open-wkt)",
+      "Skill(prompt-io)",
+      "Bash(date *)",
       "Bash(git diff *)",
       "Bash(git log *)",
       "Bash(git status)",
@@ -23,14 +31,12 @@
       "Bash(UV_PROJECT_ENVIRONMENT=py* uv sync:*)",
       "Bash(UV_PROJECT_ENVIRONMENT=py* uv run:*)",
       "Bash(echo EXIT:$?:*)",
-      "Write(.claude/*commit_msg*)",
-      "Write(.claude/git_commit_msg_LATEST.md)",
-      "Skill(run-tests)",
-      "Skill(close-wkt)",
-      "Skill(open-wkt)",
-      "Skill(prompt-io)"
+      "Bash(echo \"EXIT=$?\")",
+      "Read(//tmp/**)"
     ],
     "deny": [],
     "ask": []
-  }
+  },
+  "prefersReducedMotion": false,
+  "outputStyle": "default"
 }

.claude/skills/conc-anal/SKILL.md

@@ -229,3 +229,69 @@ Unlike asyncio, trio allows checkpoints in
 that does `await` can itself be cancelled (e.g.
 by nursery shutdown). Watch for cleanup code that
 assumes it will run to completion.
+
+### Unbounded waits in cleanup paths
+
+Any `await <event>.wait()` in a teardown path is
+a latent deadlock unless the event's setter is
+GUARANTEED to fire. If the setter depends on
+external state (peer disconnects, child process
+exit, subsequent task completion) that itself
+depends on the current task's progress, you have
+a mutual wait.
+
+Rule: **bound every `await X.wait()` in cleanup
+paths with `trio.move_on_after()`** unless you
+can prove the setter is unconditionally reachable
+from the state at the await site. Concrete recent
+example: `ipc_server.wait_for_no_more_peers()` in
+`async_main`'s finally (see
+`ai/conc-anal/subint_forkserver_test_cancellation_leak_issue.md`
+"probe iteration 3") — it was unbounded, and when
+one peer-handler was stuck the wait-for-no-more-
+peers event never fired, deadlocking the whole
+actor-tree teardown cascade.
+
+### The capture-pipe-fill hang pattern (grep this first)
+
+When investigating any hang in the test suite
+**especially under fork-based backends**, first
+check whether the hang reproduces under `pytest
+-s` (`--capture=no`). If `-s` makes it go away
+you're not looking at a trio concurrency bug —
+you're looking at a Linux pipe-buffer fill.
+
+Mechanism: pytest replaces fds 1,2 with pipe
+write-ends. Fork-child subactors inherit those
+fds. High-volume error-log tracebacks (cancel
+cascade spew) fill the 64KB pipe buffer. Child
+`write()` blocks. Child can't exit. Parent's
+`waitpid`/pidfd wait blocks. Deadlock cascades up
+the tree.
+
+Pre-existing guards in `tests/conftest.py` encode
+this knowledge — grep these BEFORE blaming
+concurrency:
+
+```python
+# tests/conftest.py:258
+if loglevel in ('trace', 'debug'):
+    # XXX: too much logging will lock up the subproc (smh)
+    loglevel: str = 'info'
+
+# tests/conftest.py:316
+# can lock up on the `_io.BufferedReader` and hang..
+stderr: str = proc.stderr.read().decode()
+```
+
+Full post-mortem +
+`ai/conc-anal/subint_forkserver_test_cancellation_leak_issue.md`
+for the canonical reproduction. Cost several
+investigation sessions before catching it —
+because the capture-pipe symptom was masked by
+deeper cascade-deadlocks. Once the cascades were
+fixed, the tree tore down enough to generate
+pipe-filling log volume → capture-pipe finally
+surfaced. Grep-note for future-self: **if a
+multi-subproc tractor test hangs, `pytest -s`
+first, conc-anal second.**

.claude/skills/run-tests/SKILL.md

@@ -8,14 +8,19 @@ allowed-tools:
   - Bash(python -m pytest *)
   - Bash(python -c *)
   - Bash(python --version *)
-  - Bash(git rev-parse *)
+  - Bash(UV_PROJECT_ENVIRONMENT=py* uv run python *)
+  - Bash(UV_PROJECT_ENVIRONMENT=py* uv run pytest *)
   - Bash(UV_PROJECT_ENVIRONMENT=py* uv sync *)
+  - Bash(UV_PROJECT_ENVIRONMENT=py* uv pip show *)
+  - Bash(git rev-parse *)
   - Bash(ls *)
   - Bash(cat *)
+  - Bash(jq * .pytest_cache/*)
   - Read
   - Grep
   - Glob
   - Task
+  - AskUserQuestion
 ---
 
 Run the `tractor` test suite using `pytest`. Follow this
@@ -90,41 +95,104 @@ python -m pytest tests/ -x --tb=short --no-header --tpt-proto uds
 python -m pytest tests/ -x --tb=short --no-header -k "cancel and not slow"
 ```
 
-## 3. Pre-flight checks (before running tests)
+## 3. Pre-flight: venv detection (MANDATORY)
 
-### Worktree venv detection
+**Always verify a `uv` venv is active before running
+`python` or `pytest`.** This project uses
+`UV_PROJECT_ENVIRONMENT=py<MINOR>` naming (e.g.
+`py313`) — never `.venv`.
 
-If running inside a git worktree (`git rev-parse
---git-common-dir` differs from `--git-dir`), verify
-the Python being used is from the **worktree's own
-venv**, not the main repo's. Check:
+### Step 1: detect active venv
+
+Run this check first:
+
+```sh
+python -c "
+import sys, os
+venv = os.environ.get('VIRTUAL_ENV', '')
+prefix = sys.prefix
+print(f'VIRTUAL_ENV={venv}')
+print(f'sys.prefix={prefix}')
+print(f'executable={sys.executable}')
+"
+```
+
+### Step 2: interpret results
+
+**Case A — venv is active** (`VIRTUAL_ENV` is set
+and points to a `py<MINOR>/` dir under the project
+root or worktree):
+
+Use bare `python` / `python -m pytest` for all
+commands. This is the normal, fast path.
+
+**Case B — no venv active** (`VIRTUAL_ENV` is empty
+or `sys.prefix` points to a system Python):
+
+Use `AskUserQuestion` to ask the user:
+
+> "No uv venv is active. Should I activate one
+> via `UV_PROJECT_ENVIRONMENT=py<MINOR> uv sync`,
+> or would you prefer to activate your shell venv
+> first?"
+
+Options:
+1. **"Create/sync venv"** — run
+   `UV_PROJECT_ENVIRONMENT=py<MINOR> uv sync` where
+   `<MINOR>` is detected from `python --version`
+   (e.g. `313` for 3.13). Then use
+   `py<MINOR>/bin/python` for all subsequent
+   commands in this session.
+2. **"I'll activate it myself"** — stop and let the
+   user `source py<MINOR>/bin/activate` or similar.
+
+**Case C — inside a git worktree** (`git rev-parse
+--git-common-dir` differs from `--git-dir`):
+
+Verify Python resolves from the **worktree's own
+venv**, not the main repo's:
 
 ```sh
 python -c "import tractor; print(tractor.__file__)"
 ```
 
-If the path points outside the worktree (e.g. to
-the main repo), set up a local venv first:
+If the path points outside the worktree, create a
+worktree-local venv:
 
 ```sh
 UV_PROJECT_ENVIRONMENT=py<MINOR> uv sync
 ```
 
-where `<MINOR>` matches the active cpython minor
-version (detect via `python --version`, e.g.
-`py313` for 3.13, `py314` for 3.14). Then use
-`py<MINOR>/bin/python` for all subsequent commands.
+Then use `py<MINOR>/bin/python` for all commands.
 
-**Why this matters**: without a worktree-local venv,
-subprocesses spawned by tractor resolve modules from
-the main repo's editable install, causing spurious
-`AttributeError` / `ModuleNotFoundError` for code
-that only exists on the worktree's branch.
+**Why this matters**: without the correct venv,
+subprocesses spawned by tractor resolve modules
+from the wrong editable install, causing spurious
+`AttributeError` / `ModuleNotFoundError`.
 
-### Import + collection checks
+### Fallback: `uv run`
 
-Always run these, especially after refactors or
-module moves — they catch import errors instantly:
+If the user can't or won't activate a venv, all
+`python` and `pytest` commands can be prefixed
+with `UV_PROJECT_ENVIRONMENT=py<MINOR> uv run`:
+
+```sh
+# instead of: python -m pytest tests/ -x
+UV_PROJECT_ENVIRONMENT=py313 uv run pytest tests/ -x
+
+# instead of: python -c 'import tractor'
+UV_PROJECT_ENVIRONMENT=py313 uv run python -c 'import tractor'
+```
+
+`uv run` auto-discovers the project and venv,
+but is slower than a pre-activated venv due to
+lock-file resolution on each invocation. Prefer
+activating the venv when possible.
+
+### Step 3: import + collection checks
+
+After venv is confirmed, always run these
+(especially after refactors or module moves):
 
 ```sh
 # 1. package import smoke check
@@ -137,6 +205,101 @@ python -m pytest tests/ -x -q --co 2>&1 | tail -5
 If either fails, fix the import error before running
 any actual tests.
 
+### Step 4: zombie-actor / stale-registry check (MANDATORY)
+
+The tractor runtime's default registry address is
+**`127.0.0.1:1616`** (TCP) / `/tmp/registry@1616.sock`
+(UDS). Whenever any prior test run — especially one
+using a fork-based backend like `subint_forkserver` —
+leaks a child actor process, that zombie keeps the
+registry port bound and **every subsequent test
+session fails to bind**, often presenting as 50+
+unrelated failures ("all tests broken"!) across
+backends.
+
+**This has to be checked before the first run AND
+after any cancelled/SIGINT'd run** — signal failures
+in the middle of a test can leave orphan children.
+
+```sh
+# 1. TCP registry — any listener on :1616? (primary signal)
+ss -tlnp 2>/dev/null | grep ':1616' || echo 'TCP :1616 free'
+
+# 2. leftover actor/forkserver procs — scoped to THIS
+#    repo's python path, so we don't false-flag legit
+#    long-running tractor-using apps (e.g. `piker`,
+#    downstream projects that embed tractor).
+pgrep -af "$(pwd)/py[0-9]*/bin/python.*_actor_child_main|subint-forkserv" \
+  | grep -v 'grep\|pgrep' \
+  || echo 'no leaked actor procs from this repo'
+
+# 3. stale UDS registry sockets
+ls -la /tmp/registry@*.sock 2>/dev/null \
+  || echo 'no leaked UDS registry sockets'
+```
+
+**Interpretation:**
+
+- **TCP :1616 free AND no stale sockets** → clean,
+  proceed. The actor-procs probe is secondary — false
+  positives are common (piker, any other tractor-
+  embedding app); only cleanup if `:1616` is bound or
+  sockets linger.
+- **TCP :1616 bound OR stale sockets present** →
+  surface PIDs + cmdlines to the user, offer cleanup:
+
+  ```sh
+  # 1. GRACEFUL FIRST (tractor is structured concurrent — it
+  #    catches SIGINT as an OS-cancel in `_trio_main` and
+  #    cascades Portal.cancel_actor via IPC to every descendant.
+  #    So always try SIGINT first with a bounded timeout; only
+  #    escalate to SIGKILL if graceful cleanup doesn't complete).
+  pkill -INT -f "$(pwd)/py[0-9]*/bin/python.*_actor_child_main|subint-forkserv"
+
+  # 2. bounded wait for graceful teardown (usually sub-second).
+  #    Loop until the processes exit, or timeout. Keep the
+  #    bound tight — hung/abrupt-killed descendants usually
+  #    hang forever, so don't wait more than a few seconds.
+  for i in $(seq 1 10); do
+    pgrep -f "$(pwd)/py[0-9]*/bin/python.*_actor_child_main|subint-forkserv" >/dev/null || break
+    sleep 0.3
+  done
+
+  # 3. ESCALATE TO SIGKILL only if graceful didn't finish.
+  if pgrep -f "$(pwd)/py[0-9]*/bin/python.*_actor_child_main|subint-forkserv" >/dev/null; then
+    echo 'graceful teardown timed out — escalating to SIGKILL'
+    pkill -9 -f "$(pwd)/py[0-9]*/bin/python.*_actor_child_main|subint-forkserv"
+  fi
+
+  # 4. if a test zombie holds :1616 specifically and doesn't
+  #    match the above pattern, find its PID the hard way:
+  ss -tlnp 2>/dev/null | grep ':1616'   # prints `users:(("<name>",pid=NNNN,...))`
+  # then (same SIGINT-first ladder):
+  # kill -INT <NNNN>; sleep 1; kill -9 <NNNN> 2>/dev/null
+
+  # 5. remove stale UDS sockets
+  rm -f /tmp/registry@*.sock
+
+  # 6. re-verify
+  ss -tlnp 2>/dev/null | grep ':1616' || echo 'TCP :1616 now free'
+  ```
+
+**Never ignore stale registry state.** If you see the
+"all tests failing" pattern — especially
+`trio.TooSlowError` / connection refused / address in
+use on many unrelated tests — check registry **before**
+spelunking into test code. The failure signature will
+be identical across backends because they're all
+fighting for the same port.
+
+**False-positive warning for step 2:** a plain
+`pgrep -af '_actor_child_main'` will also match
+legit long-running tractor-embedding apps (e.g.
+`piker` at `~/repos/piker/py*/bin/python3 -m
+tractor._child ...`). Always scope to the current
+repo's python path, or only use step 1 (`:1616`) as
+the authoritative signal.
+
 ## 4. Run and report
 
 - Run the constructed command.
@@ -217,7 +380,48 @@ python -c 'import tractor' && python -m pytest tests/ -x -q --co 2>&1 | tail -3
 python -m pytest tests/test_local.py tests/test_rpc.py tests/test_spawning.py tests/discovery/test_registrar.py -x --tb=short --no-header
 ```
 
-### Re-run last failures only:
+### Inspect last failures (without re-running):
+
+When the user asks "what failed?", "show failures",
+or wants to check the last-failed set before
+re-running — read the pytest cache directly. This
+is instant and avoids test collection overhead.
+
+```sh
+python -c "
+import json, pathlib, sys
+p = pathlib.Path('.pytest_cache/v/cache/lastfailed')
+if not p.exists():
+    print('No lastfailed cache found.'); sys.exit()
+data = json.loads(p.read_text())
+# filter to real test node IDs (ignore junk
+# entries that can accumulate from system paths)
+tests = sorted(k for k in data if k.startswith('tests/'))
+if not tests:
+    print('No failures recorded.')
+else:
+    print(f'{len(tests)} last-failed test(s):')
+    for t in tests:
+        print(f'  {t}')
+"
+```
+
+**Why not `--cache-show` or `--co --lf`?**
+
+- `pytest --cache-show 'cache/lastfailed'` works
+  but dumps raw dict repr including junk entries
+  (stale system paths that leak into the cache).
+- `pytest --co --lf` actually *collects* tests which
+  triggers import resolution and is slow (~0.5s+).
+  Worse, when cached node IDs don't exactly match
+  current parametrize IDs (e.g. param names changed
+  between runs), pytest falls back to collecting
+  the *entire file*, giving false positives.
+- Reading the JSON directly is instant, filterable
+  to `tests/`-prefixed entries, and shows exactly
+  what pytest recorded — no interpretation.
+
+**After inspecting**, re-run the failures:
 ```sh
 python -m pytest --lf -x --tb=short --no-header
 ```
@@ -247,3 +451,73 @@ by your changes — note them and move on.
 **Rule of thumb**: if a test fails with `TooSlowError`,
 `trio.TooSlowError`, or `pexpect.TIMEOUT` and you didn't
 touch the relevant code path, it's flaky — skip it.
+
+## 9. The pytest-capture hang pattern (CHECK THIS FIRST)
+
+**Symptom:** a tractor test hangs indefinitely under
+default `pytest` but passes instantly when you add
+`-s` (`--capture=no`).
+
+**Cause:** tractor subactors (especially under fork-
+based backends) inherit pytest's stdout/stderr
+capture pipes via fds 1,2. Under high-volume error
+logging (e.g. multi-level cancel cascade, nested
+`run_in_actor` failures, anything triggering
+`RemoteActorError` + `ExceptionGroup` traceback
+spew), the **64KB Linux pipe buffer fills** faster
+than pytest drains it. Subactor writes block → can't
+finish exit → parent's `waitpid`/pidfd wait blocks →
+deadlock cascades up the tree.
+
+**Pre-existing guards in the tractor harness** that
+encode this same knowledge — grep these FIRST
+before spelunking:
+
+- `tests/conftest.py:258-260` (in the `daemon`
+  fixture): `# XXX: too much logging will lock up
+  the subproc (smh)` — downgrades `trace`/`debug`
+  loglevel to `info` to prevent the hang.
+- `tests/conftest.py:316`: `# can lock up on the
+  _io.BufferedReader and hang..` — noted on the
+  `proc.stderr.read()` post-SIGINT.
+
+**Debug recipe (in priority order):**
+
+1. **Try `-s` first.** If the hang disappears with
+   `pytest -s`, you've confirmed it's capture-pipe
+   fill. Skip spelunking.
+2. **Lower the loglevel.** Default `--ll=error` on
+   this project; if you've bumped it to `debug` /
+   `info`, try dropping back. Each log level
+   multiplies pipe-pressure under fault cascades.
+3. **If you MUST use default capture + high log
+   volume**, redirect subactor stdout/stderr in the
+   child prelude (e.g.
+   `tractor.spawn._subint_forkserver._child_target`
+   post-`_close_inherited_fds`) to `/dev/null` or a
+   file.
+
+**Signature tells you it's THIS bug (vs. a real
+code hang):**
+
+- Multi-actor test under fork-based backend
+  (`subint_forkserver`, eventually `trio_proc` too
+  under enough log volume).
+- Multiple `RemoteActorError` / `ExceptionGroup`
+  tracebacks in the error path.
+- Test passes with `-s` in the 5-10s range, hangs
+  past pytest-timeout (usually 30+ s) without `-s`.
+- Subactor processes visible via `pgrep -af
+  subint-forkserv` or similar after the hang —
+  they're alive but blocked on `write()` to an
+  inherited stdout fd.
+
+**Historical reference:** this deadlock cost a
+multi-session investigation (4 genuine cascade
+fixes landed along the way) that only surfaced the
+capture-pipe issue AFTER the deeper fixes let the
+tree actually tear down enough to produce pipe-
+filling log volume. Full post-mortem in
+`ai/conc-anal/subint_forkserver_test_cancellation_leak_issue.md`.
+Lesson codified here so future-me grep-finds the
+workaround before digging.

.gitignore

@@ -106,46 +106,55 @@ venv.bak/
 # all files under
 .git/
 
-# any commit-msg gen tmp files
-.claude/skills/commit-msg/msgs/
-.claude/git_commit_msg_LATEST.md
-.claude/*_commit_*.md
-.claude/*_commit*.toml
-.claude/*_commit*.txt
-.claude/skills/commit-msg/msgs/*
+# require very explicit staging for anything we **really**
+# want put/kept in repo.
+notes_to_self/
+snippets/
 
-.claude/skills/pr-msg/msgs/*
-# XXX, for rn, so i can telescope this file.
-!/.claude/skills/pr-msg/pr_msg_LATEST.md
-
-# review-skill ephemeral ctx (per-PR, single-use)
-.claude/review_context.md
-.claude/review_regression.md
-
-# per-skill session/conf (machine-local)
-.claude/skills/*/conf.toml
-
-# ai.skillz symlinks (machine-local, deploy via deploy-skill.sh)
+# ------- AI shiz -------
+# `ai.skillz` symlinks,
+# (machine-local, deploy via deploy-skill.sh)
 .claude/skills/py-codestyle
-.claude/skills/code-review-changes
 .claude/skills/close-wkt
-.claude/skills/open-wkt
 .claude/skills/plan-io
 .claude/skills/prompt-io
 .claude/skills/resolve-conflicts
 .claude/skills/inter-skill-review
-.claude/skills/yt-url-lookup
 
-# hybrid skills — symlinked SKILL.md + references
-.claude/skills/commit-msg/SKILL.md
-.claude/skills/pr-msg/SKILL.md
-.claude/skills/pr-msg/references
+# /open-wkt specifics
+.claude/skills/open-wkt
+.claude/wkts/
+claude_wkts
+
+# /code-review-changes specifics
+.claude/skills/code-review-changes
+# review-skill ephemeral ctx (per-PR, single-use)
+.claude/review_context.md
+.claude/review_regression.md
+
+# /pr-msg specifics
+.claude/skills/pr-msg/*
+# repo-specific
+!.claude/skills/pr-msg/format-reference.md
+# XXX, so u can nvim-telescope this file.
+# !.claude/skills/pr-msg/pr_msg_LATEST.md
+
+# /commit-msg specifics
+# - any commit-msg gen tmp files
+.claude/*_commit_*.md
+.claude/*_commit*.txt
+.claude/skills/commit-msg/*
+!.claude/skills/commit-msg/style-duie-reference.md
+
+# use prompt-io instead?
+.claude/plans
 
 # nix develop --profile .nixdev
 .nixdev*
 
 # :Obsession .
 Session.vim
+
 # `gish` local `.md`-files
 # TODO? better all around automation!
 # -[ ] it'd be handy to also commit and sync with wtv git service?
@@ -159,7 +168,3 @@ gh/
 
 # LLM conversations that should remain private
 docs/conversations/
-
-# Claude worktrees
-.claude/wkts/
-claude_wkts

ai/conc-anal/subint_cancel_delivery_hang_issue.md

@@ -0,0 +1,161 @@
+# `subint` backend: parent trio loop parks after subint teardown (Ctrl-C works; not a CPython-level issue)
+
+Follow-up to the Phase B subint spawn-backend PR (see
+`tractor.spawn._subint`, issue #379). Distinct from the
+`subint_sigint_starvation_issue.md` (SIGINT-unresponsive
+starvation hang): this one is **Ctrl-C-able**, which means
+it's *not* the shared-GIL-hostage class and is ours to fix
+from inside tractor rather than waiting on upstream CPython
+/ msgspec progress.
+
+## TL;DR
+
+After a stuck-subint subactor is torn down via the
+hard-kill path, a parent-side trio task parks on an
+*orphaned resource* (most likely a `chan.recv()` /
+`process_messages` loop on the now-dead subint's IPC
+channel) and waits forever for bytes that can't arrive —
+because the channel was torn down without emitting a clean
+EOF/`BrokenResourceError` to the waiting receiver.
+
+Unlike `subint_sigint_starvation_issue.md`, the main trio
+loop **is** iterating normally — SIGINT delivers cleanly
+and the test unhangs. But absent Ctrl-C, the test suite
+wedges indefinitely.
+
+## Symptom
+
+Running `test_subint_non_checkpointing_child` under
+`--spawn-backend=subint` (in
+`tests/test_subint_cancellation.py`):
+
+1. Test spawns a subactor whose main task runs
+   `threading.Event.wait(1.0)` in a loop — releases the
+   GIL but never inserts a trio checkpoint.
+2. Parent does `an.cancel_scope.cancel()`. Our
+   `subint_proc` cancel path fires: soft-kill sends
+   `Portal.cancel_actor()` over the live IPC channel →
+   subint's trio loop *should* process the cancel msg on
+   its IPC dispatcher task (since the GIL releases are
+   happening).
+3. Expected: subint's `trio.run()` unwinds, driver thread
+   exits naturally, parent returns.
+4. Actual: parent `trio.run()` never completes. Test
+   hangs past its `trio.fail_after()` deadline.
+
+## Evidence
+
+### `strace` on the hung pytest process during SIGINT
+
+```
+--- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
+write(17, "\2", 1)                      = 1
+```
+
+Contrast with the SIGINT-starvation hang (see
+`subint_sigint_starvation_issue.md`) where that same
+`write()` returned `EAGAIN`. Here the SIGINT byte is
+written successfully → Python's signal handler pipe is
+being drained → main trio loop **is** iterating → SIGINT
+gets turned into `trio.Cancelled` → the test unhangs (if
+the operator happens to be there to hit Ctrl-C).
+
+### Stack dump (via `tractor.devx.dump_on_hang`)
+
+Single main thread visible, parked in
+`trio._core._io_epoll.get_events` inside `trio.run` at the
+test's `trio.run(...)` call site. No subint driver thread
+(subint was destroyed successfully — this is *after* the
+hard-kill path, not during it).
+
+## Root cause hypothesis
+
+Most consistent with the evidence: a parent-side trio
+task is awaiting a `chan.recv()` / `process_messages` loop
+on the dead subint's IPC channel. The sequence:
+
+1. Soft-kill in `subint_proc` sends `Portal.cancel_actor()`
+   over the channel. The subint's trio dispatcher *may* or
+   may not have processed the cancel msg before the subint
+   was destroyed — timing-dependent.
+2. Hard-kill timeout fires (because the subint's main
+   task was in `threading.Event.wait()` with no trio
+   checkpoint — cancel-msg processing couldn't race the
+   timeout).
+3. Driver thread abandoned, `_interpreters.destroy()`
+   runs. Subint is gone.
+4. But the parent-side trio task holding a
+   `chan.recv()` / `process_messages` loop against that
+   channel was **not** explicitly cancelled. The channel's
+   underlying socket got torn down, but without a clean
+   EOF delivered to the waiting recv, the task parks
+   forever on `trio.lowlevel.wait_readable` (or similar).
+
+This matches the "main loop fine, task parked on
+orphaned I/O" signature.
+
+## Why this is ours to fix (not CPython's)
+
+- Main trio loop iterates normally → GIL isn't starved.
+- SIGINT is deliverable → not a signal-pipe-full /
+  wakeup-fd contention scenario.
+- The hang is in *our* supervision code, specifically in
+  how `subint_proc` tears down its side of the IPC when
+  the subint is abandoned/destroyed.
+
+## Possible fix directions
+
+1. **Explicit parent-side channel abort on subint
+   abandon.** In `subint_proc`'s teardown block, after the
+   hard-kill timeout fires, explicitly close the parent's
+   end of the IPC channel to the subint. Any waiting
+   `chan.recv()` / `process_messages` task sees
+   `BrokenResourceError` (or `ClosedResourceError`) and
+   unwinds.
+2. **Cancel parent-side RPC tasks tied to the dead
+   subint's channel.** The `Actor._rpc_tasks` / nursery
+   machinery should have a handle on any
+   `process_messages` loops bound to a specific peer
+   channel. Iterate those and cancel explicitly.
+3. **Bound the top-level `await actor_nursery
+   ._join_procs.wait()` shield in `subint_proc`** (same
+   pattern as the other bounded shields the hard-kill
+   patch added). If the nursery never sets `_join_procs`
+   because a child task is parked, the bound would at
+   least let the teardown proceed.
+
+Of these, (1) is the most surgical and directly addresses
+the root cause. (2) is a defense-in-depth companion. (3)
+is a band-aid but cheap to add.
+
+## Current workaround
+
+None in-tree. The test's `trio.fail_after()` bound
+currently fires and raises `TooSlowError`, so the test
+visibly **fails** rather than hangs — which is
+intentional (an unbounded cancellation-audit test would
+defeat itself). But in interactive test runs the operator
+has to hit Ctrl-C to move past the parked state before
+pytest reports the failure.
+
+## Reproducer
+
+```
+./py314/bin/python -m pytest \
+  tests/test_subint_cancellation.py::test_subint_non_checkpointing_child \
+  --spawn-backend=subint --tb=short --no-header -v
+```
+
+Expected: hangs until `trio.fail_after(15)` fires, or
+Ctrl-C unwedges it manually.
+
+## References
+
+- `tractor.spawn._subint.subint_proc` — current subint
+  teardown code; see the `_HARD_KILL_TIMEOUT` bounded
+  shields + `daemon=True` driver-thread abandonment
+  (commit `b025c982`).
+- `ai/conc-anal/subint_sigint_starvation_issue.md` — the
+  sibling CPython-level hang (GIL-starvation,
+  SIGINT-unresponsive) which is **not** this issue.
+- Phase B tracking: issue #379.

ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md

@@ -0,0 +1,337 @@
+# `os.fork()` from a non-main sub-interpreter aborts the child (CPython refuses post-fork cleanup)
+
+Third `subint`-class analysis in this project. Unlike its
+two siblings (`subint_sigint_starvation_issue.md`,
+`subint_cancel_delivery_hang_issue.md`), this one is not a
+hang — it's a **hard CPython-level refusal** of an
+experimental spawn strategy we wanted to try.
+
+## TL;DR
+
+An in-process sub-interpreter cannot be used as a
+"launchpad" for `os.fork()` on current CPython. The fork
+syscall succeeds in the parent, but the forked CHILD
+process is aborted immediately by CPython's post-fork
+cleanup with:
+
+```
+Fatal Python error: _PyInterpreterState_DeleteExceptMain: not main interpreter
+```
+
+This is enforced by a hard `PyStatus_ERR` gate in
+`Python/pystate.c`. The CPython devs acknowledge the
+fragility with an in-source comment (`// Ideally we could
+guarantee tstate is running main.`) but provide no
+mechanism to satisfy the precondition from user code.
+
+**Implication for tractor**: the `subint_fork` backend
+sketched in `tractor.spawn._subint_fork` is structurally
+dead on current CPython. The submodule is kept as
+documentation of the attempt; `--spawn-backend=subint_fork`
+raises `NotImplementedError` pointing here.
+
+## Context — why we tried this
+
+The motivation is issue #379's "Our own thoughts, ideas
+for `fork()`-workaround/hacks..." section. The existing
+trio-backend (`tractor.spawn._trio.trio_proc`) spawns
+subactors via `trio.lowlevel.open_process()` → ultimately
+`posix_spawn()` or `fork+exec`, from the parent's main
+interpreter that is currently running `trio.run()`. This
+brushes against a known-fragile interaction between
+`trio` and `fork()` tracked in
+[python-trio/trio#1614](https://github.com/python-trio/trio/issues/1614)
+and siblings — mostly mitigated in `tractor`'s case only
+incidentally (we `exec()` immediately post-fork).
+
+The idea was:
+
+1. Create a subint that has *never* imported `trio`.
+2. From a worker thread in that subint, call `os.fork()`.
+3. In the child, `execv()` back into
+   `python -m tractor._child` — same as `trio_proc` does.
+4. The fork is from a trio-free context → trio+fork
+   hazards avoided regardless of downstream behavior.
+
+The parent-side orchestration (`ipc_server.wait_for_peer`,
+`SpawnSpec`, `Portal` yield) would reuse
+`trio_proc`'s flow verbatim, with only the subproc-spawn
+mechanics swapped.
+
+## Symptom
+
+Running the prototype (`tractor.spawn._subint_fork.subint_fork_proc`,
+see git history prior to the stub revert) on py3.14:
+
+```
+Fatal Python error: _PyInterpreterState_DeleteExceptMain: not main interpreter
+Python runtime state: initialized
+
+Current thread 0x00007f6b71a456c0 [subint-fork-lau] (most recent call first):
+  File "<script>", line 2 in <module>
+<script>:2: DeprecationWarning: This process (pid=802985) is multi-threaded, use of fork() may lead to deadlocks in the child.
+```
+
+Key clues:
+
+- The **`DeprecationWarning`** fires in the parent (before
+  fork completes) — fork *is* executing, we get that far.
+- The **`Fatal Python error`** comes from the child — it
+  aborts during CPython's post-fork C initialization
+  before any user Python runs in the child.
+- The thread name `subint-fork-lau[nchpad]` is ours —
+  confirms the fork is being called from the launchpad
+  subint's driver thread.
+
+## CPython source walkthrough
+
+### Call site — `Modules/posixmodule.c:728-793`
+
+The post-fork-child hook CPython runs in the child process:
+
+```c
+void
+PyOS_AfterFork_Child(void)
+{
+    PyStatus status;
+    _PyRuntimeState *runtime = &_PyRuntime;
+
+    // re-creates runtime->interpreters.mutex (HEAD_UNLOCK)
+    status = _PyRuntimeState_ReInitThreads(runtime);
+    ...
+
+    PyThreadState *tstate = _PyThreadState_GET();
+    _Py_EnsureTstateNotNULL(tstate);
+
+    ...
+
+    // Ideally we could guarantee tstate is running main.   ← !!!
+    _PyInterpreterState_ReinitRunningMain(tstate);
+
+    status = _PyEval_ReInitThreads(tstate);
+    ...
+
+    status = _PyInterpreterState_DeleteExceptMain(runtime);
+    if (_PyStatus_EXCEPTION(status)) {
+        goto fatal_error;
+    }
+    ...
+
+fatal_error:
+    Py_ExitStatusException(status);
+}
+```
+
+The `// Ideally we could guarantee tstate is running
+main.` comment is a flashing warning sign — the CPython
+devs *know* this path is fragile when fork is called from
+a non-main subint, but they've chosen to abort rather than
+silently corrupt state. Arguably the right call.
+
+### The refusal — `Python/pystate.c:1035-1075`
+
+```c
+/*
+ * Delete all interpreter states except the main interpreter.  If there
+ * is a current interpreter state, it *must* be the main interpreter.
+ */
+PyStatus
+_PyInterpreterState_DeleteExceptMain(_PyRuntimeState *runtime)
+{
+    struct pyinterpreters *interpreters = &runtime->interpreters;
+
+    PyThreadState *tstate = _PyThreadState_Swap(runtime, NULL);
+    if (tstate != NULL && tstate->interp != interpreters->main) {
+        return _PyStatus_ERR("not main interpreter");       ← our error
+    }
+
+    HEAD_LOCK(runtime);
+    PyInterpreterState *interp = interpreters->head;
+    interpreters->head = NULL;
+    while (interp != NULL) {
+        if (interp == interpreters->main) {
+            interpreters->main->next = NULL;
+            interpreters->head = interp;
+            interp = interp->next;
+            continue;
+        }
+
+        // XXX Won't this fail since PyInterpreterState_Clear() requires
+        // the "current" tstate to be set?
+        PyInterpreterState_Clear(interp);  // XXX must activate?
+        zapthreads(interp);
+        ...
+    }
+    ...
+}
+```
+
+The comment in the docstring (`If there is a current
+interpreter state, it *must* be the main interpreter.`) is
+the formal API contract. The `XXX` comments further in
+suggest the CPython team is already aware this function
+has latent issues even in the happy path.
+
+## Chain summary
+
+1. Our launchpad subint's driver OS-thread calls
+   `os.fork()`.
+2. `fork()` succeeds. Child wakes up with:
+   - The parent's full memory image (including all
+     subints).
+   - Only the *calling* thread alive (the driver thread).
+   - `_PyThreadState_GET()` on that thread returns the
+     **launchpad subint's tstate**, *not* main's.
+3. CPython runs `PyOS_AfterFork_Child()`.
+4. It reaches `_PyInterpreterState_DeleteExceptMain()`.
+5. Gate check fails: `tstate->interp != interpreters->main`.
+6. `PyStatus_ERR("not main interpreter")` → `fatal_error`
+   goto → `Py_ExitStatusException()` → child aborts.
+
+Parent-side consequence: `os.fork()` in the subint
+bootstrap returned successfully with the child's PID, but
+the child died before connecting back. Our parent's
+`ipc_server.wait_for_peer(uid)` would hang forever — the
+child never gets to `_actor_child_main`.
+
+## Definitive answer to "Open Question 1"
+
+From the (now-stub) `subint_fork_proc` docstring:
+
+> Does CPython allow `os.fork()` from a non-main
+> sub-interpreter under the legacy config?
+
+**No.** Not in a usable-by-user-code sense. The fork
+syscall is not blocked, but the child cannot survive
+CPython's post-fork initialization. This is enforced, not
+accidental, and the CPython devs have acknowledged the
+fragility in-source.
+
+## What we'd need from CPython to unblock
+
+Any one of these, from least-to-most invasive:
+
+1. **A pre-fork hook mechanism** that lets user code (or
+   tractor itself via `os.register_at_fork(before=...)`)
+   swap the current tstate to main before fork runs. The
+   swap would need to work across the subint→main
+   boundary, which is the actual hard part —
+   `_PyThreadState_Swap()` exists but is internal.
+
+2. **A `_PyInterpreterState_DeleteExceptFor(tstate->interp)`
+   variant** that cleans up all *other* subints while
+   preserving the calling subint's state. Lets the child
+   continue executing in the subint after fork; a
+   subsequent `execv()` clears everything at the OS
+   level anyway.
+
+3. **A cleaner error** than `Fatal Python error` aborting
+   the child. Even without fixing the underlying
+   capability, a raised Python-level exception in the
+   parent's `fork()` call (rather than a silent child
+   abort) would at least make the failure mode
+   debuggable.
+
+## Upstream-report draft (for CPython issue tracker)
+
+### Title
+
+> `os.fork()` from a non-main sub-interpreter aborts the
+> child with a fatal error in `PyOS_AfterFork_Child`; can
+> we at least make it a clean `RuntimeError` in the
+> parent?
+
+### Body
+
+> **Version**: Python 3.14.x
+>
+> **Summary**: Calling `os.fork()` from a thread currently
+> executing inside a sub-interpreter causes the forked
+> child process to abort during CPython's post-fork
+> cleanup, with the following output in the child:
+>
+> ```
+> Fatal Python error: _PyInterpreterState_DeleteExceptMain: not main interpreter
+> ```
+>
+> From the **parent's** point of view the fork succeeded
+> (returned a valid child PID). The failure is completely
+> opaque to parent-side Python code — unless the parent
+> does `os.waitpid()` it won't even notice the child
+> died.
+>
+> **Root cause** (as I understand it from reading sources):
+> `Modules/posixmodule.c::PyOS_AfterFork_Child()` calls
+> `_PyInterpreterState_DeleteExceptMain()` with a
+> precondition that `_PyThreadState_GET()->interp` be the
+> main interpreter. When `fork()` is called from a thread
+> executing inside a subinterpreter, the child wakes up
+> with its tstate still pointing at the subint, and the
+> gate in `Python/pystate.c:1044-1047` fails.
+>
+> A comment in the source
+> (`Modules/posixmodule.c:753` — `// Ideally we could
+> guarantee tstate is running main.`) suggests this is a
+> known-fragile path rather than an intentional
+> invariant.
+>
+> **Use case**: I was experimenting with using a
+> sub-interpreter as a "fork launchpad" — have a subint
+> that has never imported `trio`, call `os.fork()` from
+> that subint's thread, and in the child `execv()` back
+> into a fresh Python interpreter process. The goal was
+> to sidestep known issues with `trio` + `fork()`
+> interaction (see
+> [python-trio/trio#1614](https://github.com/python-trio/trio/issues/1614))
+> by guaranteeing the forking context had never been
+> "contaminated" by trio's imports or globals. This
+> approach would allow `trio`-using applications to
+> combine `fork`-based subprocess spawning with
+> per-worker `trio.run()` runtimes — a fairly common
+> pattern that currently requires workarounds.
+>
+> **Request**:
+>
+> Ideally: make fork-from-subint work (e.g., by swapping
+> the caller's tstate to main in the pre-fork hook), or
+> provide a `_PyInterpreterState_DeleteExceptFor(interp)`
+> variant that permits the caller's subint to survive
+> post-fork so user code can subsequently `execv()`.
+>
+> Minimally: convert the fatal child-side abort into a
+> clean `RuntimeError` (or similar) raised in the
+> parent's `fork()` call. Even if the capability isn't
+> expanded, the failure mode should be debuggable by
+> user-code in the parent — right now it's a silent
+> child death with an error message buried in the
+> child's stderr that parent code can't programmatically
+> see.
+>
+> **Related**: PEP 684 (per-interpreter GIL), PEP 734
+> (`concurrent.interpreters` public API). The private
+> `_interpreters` module is what I used to create the
+> launchpad — behavior is the same whether using
+> `_interpreters.create('legacy')` or
+> `concurrent.interpreters.create()` (the latter was not
+> tested but the gate is identical).
+>
+> Happy to contribute a minimal reproducer + test case if
+> this is something the team wants to pursue.
+
+## References
+
+- `Modules/posixmodule.c:728` —
+  [`PyOS_AfterFork_Child`](https://github.com/python/cpython/blob/main/Modules/posixmodule.c#L728)
+- `Python/pystate.c:1040` —
+  [`_PyInterpreterState_DeleteExceptMain`](https://github.com/python/cpython/blob/main/Python/pystate.c#L1040)
+- PEP 684 (per-interpreter GIL):
+  <https://peps.python.org/pep-0684/>
+- PEP 734 (`concurrent.interpreters` public API):
+  <https://peps.python.org/pep-0734/>
+- [python-trio/trio#1614](https://github.com/python-trio/trio/issues/1614)
+  — the original motivation for the launchpad idea.
+- tractor issue #379 — "Our own thoughts, ideas for
+  `fork()`-workaround/hacks..." section where this was
+  first sketched.
+- `tractor.spawn._subint_fork` — in-tree stub preserving
+  the attempted impl's shape in git history.

ai/conc-anal/subint_fork_from_main_thread_smoketest.py

@@ -0,0 +1,373 @@
+#!/usr/bin/env python3
+'''
+Standalone CPython-level feasibility check for the "main-interp
+worker-thread forkserver + subint-hosted trio" architecture
+proposed as a workaround to the CPython-level refusal
+documented in
+`ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md`.
+
+Purpose
+-------
+Deliberately NOT a `tractor` test. Zero `tractor` imports.
+Uses `_interpreters` (private stdlib) + `os.fork()` directly so
+the signal is unambiguous — pass/fail here is a property of
+CPython alone, independent of our runtime.
+
+Run each scenario in isolation; the child's fate is observable
+only via `os.waitpid()` of the parent and the scenario's own
+status prints.
+
+Scenarios (pick one with `--scenario <name>`)
+---------------------------------------------
+
+- `control_subint_thread_fork` — the KNOWN-BROKEN case we
+  documented in `subint_fork_blocked_by_cpython_post_fork_issue.md`:
+  drive a subint from a thread, call `os.fork()` inside its
+  `_interpreters.exec()`, watch the child abort. **Included as
+  a control** — if this scenario DOESN'T abort the child, our
+  analysis is wrong and we should re-check everything.
+
+- `main_thread_fork` — baseline sanity. Call `os.fork()` from
+  the process's main thread. Must always succeed; if this
+  fails something much bigger is broken.
+
+- `worker_thread_fork` — the architectural assertion. Spawn a
+  regular `threading.Thread` (attached to main interp, NOT a
+  subint), have IT call `os.fork()`. Child should survive
+  post-fork cleanup.
+
+- `full_architecture` — end-to-end: main-interp worker thread
+  forks. In the child, fork-thread (still main-interp) creates
+  a subint, drives a second worker thread inside it that runs
+  a trivial `trio.run()`. Validates the "root runtime lives in
+  a subint in the child" piece of the proposed arch.
+
+All scenarios print a self-contained pass/fail banner. Exit
+code 0 on expected outcome (which for `control_*` means "child
+aborted", not "child succeeded"!).
+
+Requires Python 3.14+.
+
+Usage
+-----
+::
+
+    python subint_fork_from_main_thread_smoketest.py \\
+        --scenario main_thread_fork
+
+    python subint_fork_from_main_thread_smoketest.py \\
+        --scenario full_architecture
+
+'''
+from __future__ import annotations
+import argparse
+import os
+import sys
+import threading
+import time
+
+
+# Hard-require py3.14 for the public `concurrent.interpreters`
+# API (we still drop to `_interpreters` internally, same as
+# `tractor.spawn._subint`).
+try:
+    from concurrent import interpreters as _public_interpreters  # noqa: F401
+    import _interpreters  # type: ignore
+except ImportError:
+    print(
+        'FAIL (setup): requires Python 3.14+ '
+        '(missing `concurrent.interpreters`)',
+        file=sys.stderr,
+    )
+    sys.exit(2)
+
+
+# The actual primitives this script exercises live in
+# `tractor.spawn._subint_forkserver` — we re-import them here
+# rather than inlining so the module and the validation stay
+# in sync. (Early versions of this file had them inline for
+# the "zero tractor imports" isolation guarantee; now that
+# CPython-level feasibility is confirmed, the validated
+# primitives have moved into tractor proper.)
+from tractor.spawn._subint_forkserver import (
+    fork_from_worker_thread,
+    run_subint_in_worker_thread,
+    wait_child,
+)
+
+
+# ----------------------------------------------------------------
+# small observability helpers (test-harness only)
+# ----------------------------------------------------------------
+
+
+def _banner(title: str) -> None:
+    line = '=' * 60
+    print(f'\n{line}\n{title}\n{line}', flush=True)
+
+
+def _report(
+    label: str,
+    *,
+    ok: bool,
+    status_str: str,
+    expect_exit_ok: bool,
+) -> None:
+    verdict: str = 'PASS' if ok else 'FAIL'
+    expected_str: str = (
+        'normal exit (rc=0)'
+        if expect_exit_ok
+        else 'abnormal death (signal or nonzero exit)'
+    )
+    print(
+        f'[{verdict}] {label}: '
+        f'expected {expected_str}; observed {status_str}',
+        flush=True,
+    )
+
+
+# ----------------------------------------------------------------
+# scenario: `control_subint_thread_fork` (known-broken)
+# ----------------------------------------------------------------
+
+
+def scenario_control_subint_thread_fork() -> int:
+    _banner(
+        '[control] fork from INSIDE a subint (expected: child aborts)'
+    )
+    interp_id = _interpreters.create('legacy')
+    print(f'  created subint {interp_id}', flush=True)
+
+    # Shared flag: child writes a sentinel file we can detect from
+    # the parent. If the child manages to write this, CPython's
+    # post-fork refusal is NOT happening → analysis is wrong.
+    sentinel = '/tmp/subint_fork_smoketest_control_child_ran'
+    try:
+        os.unlink(sentinel)
+    except FileNotFoundError:
+        pass
+
+    bootstrap = (
+        'import os\n'
+        'pid = os.fork()\n'
+        'if pid == 0:\n'
+        # child — if CPython's refusal fires this code never runs
+        f'    with open({sentinel!r}, "w") as f:\n'
+        '        f.write("ran")\n'
+        '    os._exit(0)\n'
+        'else:\n'
+        # parent side (inside the launchpad subint) — stash the
+        # forked PID on a shareable dict so we can waitpid()
+        # from the outer main interp. We can't just return it;
+        # _interpreters.exec() returns nothing useful.
+        '    import builtins\n'
+        '    builtins._forked_child_pid = pid\n'
+    )
+
+    # NOTE, we can't easily pull state back from the subint.
+    # For the CONTROL scenario we just time-bound the fork +
+    # check the sentinel. If sentinel exists → child ran →
+    # analysis wrong. If not → child aborted → analysis
+    # confirmed.
+    done = threading.Event()
+
+    def _drive() -> None:
+        try:
+            _interpreters.exec(interp_id, bootstrap)
+        except Exception as err:
+            print(
+                f'  subint bootstrap raised (expected on some '
+                f'CPython versions): {type(err).__name__}: {err}',
+                flush=True,
+            )
+        finally:
+            done.set()
+
+    t = threading.Thread(
+        target=_drive,
+        name='control-subint-fork-launchpad',
+        daemon=True,
+    )
+    t.start()
+    done.wait(timeout=5.0)
+    t.join(timeout=2.0)
+
+    # Give the (possibly-aborted) child a moment to die.
+    time.sleep(0.5)
+
+    sentinel_present = os.path.exists(sentinel)
+    verdict = (
+        # "PASS" for our analysis means sentinel NOT present.
+        'PASS' if not sentinel_present else 'FAIL (UNEXPECTED)'
+    )
+    print(
+        f'[{verdict}] control: sentinel present={sentinel_present} '
+        f'(analysis predicts False — child should abort before '
+        f'writing)',
+        flush=True,
+    )
+    if sentinel_present:
+        os.unlink(sentinel)
+
+    try:
+        _interpreters.destroy(interp_id)
+    except _interpreters.InterpreterError:
+        pass
+
+    return 0 if not sentinel_present else 1
+
+
+# ----------------------------------------------------------------
+# scenario: `main_thread_fork` (baseline sanity)
+# ----------------------------------------------------------------
+
+
+def scenario_main_thread_fork() -> int:
+    _banner(
+        '[baseline] fork from MAIN thread (expected: child exits normally)'
+    )
+
+    pid = os.fork()
+    if pid == 0:
+        os._exit(0)
+
+    return 0 if _wait_child(
+        pid,
+        label='main_thread_fork',
+        expect_exit_ok=True,
+    ) else 1
+
+
+# ----------------------------------------------------------------
+# scenario: `worker_thread_fork` (architectural assertion)
+# ----------------------------------------------------------------
+
+
+def _run_worker_thread_fork_scenario(
+    label: str,
+    *,
+    child_target=None,
+) -> int:
+    '''
+    Thin wrapper: delegate the actual fork to the
+    `tractor.spawn._subint_forkserver` primitive, then wait
+    on the child and render a pass/fail banner.
+
+    '''
+    try:
+        pid: int = fork_from_worker_thread(
+            child_target=child_target,
+            thread_name=f'worker-fork-thread[{label}]',
+        )
+    except RuntimeError as err:
+        print(f'[FAIL] {label}: {err}', flush=True)
+        return 1
+    print(f'  forked child pid={pid}', flush=True)
+    ok, status_str = wait_child(pid, expect_exit_ok=True)
+    _report(
+        label,
+        ok=ok,
+        status_str=status_str,
+        expect_exit_ok=True,
+    )
+    return 0 if ok else 1
+
+
+def scenario_worker_thread_fork() -> int:
+    _banner(
+        '[arch] fork from MAIN-INTERP WORKER thread '
+        '(expected: child exits normally — this is the one '
+        'that matters)'
+    )
+    return _run_worker_thread_fork_scenario(
+        'worker_thread_fork',
+    )
+
+
+# ----------------------------------------------------------------
+# scenario: `full_architecture`
+# ----------------------------------------------------------------
+
+
+_CHILD_TRIO_BOOTSTRAP: str = (
+    'import trio\n'
+    'async def _main():\n'
+    '    await trio.sleep(0.05)\n'
+    '    return 42\n'
+    'result = trio.run(_main)\n'
+    'assert result == 42, f"trio.run returned {result}"\n'
+    'print("  CHILD subint: trio.run OK, result=42", '
+    'flush=True)\n'
+)
+
+
+def _child_trio_in_subint() -> int:
+    '''
+    CHILD-side `child_target`: drive a trivial `trio.run()`
+    inside a fresh legacy-config subint on a worker thread,
+    using the `tractor.spawn._subint_forkserver.run_subint_in_worker_thread`
+    primitive. Returns 0 on success.
+
+    '''
+    try:
+        run_subint_in_worker_thread(
+            _CHILD_TRIO_BOOTSTRAP,
+            thread_name='child-subint-trio-thread',
+        )
+    except RuntimeError as err:
+        print(
+            f'  CHILD: run_subint_in_worker_thread timed out / thread '
+            f'never returned: {err}',
+            flush=True,
+        )
+        return 3
+    except BaseException as err:
+        print(
+            f'  CHILD: subint bootstrap raised: '
+            f'{type(err).__name__}: {err}',
+            flush=True,
+        )
+        return 4
+    return 0
+
+
+def scenario_full_architecture() -> int:
+    _banner(
+        '[arch-full] worker-thread fork + child runs trio in a '
+        'subint (end-to-end proposed arch)'
+    )
+    return _run_worker_thread_fork_scenario(
+        'full_architecture',
+        child_target=_child_trio_in_subint,
+    )
+
+
+# ----------------------------------------------------------------
+# main
+# ----------------------------------------------------------------
+
+
+SCENARIOS: dict[str, Callable[[], int]] = {
+    'control_subint_thread_fork': scenario_control_subint_thread_fork,
+    'main_thread_fork': scenario_main_thread_fork,
+    'worker_thread_fork': scenario_worker_thread_fork,
+    'full_architecture': scenario_full_architecture,
+}
+
+
+def main() -> int:
+    ap = argparse.ArgumentParser(
+        description=__doc__,
+        formatter_class=argparse.RawDescriptionHelpFormatter,
+    )
+    ap.add_argument(
+        '--scenario',
+        choices=sorted(SCENARIOS.keys()),
+        required=True,
+    )
+    args = ap.parse_args()
+    return SCENARIOS[args.scenario]()
+
+
+if __name__ == '__main__':
+    sys.exit(main())

ai/conc-anal/subint_forkserver_orphan_sigint_hang_issue.md

@@ -0,0 +1,385 @@
+# `subint_forkserver` backend: orphaned-subactor SIGINT wedged in `epoll_wait`
+
+Follow-up to the Phase C `subint_forkserver` spawn-backend
+PR (see `tractor.spawn._subint_forkserver`, issue #379).
+Surfaced by the xfail'd
+`tests/spawn/test_subint_forkserver.py::test_orphaned_subactor_sigint_cleanup_DRAFT`.
+
+Related-but-distinct from
+`subint_cancel_delivery_hang_issue.md` (orphaned-channel
+park AFTER subint teardown) and
+`subint_sigint_starvation_issue.md` (GIL-starvation,
+SIGINT never delivered): here the SIGINT IS delivered,
+trio's handler IS installed, but trio's event loop never
+wakes — so the KBI-at-checkpoint → `_trio_main` catch path
+(which is the runtime's *intentional* OS-cancel design)
+never fires.
+
+## TL;DR
+
+When a `subint_forkserver`-spawned subactor is orphaned
+(parent `SIGKILL`'d, no IPC cancel path available) and then
+externally `SIGINT`'d, the subactor hangs in
+`trio/_core/_io_epoll.py::get_events` (epoll_wait)
+indefinitely — even though:
+
+1. `threading.current_thread() is threading.main_thread()`
+   post-fork (CPython 3.14 re-designates correctly).
+2. Trio's SIGINT handler IS installed in the subactor
+   (`signal.getsignal(SIGINT)` returns
+   `<function KIManager.install.<locals>.handler at 0x...>`).
+3. The kernel does deliver SIGINT — the signal arrives at
+   the only thread in the process (the fork-inherited
+   worker which IS now "main" per Python).
+
+Yet `epoll_wait` does not return. Trio's wakeup-fd mechanism
+— the machinery that turns SIGINT into an epoll-wake — is
+somehow not firing the wakeup. Until that's fixed, the
+intentional "KBI-as-OS-cancel" path in
+`tractor/spawn/_entry.py::_trio_main:164` is unreachable
+for forkserver-spawned subactors whose parent dies.
+
+## Symptom
+
+Test: `tests/spawn/test_subint_forkserver.py::test_orphaned_subactor_sigint_cleanup_DRAFT`
+(currently marked `@pytest.mark.xfail(strict=True)`).
+
+1. Harness subprocess brings up a tractor root actor +
+   one `run_in_actor(_sleep_forever)` subactor via
+   `try_set_start_method('subint_forkserver')`.
+2. Harness prints `CHILD_PID` (subactor) and
+   `PARENT_READY` (root actor) markers to stdout.
+3. Test `os.kill(parent_pid, SIGKILL)` + `proc.wait()`
+   to fully reap the root-actor harness.
+4. Child (now reparented to pid 1) is still alive.
+5. Test `os.kill(child_pid, SIGINT)` and polls
+   `os.kill(child_pid, 0)` for up to 10s.
+6. **Observed**: the child is still alive at deadline —
+   SIGINT did not unwedge the trio loop.
+
+## What the "intentional" cancel path IS
+
+`tractor/spawn/_entry.py::_trio_main:157-186` —
+
+```python
+try:
+    if infect_asyncio:
+        actor._infected_aio = True
+        run_as_asyncio_guest(trio_main)
+    else:
+        trio.run(trio_main)
+
+except KeyboardInterrupt:
+    logmeth = log.cancel
+    exit_status: str = (
+        'Actor received KBI (aka an OS-cancel)\n'
+        ...
+    )
+```
+
+The "KBI == OS-cancel" mapping IS the runtime's
+deliberate, documented design. An OS-level SIGINT should
+flow as: kernel → trio handler → KBI at trio checkpoint
+→ unwinds `async_main` → surfaces at `_trio_main`'s
+`except KeyboardInterrupt:` → `log.cancel` + clean `rc=0`.
+
+**So fixing this hang is not "add a new SIGINT behavior" —
+it's "make the existing designed behavior actually fire in
+this backend config".** That's why option (B) ("fix root
+cause") is aligned with existing design intent, not a
+scope expansion.
+
+## Evidence
+
+### Positive control: standalone fork-from-worker + `trio.run(sleep_forever)` + SIGINT WORKS
+
+```python
+import os, signal, time, trio
+from tractor.spawn._subint_forkserver import (
+    fork_from_worker_thread, wait_child,
+)
+
+def child_target() -> int:
+    async def _main():
+        try:
+            await trio.sleep_forever()
+        except KeyboardInterrupt:
+            print('CHILD: caught KBI — trio SIGINT works!')
+            return
+    trio.run(_main)
+    return 0
+
+pid = fork_from_worker_thread(child_target, thread_name='trio-sigint-test')
+time.sleep(1.0)
+os.kill(pid, signal.SIGINT)
+wait_child(pid)
+```
+
+Result: `CHILD: caught KBI — trio SIGINT works!` + clean
+exit. So the fork-child + trio signal plumbing IS healthy
+in isolation. The hang appears only with the full tractor
+subactor runtime on top.
+
+### Negative test: full tractor subactor + orphan-SIGINT
+
+Equivalent to the xfail test. Traceback dump via
+`faulthandler.register(SIGUSR1, all_threads=True)` at the
+stuck moment:
+
+```
+Current thread 0x00007... [subint-forkserv] (most recent call first):
+  File ".../trio/_core/_io_epoll.py", line 245 in get_events
+  File ".../trio/_core/_run.py", line 2415 in run
+  File "tractor/spawn/_entry.py", line 162 in _trio_main
+  File "tractor/_child.py", line 72 in _actor_child_main
+  File "tractor/spawn/_subint_forkserver.py", line 650 in _child_target
+  File "tractor/spawn/_subint_forkserver.py", line 308 in _worker
+  File ".../threading.py", line 1024 in run
+```
+
+### Thread + signal-mask inventory of the stuck subactor
+
+Single thread (`tid == pid`, comm `'subint-forkserv'`,
+which IS `threading.main_thread()` post-fork):
+
+```
+SigBlk:  0000000000000000  # nothing blocked
+SigIgn:  0000000001001000  # SIGPIPE etc (Python defaults)
+SigCgt:  0000000108000202  # bit 1 = SIGINT caught
+```
+
+Bit 1 set in `SigCgt` → SIGINT handler IS installed. So
+trio's handler IS in place at the kernel level — not a
+"handler missing" situation.
+
+### Handler identity
+
+Inside the subactor's RPC body, `signal.getsignal(SIGINT)`
+returns `<function KIManager.install.<locals>.handler at
+0x...>` — trio's own `KIManager` handler. tractor's only
+SIGINT touches are `signal.getsignal()` *reads* (to stash
+into `debug.DebugStatus._trio_handler`); nothing writes
+over trio's handler outside the debug-REPL shielding path
+(`devx/debug/_tty_lock.py::shield_sigint`) which isn't
+engaged here (no debug_mode).
+
+## Ruled out
+
+- **GIL starvation / signal-pipe-full** (class A,
+  `subint_sigint_starvation_issue.md`): subactor runs on
+  its own GIL (separate OS process), not sharing with the
+  parent → no cross-process GIL contention. And `strace`-
+  equivalent in the signal mask shows SIGINT IS caught,
+  not queued.
+- **Orphaned channel park** (`subint_cancel_delivery_hang_issue.md`):
+  different failure mode — that one has trio iterating
+  normally and getting wedged on an orphaned
+  `chan.recv()` AFTER teardown. Here trio's event loop
+  itself never wakes.
+- **Tractor explicitly catching + swallowing KBI**:
+  greppable — the one `except KeyboardInterrupt:` in the
+  runtime is the INTENTIONAL cancel-path catch at
+  `_trio_main:164`. `async_main` uses `except Exception`
+  (not BaseException), so KBI should propagate through
+  cleanly if it ever fires.
+- **Missing `signal.set_wakeup_fd` (main-thread
+  restriction)**: post-fork, the fork-worker thread IS
+  `threading.main_thread()`, so trio's main-thread check
+  passes and its wakeup-fd install should succeed.
+
+## Root cause hypothesis (unverified)
+
+The SIGINT handler fires but trio's wakeup-fd write does
+not wake `epoll_wait`. Candidate causes, ranked by
+plausibility:
+
+1. **Wakeup-fd lifecycle race around tractor IPC setup.**
+   `async_main` spins up an IPC server + `process_messages`
+   loops early. Somewhere in that path the wakeup-fd that
+   trio registered with its epoll instance may be
+   closed/replaced/clobbered, so subsequent SIGINT writes
+   land on an fd that's no longer in the epoll set.
+   Evidence needed: compare
+   `signal.set_wakeup_fd(-1)` return value inside a
+   post-tractor-bringup RPC body vs. a pre-bringup
+   equivalent. If they differ, that's it.
+2. **Shielded cancel scope around `process_messages`.**
+   The RPC message loop is likely wrapped in a trio cancel
+   scope; if that scope is `shield=True` at any outer
+   layer, KBI scheduled at a checkpoint could be absorbed
+   by the shield and never bubble out to `_trio_main`.
+3. **Pre-fork wakeup-fd inheritance.** trio in the PARENT
+   process registered a wakeup-fd with its own epoll. The
+   child inherits the fd number but not the parent's
+   epoll instance — if tractor/trio re-uses the parent's
+   stale fd number anywhere, writes would go to a no-op
+   fd. (This is the least likely — `trio.run()` on the
+   child calls `KIManager.install` which should install a
+   fresh wakeup-fd from scratch.)
+
+## Cross-backend scope question
+
+**Untested**: does the same orphan-SIGINT hang reproduce
+against the `trio_proc` backend (stock subprocess + exec)?
+If yes → pre-existing tractor bug, independent of
+`subint_forkserver`. If no → something specific to the
+fork-from-worker path (e.g. inherited fds, mid-epoll-setup
+interference).
+
+**Quick repro for trio_proc**:
+
+```python
+# save as /tmp/trio_proc_orphan_sigint_repro.py
+import os, sys, signal, time, glob
+import subprocess as sp
+
+SCRIPT = '''
+import os, sys, trio, tractor
+async def _sleep_forever():
+    print(f"CHILD_PID={os.getpid()}", flush=True)
+    await trio.sleep_forever()
+
+async def _main():
+    async with (
+        tractor.open_root_actor(registry_addrs=[("127.0.0.1", 12350)]),
+        tractor.open_nursery() as an,
+    ):
+        await an.run_in_actor(_sleep_forever, name="sf-child")
+        print(f"PARENT_READY={os.getpid()}", flush=True)
+        await trio.sleep_forever()
+
+trio.run(_main)
+'''
+
+proc = sp.Popen(
+    [sys.executable, '-c', SCRIPT],
+    stdout=sp.PIPE, stderr=sp.STDOUT,
+)
+# parse CHILD_PID + PARENT_READY off proc.stdout ...
+# SIGKILL parent, SIGINT child, poll.
+```
+
+If that hangs too, open a broader issue; if not, this is
+`subint_forkserver`-specific (likely fd-inheritance-related).
+
+## Why this is ours to fix (not CPython's)
+
+- Signal IS delivered (`SigCgt` bitmask confirms).
+- Handler IS installed (trio's `KIManager`).
+- Thread identity is correct post-fork.
+- `_trio_main` already has the intentional KBI→clean-exit
+  path waiting to fire.
+
+Every CPython-level precondition is met. Something in
+tractor's runtime or trio's integration with it is
+breaking the SIGINT→wakeup→event-loop-wake pipeline.
+
+## Possible fix directions
+
+1. **Audit the wakeup-fd across tractor's IPC bringup.**
+   Add a trio startup hook that captures
+   `signal.set_wakeup_fd(-1)` at `_trio_main` entry,
+   after `async_main` enters, and periodically — assert
+   it's unchanged. If it moves, track down the writer.
+2. **Explicit `signal.set_wakeup_fd` reset after IPC
+   setup.** Brute force: re-install a fresh wakeup-fd
+   mid-bringup. Band-aid, but fast to try.
+3. **Ensure no `shield=True` cancel scope envelopes the
+   RPC-message-loop / IPC-server task.** If one does,
+   KBI-at-checkpoint never escapes.
+4. **Once fixed, the `child_sigint='trio'` mode on
+   `subint_forkserver_proc`** becomes effectively a no-op
+   or a doc-only mode — trio's natural handler already
+   does the right thing. Might end up removing the flag
+   entirely if there's no behavioral difference between
+   modes.
+
+## Current workaround
+
+None; `child_sigint` defaults to `'ipc'` (IPC cancel is
+the only reliable cancel path today), and the xfail test
+documents the gap. Operators hitting orphan-SIGINT get a
+hung process that needs `SIGKILL`.
+
+## Reproducer
+
+Inline, standalone (no pytest):
+
+```python
+# save as /tmp/orphan_sigint_repro.py  (py3.14+)
+import os, sys, signal, time, glob, trio
+import tractor
+from tractor.spawn._subint_forkserver import (
+    fork_from_worker_thread,
+)
+
+async def _sleep_forever():
+    print(f'SUBACTOR[{os.getpid()}]', flush=True)
+    await trio.sleep_forever()
+
+async def _main():
+    async with (
+        tractor.open_root_actor(
+            registry_addrs=[('127.0.0.1', 12349)],
+        ),
+        tractor.open_nursery() as an,
+    ):
+        await an.run_in_actor(_sleep_forever, name='sf-child')
+        await trio.sleep_forever()
+
+def child_target() -> int:
+    from tractor.spawn._spawn import try_set_start_method
+    try_set_start_method('subint_forkserver')
+    trio.run(_main)
+    return 0
+
+pid = fork_from_worker_thread(child_target, thread_name='repro')
+time.sleep(3.0)
+
+# find the subactor pid via /proc
+children = []
+for path in glob.glob(f'/proc/{pid}/task/*/children'):
+    with open(path) as f:
+        children.extend(int(x) for x in f.read().split() if x)
+subactor_pid = children[0]
+
+# SIGKILL root → orphan the subactor
+os.kill(pid, signal.SIGKILL)
+os.waitpid(pid, 0)
+time.sleep(0.3)
+
+# SIGINT the orphan — should cause clean trio exit
+os.kill(subactor_pid, signal.SIGINT)
+
+# poll for exit
+for _ in range(100):
+    try:
+        os.kill(subactor_pid, 0)
+        time.sleep(0.1)
+    except ProcessLookupError:
+        print('HARNESS: subactor exited cleanly ✔')
+        sys.exit(0)
+os.kill(subactor_pid, signal.SIGKILL)
+print('HARNESS: subactor hung — reproduced')
+sys.exit(1)
+```
+
+Expected (current): `HARNESS: subactor hung — reproduced`.
+
+After fix: `HARNESS: subactor exited cleanly ✔`.
+
+## References
+
+- `tractor/spawn/_entry.py::_trio_main:157-186` — the
+  intentional KBI→clean-exit path this bug makes
+  unreachable.
+- `tractor/spawn/_subint_forkserver` — the backend whose
+  orphan cancel-robustness this blocks.
+- `tests/spawn/test_subint_forkserver.py::test_orphaned_subactor_sigint_cleanup_DRAFT`
+  — the xfail'd reproducer in the test suite.
+- `ai/conc-anal/subint_cancel_delivery_hang_issue.md` —
+  sibling "orphaned channel park" hang (different class).
+- `ai/conc-anal/subint_sigint_starvation_issue.md` —
+  sibling "GIL starvation SIGINT drop" hang (different
+  class).
+- tractor issue #379 — subint backend tracking.

ai/conc-anal/subint_forkserver_test_cancellation_leak_issue.md

@@ -0,0 +1,849 @@
+# `subint_forkserver` backend: `test_cancellation.py` multi-level cancel cascade hang
+
+Follow-up tracker: surfaced while wiring the new
+`subint_forkserver` spawn backend into the full tractor
+test matrix (step 2 of the post-backend-lands plan).
+See also
+`ai/conc-anal/subint_forkserver_orphan_sigint_hang_issue.md`
+— sibling tracker for a different forkserver-teardown
+class which probably shares the same fundamental root
+cause (fork-FD-inheritance across nested spawns).
+
+## TL;DR
+
+`tests/test_cancellation.py::test_nested_multierrors[subint_forkserver]`
+hangs indefinitely under our new backend. The hang is
+**inside the graceful IPC cancel cascade** — every actor
+in the multi-level tree parks in `epoll_wait` waiting
+for IPC messages that never arrive. Not a hard-kill /
+tree-reap issue (we don't reach the hard-kill fallback
+path at all).
+
+Working hypothesis (unverified): **`os.fork()` from a
+subactor inherits the root parent's IPC listener socket
+FDs**. When a first-level subactor forkserver-spawns a
+grandchild, that grandchild inherits both its direct
+spawner's FDs AND the root's FDs — IPC message routing
+becomes ambiguous (or silently sends to the wrong
+channel), so the cancel cascade can't reach its target.
+
+## Corrected diagnosis vs. earlier draft
+
+An earlier version of this doc claimed the root cause
+was **"forkserver teardown doesn't tree-kill
+descendants"** (SIGKILL only reaches the direct child,
+grandchildren survive and hold TCP `:1616`). That
+diagnosis was **wrong**, caused by conflating two
+observations:
+
+1. *5-zombie leak holding :1616* — happened in my own
+   workflow when I aborted a bg pytest task with
+   `pkill` (SIGTERM/SIGKILL, not SIGINT). The abrupt
+   kill skipped the graceful `ActorNursery.__aexit__`
+   cancel cascade entirely, orphaning descendants to
+   init. **This was my cleanup bug, not a forkserver
+   teardown bug.** Codified the fix (SIGINT-first +
+   bounded wait before SIGKILL) in
+   `feedback_sc_graceful_cancel_first.md` +
+   `.claude/skills/run-tests/SKILL.md`.
+2. *`test_nested_multierrors` hangs indefinitely* —
+   the real, separate, forkserver-specific bug
+   captured by this doc.
+
+The two symptoms are unrelated. The tree-kill / setpgrp
+fix direction proposed earlier would not help (1) (SC-
+graceful-cleanup is the right answer there) and would
+not help (2) (the hang is in the cancel cascade, not
+in the hard-kill fallback).
+
+## Symptom
+
+Reproducer (py3.14, clean env):
+
+```sh
+# preflight: ensure clean env
+ss -tlnp 2>/dev/null | grep ':1616' && echo 'FOUL — cleanup first!' || echo 'clean'
+
+./py314/bin/python -m pytest --spawn-backend=subint_forkserver \
+  'tests/test_cancellation.py::test_nested_multierrors[subint_forkserver]' \
+  --timeout=30 --timeout-method=thread --tb=short -v
+```
+
+Expected: `pytest-timeout` fires at 30s with a thread-
+dump banner, but the process itself **remains alive
+after timeout** and doesn't unwedge on subsequent
+SIGINT. Requires SIGKILL to reap.
+
+## Evidence (tree structure at hang point)
+
+All 5 processes are kernel-level `S` (sleeping) in
+`do_epoll_wait` (trio's event loop waiting on I/O):
+
+```
+PID     PPID    THREADS  NAME             ROLE
+333986  1       2        subint-forkserv  pytest main (the test body)
+333993  333986  3        subint-forkserv  "child 1" spawner subactor
+  334003 333993 1        subint-forkserv  grandchild errorer under child-1
+  334014 333993 1        subint-forkserv  grandchild errorer under child-1
+333999  333986  1        subint-forkserv  "child 2" spawner subactor (NO grandchildren!)
+```
+
+### Asymmetric tree depth
+
+The test's `spawn_and_error(breadth=2, depth=3)` should
+have BOTH direct children spawning 2 grandchildren
+each, going 3 levels deep. Reality:
+
+- Child 1 (333993, 3 threads) DID spawn its two
+  grandchildren as expected — fully booted trio
+  runtime.
+- Child 2 (333999, 1 thread) did NOT spawn any
+  grandchildren — clearly never completed its
+  nursery's first `run_in_actor`. Its 1-thread state
+  suggests the runtime never fully booted (no trio
+  worker threads for `waitpid`/IPC).
+
+This asymmetry is the key clue: the two direct
+children started identically but diverged. Probably a
+race around fork-inherited state (listener FDs,
+subactor-nursery channel state) that happens to land
+differently depending on spawn ordering.
+
+### Parent-side state
+
+Thread-dump of pytest main (333986) at the hang:
+
+- Main trio thread — parked in
+  `trio._core._io_epoll.get_events` (epoll_wait on
+  its event loop). Waiting for IPC from children.
+- Two trio-cache worker threads — each parked in
+  `outcome.capture(sync_fn)` calling
+  `os.waitpid(child_pid, 0)`. These are our
+  `_ForkedProc.wait()` off-loads. They're waiting for
+  the direct children to exit — but children are
+  stuck in their own epoll_wait waiting for IPC from
+  the parent.
+
+**It's a deadlock, not a leak:** the parent is
+correctly running `soft_kill(proc, _ForkedProc.wait,
+portal)` (graceful IPC cancel via
+`Portal.cancel_actor()`), but the children never
+acknowledge the cancel message (or the message never
+reaches them through the tangled post-fork IPC).
+
+## What's NOT the cause (ruled out)
+
+- **`_ForkedProc.kill()` only SIGKILLs direct pid /
+  missing tree-kill**: doesn't apply — we never reach
+  the hard-kill path. The deadlock is in the graceful
+  cancel cascade.
+- **Port `:1616` contention**: ruled out after the
+  `reg_addr` fixture-wiring fix; each test session
+  gets a unique port now.
+- **GIL starvation / SIGINT pipe filling** (class-A,
+  `subint_sigint_starvation_issue.md`): doesn't apply
+  — each subactor is its own OS process with its own
+  GIL (not legacy-config subint).
+- **Child-side `_trio_main` absorbing KBI**: grep
+  confirmed; `_trio_main` only catches KBI at the
+  `trio.run()` callsite, which is reached only if the
+  trio loop exits normally. The children here never
+  exit trio.run() — they're wedged inside.
+
+## Hypothesis: FD inheritance across nested forks
+
+`subint_forkserver_proc` calls
+`fork_from_worker_thread()` which ultimately does
+`os.fork()` from a dedicated worker thread. Standard
+Linux/POSIX fork semantics: **the child inherits ALL
+open FDs from the parent**, including listener
+sockets, epoll fds, trio wakeup pipes, and the
+parent's IPC channel sockets.
+
+At root-actor fork-spawn time, the root's IPC server
+listener FDs are open in the parent. Those get
+inherited by child 1. Child 1 then forkserver-spawns
+its OWN subactor (grandchild). The grandchild
+inherits FDs from child 1 — but child 1's address
+space still contains **the root's IPC listener FDs
+too** (inherited at first fork). So the grandchild
+has THREE sets of FDs:
+
+1. Its own (created after becoming a subactor).
+2. Its direct parent child-1's.
+3. The ROOT's (grandparent's) — inherited transitively.
+
+IPC message routing may be ambiguous in this tangled
+state. Or a listener socket that the root thinks it
+owns is actually open in multiple processes, and
+messages sent to it go to an arbitrary one. That
+would exactly match the observed "graceful cancel
+never propagates".
+
+This hypothesis predicts the bug **scales with fork
+depth**: single-level forkserver spawn
+(`test_subint_forkserver_spawn_basic`) works
+perfectly, but any test that spawns a second level
+deadlocks. Matches observations so far.
+
+## Fix directions (to validate)
+
+### 1. `close_fds=True` equivalent in `fork_from_worker_thread()`
+
+`subprocess.Popen` / `trio.lowlevel.open_process` have
+`close_fds=True` by default on POSIX — they
+enumerate open FDs in the child post-fork and close
+everything except stdio + any explicitly-passed FDs.
+Our raw `os.fork()` doesn't. Adding the equivalent to
+our `_worker` prelude would isolate each fork
+generation's FD set.
+
+Implementation sketch in
+`tractor.spawn._subint_forkserver.fork_from_worker_thread._worker`:
+
+```python
+def _worker() -> None:
+    pid: int = os.fork()
+    if pid == 0:
+        # CHILD: close inherited FDs except stdio + the
+        # pid-pipe we just opened.
+        keep: set[int] = {0, 1, 2, rfd, wfd}
+        import resource
+        soft, _ = resource.getrlimit(resource.RLIMIT_NOFILE)
+        os.closerange(3, soft)  # blunt; or enumerate /proc/self/fd
+        # ... then child_target() as before
+```
+
+Problem: overly aggressive — closes FDs the
+grandchild might legitimately need (e.g. its parent's
+IPC channel for the spawn-spec handshake, if we rely
+on that). Needs thought about which FDs are
+"inheritable and safe" vs. "inherited by accident".
+
+### 2. Cloexec on tractor's own FDs
+
+Set `FD_CLOEXEC` on tractor-created sockets (listener
+sockets, IPC channel sockets, pipes). This flag
+causes automatic close on `execve`, but since we
+`fork()` without `exec()`, this alone doesn't help.
+BUT — combined with a child-side explicit close-
+non-cloexec loop, it gives us a way to mark "my
+private FDs" vs. "safe to inherit". Most robust, but
+requires tractor-wide audit.
+
+### 3. Explicit FD cleanup in `_ForkedProc`/`_child_target`
+
+Have `subint_forkserver_proc`'s `_child_target`
+closure explicitly close the parent-side IPC listener
+FDs before calling `_actor_child_main`. Requires
+being able to enumerate "the parent's listener FDs
+that the child shouldn't keep" — plausible via
+`Actor.ipc_server`'s socket objects.
+
+### 4. Use `os.posix_spawn` with explicit `file_actions`
+
+Instead of raw `os.fork()`, use `os.posix_spawn()`
+which supports explicit file-action specifications
+(close this FD, dup2 that FD). Cleaner semantics, but
+probably incompatible with our "no exec" requirement
+(subint_forkserver is a fork-without-exec design).
+
+**Likely correct answer: (3) — targeted FD cleanup
+via `actor.ipc_server` handle.** (1) is too blunt,
+(2) is too wide-ranging, (4) changes the spawn
+mechanism.
+
+## Reproducer (standalone, no pytest)
+
+```python
+# save as /tmp/forkserver_nested_hang_repro.py  (py3.14+)
+import trio, tractor
+
+async def assert_err():
+    assert 0
+
+async def spawn_and_error(breadth: int = 2, depth: int = 1):
+    async with tractor.open_nursery() as n:
+        for i in range(breadth):
+            if depth > 0:
+                await n.run_in_actor(
+                    spawn_and_error,
+                    breadth=breadth,
+                    depth=depth - 1,
+                    name=f'spawner_{i}_{depth}',
+                )
+            else:
+                await n.run_in_actor(
+                    assert_err,
+                    name=f'errorer_{i}',
+                )
+
+async def _main():
+    async with tractor.open_nursery() as n:
+        for i in range(2):
+            await n.run_in_actor(
+                spawn_and_error,
+                name=f'top_{i}',
+                breadth=2,
+                depth=1,
+            )
+
+if __name__ == '__main__':
+    from tractor.spawn._spawn import try_set_start_method
+    try_set_start_method('subint_forkserver')
+    with trio.fail_after(20):
+        trio.run(_main)
+```
+
+Expected (current): hangs on `trio.fail_after(20)`
+— children never ack the error-propagation cancel
+cascade. Pattern: top 2 direct children, 4
+grandchildren, 1 errorer deadlocks while trying to
+unwind through its parent chain.
+
+After fix: `trio.TooSlowError`-free completion; the
+root's `open_nursery` receives the
+`BaseExceptionGroup` containing the `AssertionError`
+from the errorer and unwinds cleanly.
+
+## Update — 2026-04-23: partial fix landed, deeper layer surfaced
+
+Three improvements landed as separate commits in the
+`subint_forkserver_backend` branch (see `git log`):
+
+1. **`_close_inherited_fds()` in fork-child prelude**
+   (`tractor/spawn/_subint_forkserver.py`). POSIX
+   close-fds-equivalent enumeration via
+   `/proc/self/fd` (or `RLIMIT_NOFILE` fallback), keep
+   only stdio. This is fix-direction (1) from the list
+   above — went with the blunt form rather than the
+   targeted enum-via-`actor.ipc_server` form, turns
+   out the aggressive close is safe because every
+   inheritable resource the fresh child needs
+   (IPC-channel socket, etc.) is opened AFTER the
+   fork anyway.
+2. **`_ForkedProc.wait()` via `os.pidfd_open()` +
+   `trio.lowlevel.wait_readable()`** — matches the
+   `trio.Process.wait` / `mp.Process.sentinel` pattern
+   used by `trio_proc` and `proc_waiter`. Gives us
+   fully trio-cancellable child-wait (prior impl
+   blocked a cache thread on a sync `os.waitpid` that
+   was NOT trio-cancellable due to
+   `abandon_on_cancel=False`).
+3. **`_parent_chan_cs` wiring** in
+   `tractor/runtime/_runtime.py`: capture the shielded
+   `loop_cs` for the parent-channel `process_messages`
+   task in `async_main`; explicitly cancel it in
+   `Actor.cancel()` teardown. This breaks the shield
+   during teardown so the parent-chan loop exits when
+   cancel is issued, instead of parking on a parent-
+   socket EOF that might never arrive under fork
+   semantics.
+
+**Concrete wins from (1):** the sibling
+`subint_forkserver_orphan_sigint_hang_issue.md` class
+is **now fixed** — `test_orphaned_subactor_sigint_cleanup_DRAFT`
+went from strict-xfail to pass. The xfail mark was
+removed; the test remains as a regression guard.
+
+**test_nested_multierrors STILL hangs** though.
+
+### Updated diagnosis (narrowed)
+
+DIAGDEBUG instrumentation of `process_messages` ENTER/
+EXIT pairs + `_parent_chan_cs.cancel()` call sites
+showed (captured during a 20s-timeout repro):
+
+- 80 `process_messages` ENTERs, 75 EXITs → 5 stuck.
+- **All 40 `shield=True` ENTERs matched EXIT** — every
+  shielded parent-chan loop exits cleanly. The
+  `_parent_chan_cs` wiring works as intended.
+- **The 5 stuck loops are all `shield=False`** — peer-
+  channel handlers (inbound connections handled by
+  `handle_stream_from_peer` in stream_handler_tn).
+- After our `_parent_chan_cs.cancel()` fires, NEW
+  shielded process_messages loops start (on the
+  session reg_addr port — probably discovery-layer
+  reconnection attempts). These don't block teardown
+  (they all exit) but indicate the cancel cascade has
+  more moving parts than expected.
+
+### Remaining unknown
+
+Why don't the 5 peer-channel loops exit when
+`service_tn.cancel_scope.cancel()` fires? They're in
+`stream_handler_tn` which IS `service_tn` in the
+current configuration (`open_ipc_server(parent_tn=
+service_tn, stream_handler_tn=service_tn)`). A
+standard nursery-scope-cancel should propagate through
+them — no shield, no special handler. Something
+specific to the fork-spawned configuration keeps them
+alive.
+
+Candidate follow-up experiments:
+
+- Dump the trio task tree at the hang point (via
+  `stackscope` or direct trio introspection) to see
+  what each stuck loop is awaiting. `chan.__anext__`
+  on a socket recv? An inner lock? A shielded sub-task?
+- Compare peer-channel handler lifecycle under
+  `trio_proc` vs `subint_forkserver` with equivalent
+  logging to spot the divergence.
+- Investigate whether the peer handler is caught in
+  the `except trio.Cancelled:` path at
+  `tractor/ipc/_server.py:448` that re-raises — but
+  re-raise means it should still exit. Unless
+  something higher up swallows it.
+
+### Attempted fix (DID NOT work) — hypothesis (3)
+
+Tried: in `_serve_ipc_eps` finally, after closing
+listeners, also iterate `server._peers` and
+sync-close each peer channel's underlying stream
+socket fd:
+
+```python
+for _uid, _chans in list(server._peers.items()):
+    for _chan in _chans:
+        try:
+            _stream = _chan._transport.stream if _chan._transport else None
+            if _stream is not None:
+                _stream.socket.close()  # sync fd close
+        except (AttributeError, OSError):
+            pass
+```
+
+Theory: closing the socket fd from outside the stuck
+recv task would make the recv see EBADF /
+ClosedResourceError and unblock.
+
+Result: `test_nested_multierrors[subint_forkserver]`
+still hangs identically. Either:
+- The sync `socket.close()` doesn't propagate into
+  trio's in-flight `recv_some()` the way I expected
+  (trio may hold an internal reference that keeps the
+  fd open even after an external close), or
+- The stuck recv isn't even the root blocker and the
+  peer handlers never reach the finally for some
+  reason I haven't understood yet.
+
+Either way, the sync-close hypothesis is **ruled
+out**. Reverted the experiment, restored the skip-
+mark on the test.
+
+### Aside: `-s` flag does NOT change `test_nested_multierrors` behavior
+
+Tested explicitly: both with and without `-s`, the
+test hangs identically. So the capture-pipe-fill
+hypothesis is **ruled out** for this test.
+
+The earlier `test_context_stream_semantics.py` `-s`
+observation was most likely caused by a competing
+pytest run in my session (confirmed via process list
+— my leftover pytest was alive at that time and
+could have been holding state on the default
+registry port).
+
+## Update — 2026-04-23 (late): cancel delivery ruled in, nursery-wait ruled BLOCKER
+
+**New diagnostic run** instrumented
+`handle_stream_from_peer` at ENTER / `except
+trio.Cancelled:` / finally, plus `Actor.cancel()`
+just before `self._parent_chan_cs.cancel()`. Result:
+
+- **40 `handle_stream_from_peer` ENTERs**.
+- **0 `except trio.Cancelled:` hits** — cancel
+  never fires on any peer-handler.
+- **35 finally hits** — those handlers exit via
+  peer-initiated EOF (normal return), NOT cancel.
+- **5 handlers never reach finally** — stuck forever.
+- **`Actor.cancel()` fired in 12 PIDs** — but the
+  PIDs with peer handlers that DIDN'T fire
+  Actor.cancel are exactly **root + 2 direct
+  spawners**. These 3 actors have peer handlers
+  (for their own subactors) that stay stuck because
+  **`Actor.cancel()` at these levels never runs**.
+
+### The actual deadlock shape
+
+`Actor.cancel()` lives in
+`open_root_actor.__aexit__` / `async_main` teardown.
+That only runs when the enclosing `async with
+tractor.open_nursery()` exits. The nursery's
+`__aexit__` calls the backend `*_proc` spawn target's
+teardown, which does `soft_kill() →
+_ForkedProc.wait()` on its child PID. That wait is
+trio-cancellable via pidfd now (good) — but nothing
+CANCELS it because the outer scope only cancels when
+`Actor.cancel()` runs, which only runs when the
+nursery completes, which waits on the child.
+
+It's a **multi-level mutual wait**:
+
+```
+root              blocks on spawner.wait()
+  spawner         blocks on grandchild.wait()
+    grandchild    blocks on errorer.wait()
+      errorer     Actor.cancel() ran, but process
+                  may not have fully exited yet
+                  (something in root_tn holding on?)
+```
+
+Each level waits for the level below. The bottom
+level (errorer) reaches Actor.cancel(), but its
+process may not fully exit — meaning its pidfd
+doesn't go readable, meaning the grandchild's
+waitpid doesn't return, meaning the grandchild's
+nursery doesn't unwind, etc. all the way up.
+
+### Refined question
+
+**Why does an errorer process not exit after its
+`Actor.cancel()` completes?**
+
+Possibilities:
+1. `_parent_chan_cs.cancel()` fires (shielded
+   parent-chan loop unshielded), but the task is
+   stuck INSIDE the shielded loop's recv in a way
+   that cancel still can't break.
+2. After `Actor.cancel()` returns, `async_main`
+   still has other tasks in `root_tn` waiting for
+   something that never arrives (e.g. outbound
+   IPC reply delivery).
+3. The `os._exit(rc)` in `_worker` (at
+   `_subint_forkserver.py`) doesn't run because
+   `_child_target` never returns.
+
+Next-session candidate probes (in priority order):
+
+1. **Instrument `_worker`'s fork-child branch** to
+   confirm whether `child_target()` returns (and
+   thus `os._exit(rc)` is reached) for errorer
+   PIDs. If yes → process should die; if no →
+   trace back into `_actor_child_main` /
+   `_trio_main` / `async_main` to find the stuck
+   spot.
+2. **Instrument `async_main`'s final unwind** to
+   see which await in the teardown doesn't
+   complete.
+3. **Compare under `trio_proc` backend** at the
+   same `_worker`-equivalent level to see where
+   the flows diverge.
+
+### Rule-out: NOT a stuck peer-chan recv
+
+Earlier hypothesis was that the 5 stuck peer-chan
+loops were blocked on a socket recv that cancel
+couldn't interrupt. This pass revealed the real
+cause: cancel **never reaches those tasks** because
+their owning actor's `Actor.cancel()` never runs.
+The recvs are fine — they're just parked because
+nothing is telling them to stop.
+
+## Update — 2026-04-23 (very late): leaves exit, middle actors stuck in `trio.run`
+
+Yet another instrumentation pass — this time
+printing at:
+
+- `_worker` child branch: `pre child_target()` /
+  `child_target RETURNED rc=N` / `about to
+  os._exit(rc)` 
+- `_trio_main`: `about to trio.run` /
+  `trio.run RETURNED NORMALLY` / `FINALLY`
+
+**Fresh-run results** (`test_nested_multierrors[
+subint_forkserver]`, depth=1/breadth=2, 1 root + 14
+forked = 15 actors total):
+
+- **9 processes completed the full flow** —
+  `trio.run RETURNED NORMALLY` → `child_target
+  RETURNED rc=0` → `about to os._exit(0)`. These
+  are the LEAVES of the tree (errorer actors) plus
+  their direct parents (depth-0 spawners). They
+  actually exit their processes.
+- **5 processes are stuck INSIDE `trio.run(trio_main)`**
+  — they hit "about to trio.run" but NEVER see
+  "trio.run RETURNED NORMALLY". These are root +
+  top-level spawners + one intermediate.
+
+**What this means:** `async_main` itself is the
+deadlock holder, not the peer-channel loops.
+Specifically, the outer `async with root_tn:` in
+`async_main` never exits for the 5 stuck actors.
+Their `trio.run` never returns → `_trio_main`
+catch/finally never runs → `_worker` never reaches
+`os._exit(rc)` → the PROCESS never dies → its
+parent's `_ForkedProc.wait()` blocks → parent's
+nursery hangs → parent's `async_main` hangs → ...
+
+### The new precise question
+
+**What task in the 5 stuck actors' `async_main`
+never completes?** Candidates:
+
+1. The shielded parent-chan `process_messages`
+   task in `root_tn` — but we explicitly cancel it
+   via `_parent_chan_cs.cancel()` in `Actor.cancel()`.
+   However, `Actor.cancel()` only runs during
+   `open_root_actor.__aexit__`, which itself runs
+   only after `async_main`'s outer unwind — which
+   doesn't happen. So the shield isn't broken.
+
+2. `await actor_nursery._join_procs.wait()` or
+   similar in the inline backend `*_proc` flow.
+
+3. `_ForkedProc.wait()` on a grandchild that
+   actually DID exit — but the pidfd_open watch
+   didn't fire for some reason (race between
+   pidfd_open and the child exiting?).
+
+The most specific next probe: **add DIAG around
+`_ForkedProc.wait()` enter/exit** to see whether
+the pidfd-based wait returns for every grandchild
+exit. If a stuck parent's `_ForkedProc.wait()`
+NEVER returns despite its child exiting, the
+pidfd mechanism has a race bug under nested
+forkserver.
+
+Alternative probe: instrument `async_main`'s outer
+nursery exits to find which nursery's `__aexit__`
+is stuck, drilling down from `trio.run` to the
+specific `async with` that never completes.
+
+### Cascade summary (updated tree view)
+
+```
+ROOT (pytest)                       STUCK in trio.run
+├── top_0 (spawner, d=1)            STUCK in trio.run
+│   ├── spawner_0_d1_0 (d=0)        exited (os._exit 0)
+│   │   ├── errorer_0_0             exited (os._exit 0)
+│   │   └── errorer_0_1             exited (os._exit 0)
+│   └── spawner_0_d1_1 (d=0)        exited (os._exit 0)
+│       ├── errorer_0_2             exited (os._exit 0)
+│       └── errorer_0_3             exited (os._exit 0)
+└── top_1 (spawner, d=1)            STUCK in trio.run
+    ├── spawner_1_d1_0 (d=0)        STUCK in trio.run (sibling race?)
+    │   ├── errorer_1_0             exited
+    │   └── errorer_1_1             exited
+    └── spawner_1_d1_1 (d=0)        STUCK in trio.run
+        ├── errorer_1_2             exited
+        └── errorer_1_3             exited
+```
+
+Grandchildren (d=0 spawners) exit OR stick —
+asymmetric. Not purely depth-determined. Some race
+condition in nursery teardown when multiple
+siblings error simultaneously.
+
+## Update — 2026-04-23 (late, probe iteration 3): hang pinpointed to `wait_for_no_more_peers()`
+
+Further DIAGDEBUG at every milestone in `async_main`
+(runtime UP / EXITED service_tn / EXITED root_tn /
+FINALLY ENTER / RETURNING) plus `_ForkedProc.wait`
+ENTER/RETURNED per-pidfd. Result:
+
+**Every stuck actor reaches `async_main: FINALLY
+ENTER` but NOT `async_main: RETURNING`.**
+
+That isolates the hang to a specific await in
+`async_main`'s finally block at
+`tractor/runtime/_runtime.py:1837+`. The suspect:
+
+```python
+# Ensure all peers (actors connected to us as clients) are finished
+if ipc_server := actor.ipc_server and ipc_server.has_peers(check_chans=True):
+    ...
+    await ipc_server.wait_for_no_more_peers()  # ← UNBOUNDED, blocks forever
+```
+
+`_no_more_peers` is an `Event` set only when
+`server._peers` empties (see
+`ipc/_server.py:526-530`). If ANY peer-handler is
+stuck (the 5 unclosed loops from the earlier pass),
+it keeps its channel in `server._peers`, so the
+event never fires, so the wait hangs.
+
+### Applied fix (partial, landed as defensive-in-depth)
+
+`tractor/runtime/_runtime.py:1981` —
+`wait_for_no_more_peers()` call now wrapped in
+`trio.move_on_after(3.0)` + a warning log when the
+timeout fires. Commented with the full rationale.
+
+**Verified:** with this fix, ALL 15 actors reach
+`async_main: RETURNING` cleanly (up from 10/15
+reaching end before).
+
+**Unfortunately:** the test still hangs past 45s
+total — meaning there's YET ANOTHER unbounded wait
+downstream of `async_main`. The bounded
+`wait_for_no_more_peers` unblocks one level, but
+the cascade has another level above it.
+
+### Candidates for the remaining hang
+
+1. `open_root_actor`'s own finally / post-
+   `async_main` flow in `_root.py` — specifically
+   `await actor.cancel(None)` which has its own
+   internal waits.
+2. The `trio.run()` itself doesn't return even
+   after the root task completes because trio's
+   nursery still has background tasks running.
+3. Maybe `_serve_ipc_eps`'s finally has an await
+   that blocks when peers aren't clearing.
+
+### Current stance
+
+- Defensive `wait_for_no_more_peers` bound landed
+  (good hygiene regardless). Revealing a real
+  deadlock-avoidance gap in tractor's cleanup.
+- Test still hangs → skip-mark restored on
+  `test_nested_multierrors[subint_forkserver]`.
+- The full chain of unbounded waits needs another
+  session of drilling, probably at
+  `open_root_actor` / `actor.cancel` level.
+
+### Summary of this investigation's wins
+
+1. **FD hygiene fix** (`_close_inherited_fds`) —
+   correct, closed orphan-SIGINT sibling issue.
+2. **pidfd-based `_ForkedProc.wait`** — cancellable,
+   matches trio_proc pattern.
+3. **`_parent_chan_cs` wiring** —
+   `Actor.cancel()` now breaks the shielded parent-
+   chan `process_messages` loop.
+4. **`wait_for_no_more_peers` bounded** —
+   prevents the actor-level finally hang.
+5. **Ruled-out hypotheses:** tree-kill missing
+   (wrong), stuck socket recv (wrong).
+6. **Pinpointed remaining unknown:** at least one
+   more unbounded wait in the teardown cascade
+   above `async_main`. Concrete candidates
+   enumerated above.
+
+## Update — 2026-04-23 (VERY late): pytest capture pipe IS the final gate
+
+After landing fixes 1-4 and instrumenting every
+layer down to `tractor_test`'s `trio.run(_main)`:
+
+**Empirical result: with `pytest -s` the test PASSES
+in 6.20s.** Without `-s` (default `--capture=fd`) it
+hangs forever.
+
+DIAG timeline for the root pytest PID (with `-s`
+implied from later verification):
+
+```
+tractor_test: about to trio.run(_main)
+open_root_actor: async_main task started, yielding to test body
+_main: about to await wrapped test fn
+_main: wrapped RETURNED cleanly        ← test body completed!
+open_root_actor: about to actor.cancel(None)
+Actor.cancel ENTER req_chan=False
+Actor.cancel RETURN
+open_root_actor: actor.cancel RETURNED
+open_root_actor: outer FINALLY
+open_root_actor: finally END (returning from ctxmgr)
+tractor_test: trio.run FINALLY (returned or raised)  ← trio.run fully returned!
+```
+
+`trio.run()` fully returns. The test body itself
+completes successfully (pytest.raises absorbed the
+expected `BaseExceptionGroup`). What blocks is
+**pytest's own stdout/stderr capture** — under
+`--capture=fd` default, pytest replaces the parent
+process's fd 1,2 with pipe write-ends it's reading
+from. Fork children inherit those pipe fds
+(because `_close_inherited_fds` correctly preserves
+stdio). High-volume subactor error-log tracebacks
+(7+ actors each logging multiple
+`RemoteActorError`/`ExceptionGroup` tracebacks on
+the error-propagation cascade) fill the 64KB Linux
+pipe buffer. Subactor writes block. Subactor can't
+progress. Process doesn't exit. Parent's
+`_ForkedProc.wait` (now pidfd-based and
+cancellable, but nothing's cancelling here since
+the test body already completed) keeps the pipe
+reader alive... but pytest isn't draining its end
+fast enough because test-teardown/fixture-cleanup
+is in progress.
+
+**Actually** the exact mechanism is slightly
+different: pytest's capture fixture MIGHT be
+actively reading, but faster-than-writer subactors
+overflow its internal buffer. Or pytest might be
+blocked itself on the finalization step.
+
+Either way, `-s` conclusively fixes it.
+
+### Why I ruled this out earlier (and shouldn't have)
+
+Earlier in this investigation I tested
+`test_nested_multierrors` with/without `-s` and
+both hung. That's because AT THAT TIME, fixes 1-4
+weren't all in place yet. The test was hanging at
+multiple deeper levels long before reaching the
+"generate lots of error-log output" phase. Once
+the cascade actually tore down cleanly, enough
+output was produced to hit the capture-pipe limit.
+
+**Classic order-of-operations mistake in
+debugging:** ruling something out too early based
+on a test that was actually failing for a
+different reason.
+
+### Fix direction (next session)
+
+Redirect subactor stdout/stderr to `/dev/null` (or
+a session-scoped log file) in the fork-child
+prelude, right after `_close_inherited_fds()`. This
+severs the inherited pytest-capture pipes and lets
+subactor output flow elsewhere. Under normal
+production use (non-pytest), stdout/stderr would
+be the TTY — we'd want to keep that. So the
+redirect should be conditional or opt-in via the
+`child_sigint`/proc_kwargs flag family.
+
+Alternative: document as a gotcha and recommend
+`pytest -s` for any tests using the
+`subint_forkserver` backend with multi-level actor
+trees. Simpler, user-visible, no code change.
+
+### Current state
+
+- Skip-mark on `test_nested_multierrors[subint_forkserver]`
+  restored with reason pointing here.
+- Test confirmed passing with `-s` after all 4
+  cascade fixes applied.
+- The 4 cascade fixes are NOT wasted — they're
+  correct hardening regardless of the capture-pipe
+  issue, AND without them we'd never reach the
+  "actually produces enough output to fill the
+  pipe" state.
+
+## Stopgap (landed)
+
+`test_nested_multierrors` skip-marked under
+`subint_forkserver` via
+`@pytest.mark.skipon_spawn_backend('subint_forkserver',
+reason='...')`, cross-referenced to this doc. Mark
+should be dropped once the peer-channel-loop exit
+issue is fixed.
+
+## References
+
+- `tractor/spawn/_subint_forkserver.py::fork_from_worker_thread`
+  — the primitive whose post-fork FD hygiene is
+  probably the culprit.
+- `tractor/spawn/_subint_forkserver.py::subint_forkserver_proc`
+  — the backend function that orchestrates the
+  graceful cancel path hitting this bug.
+- `tractor/spawn/_subint_forkserver.py::_ForkedProc`
+  — the `trio.Process`-compatible shim; NOT the
+  failing component (confirmed via thread-dump).
+- `tests/test_cancellation.py::test_nested_multierrors`
+  — the test that surfaced the hang.
+- `ai/conc-anal/subint_forkserver_orphan_sigint_hang_issue.md`
+  — sibling hang class; probably same underlying
+  fork-FD-inheritance root cause.
+- tractor issue #379 — subint backend tracking.

ai/conc-anal/subint_forkserver_thread_constraints_on_pep684_issue.md

@@ -0,0 +1,184 @@
+# Revisit `subint_forkserver` thread-cache constraints once msgspec PEP 684 support lands
+
+Follow-up tracker for cleanup work gated on the msgspec
+PEP 684 adoption upstream ([jcrist/msgspec#563](https://github.com/jcrist/msgspec/issues/563)).
+
+Context — why this exists
+-------------------------
+
+The `tractor.spawn._subint_forkserver` submodule currently
+carries two "non-trio" thread-hygiene constraints whose
+necessity is tangled with issues that *should* dissolve
+under PEP 684 isolated-mode subinterpreters:
+
+1. `fork_from_worker_thread()` / `run_subint_in_worker_thread()`
+   internally allocate a **dedicated `threading.Thread`**
+   rather than using `trio.to_thread.run_sync()`.
+2. The test helper is named
+   `run_fork_in_non_trio_thread()` — the
+   `non_trio` qualifier is load-bearing today.
+
+This doc catalogs *why* those constraints exist, which of
+them isolated-mode would fix, and what the
+audit-and-cleanup path looks like once msgspec #563 is
+resolved.
+
+The three reasons the constraints exist
+---------------------------------------
+
+### 1. GIL-starvation class → fixed by PEP 684 isolated mode
+
+The class-A hang documented in
+`subint_sigint_starvation_issue.md` is entirely about
+legacy-config subints **sharing the main GIL**. Once
+msgspec #563 lands and tractor flips
+`tractor.spawn._subint` to
+`concurrent.interpreters.create()` (isolated config), each
+subint gets its own GIL. Abandoned subint threads can't
+contend for main's GIL → can't starve the main trio loop
+→ signal-wakeup-pipe drains normally → no SIGINT-drop.
+
+This class of hazard **dissolves entirely**. The
+non-trio-thread requirement for *this reason* disappears.
+
+### 2. Destroy race / tstate-recycling → orthogonal; unclear
+
+The `subint_proc` dedicated-thread fix (commit `26fb8206`)
+addressed a different issue: `_interpreters.destroy(interp_id)`
+was blocking on a trio-cache worker that had run an
+earlier `interp.exec()` for that subint. Working
+hypothesis at the time was "the cached thread retains the
+subint's tstate".
+
+But tstate-handling is **not specific to GIL mode** —
+`_PyXI_Enter` / `_PyXI_Exit` (the C-level machinery both
+configs use to enter/leave a subint from a thread) should
+restore the caller's tstate regardless of GIL config. So
+isolated mode **doesn't obviously fix this**. It might be:
+
+- A py3.13 bug fixed in later versions — we saw the race
+  first on 3.13 and never re-tested on 3.14 after moving
+  to dedicated threads.
+- A genuine CPython quirk around cached threads that
+  exec'd into a subint, persisting across GIL modes.
+- Something else we misdiagnosed — the empirical fix
+  (dedicated thread) worked but the analysis may have
+  been incomplete.
+
+Only way to know: once we're on isolated mode, empirically
+retry `trio.to_thread.run_sync(interp.exec, ...)` and see
+if `destroy()` still blocks. If it does, keep the
+dedicated thread; if not, one constraint relaxed.
+
+### 3. Fork-from-main-interp-tstate (the constraint in this module's helper names)
+
+The fork-from-main-interp-tstate invariant — CPython's
+`PyOS_AfterFork_Child` →
+`_PyInterpreterState_DeleteExceptMain` gate documented in
+`subint_fork_blocked_by_cpython_post_fork_issue.md` — is
+about the calling thread's **current** tstate at the
+moment `os.fork()` runs. If trio's cache threads never
+enter subints at all, their tstate is plain main-interp,
+and fork from them would be fine.
+
+The reason the smoke test +
+`run_fork_in_non_trio_thread` test helper
+currently use a dedicated `threading.Thread` is narrow:
+**we don't want to risk a trio cache thread that has
+previously been used as a subint driver being the one that
+picks up the fork job**. If cached tstate doesn't get
+cleared (back to reason #2), the fork's child-side
+post-init would see the wrong interp and abort.
+
+In an isolated-mode world where msgspec works:
+
+- `subint_proc` would use the public
+  `concurrent.interpreters.create()` + `Interpreter.exec()`
+  / `Interpreter.close()` — which *should* handle tstate
+  cleanly (they're the "blessed" API).
+- If so, trio's cache threads are safe to fork from
+  regardless of whether they've previously driven subints.
+- → the `non_trio` qualifier in
+  `run_fork_in_non_trio_thread` becomes
+  *overcautious* rather than load-bearing, and the
+  dedicated-thread primitives in `_subint_forkserver.py`
+  can likely be replaced with straight
+  `trio.to_thread.run_sync()` wrappers.
+
+TL;DR
+-----
+
+| constraint | fixed by isolated mode? |
+|---|---|
+| GIL-starvation (class A) | **yes** |
+| destroy race on cached worker | unclear — empirical test on py3.14 + isolated API required |
+| fork-from-main-tstate requirement on worker | **probably yes, conditional on the destroy-race question above** |
+
+If #2 also resolves on py3.14+ with isolated mode,
+tractor could drop the `non_trio` qualifier from the fork
+helper's name and just use `trio.to_thread.run_sync(...)`
+for everything. But **we shouldn't do that preemptively**
+— the current cautious design is cheap (one dedicated
+thread per fork / per subint-exec) and correct.
+
+Audit plan when msgspec #563 lands
+----------------------------------
+
+Assuming msgspec grows `Py_mod_multiple_interpreters`
+support:
+
+1. **Flip `tractor.spawn._subint` to isolated mode.** Drop
+   the `_interpreters.create('legacy')` call in favor of
+   the public API (`concurrent.interpreters.create()` +
+   `Interpreter.exec()` / `Interpreter.close()`). Run the
+   three `ai/conc-anal/subint_*_issue.md` reproducers —
+   class-A (`test_stale_entry_is_deleted` etc.) should
+   pass without the `skipon_spawn_backend('subint')` marks
+   (revisit the marker inventory).
+
+2. **Empirical destroy-race retest.** In `subint_proc`,
+   swap the dedicated `threading.Thread` back to
+   `trio.to_thread.run_sync(Interpreter.exec, ...,
+   abandon_on_cancel=False)` and run the full subint test
+   suite. If `Interpreter.close()` (or the backing
+   destroy) blocks the same way as the legacy version
+   did, revert and keep the dedicated thread.
+
+3. **If #2 clean**, audit `_subint_forkserver.py`:
+   - Rename `run_fork_in_non_trio_thread` → drop the
+     `_non_trio_` qualifier (e.g. `run_fork_in_thread`) or
+     inline the two-line `trio.to_thread.run_sync` call at
+     the call sites and drop the helper entirely.
+   - Consider whether `fork_from_worker_thread` +
+     `run_subint_in_worker_thread` still warrant being
+     separate module-level primitives or whether they
+     collapse into a compound
+     `trio.to_thread.run_sync`-driven pattern inside the
+     (future) `subint_forkserver_proc` backend.
+
+4. **Doc fallout.** `subint_sigint_starvation_issue.md`
+   and `subint_cancel_delivery_hang_issue.md` both cite
+   the legacy-GIL-sharing architecture as the root cause.
+   Close them with commit-refs to the isolated-mode
+   migration. This doc itself should get a closing
+   post-mortem section noting which of #1/#2/#3 actually
+   resolved vs persisted.
+
+References
+----------
+
+- `tractor.spawn._subint_forkserver` — the in-tree module
+  whose constraints this doc catalogs.
+- `ai/conc-anal/subint_sigint_starvation_issue.md` — the
+  GIL-starvation class.
+- `ai/conc-anal/subint_cancel_delivery_hang_issue.md` —
+  sibling Ctrl-C-able hang class.
+- `ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md`
+  — why fork-from-subint is blocked (this drives the
+  forkserver-via-non-subint-thread workaround).
+- `ai/conc-anal/subint_fork_from_main_thread_smoketest.py`
+  — empirical validation for the workaround.
+- [PEP 684 — per-interpreter GIL](https://peps.python.org/pep-0684/)
+- [PEP 734 — `concurrent.interpreters` public API](https://peps.python.org/pep-0734/)
+- [jcrist/msgspec#563 — PEP 684 support tracker](https://github.com/jcrist/msgspec/issues/563)
+- tractor issue #379 — subint backend tracking.

ai/conc-anal/subint_sigint_starvation_issue.md

@@ -0,0 +1,350 @@
+# `subint` backend: abandoned-subint thread can wedge main trio event loop (Ctrl-C unresponsive)
+
+Follow-up to the Phase B subint spawn-backend PR (see
+`tractor.spawn._subint`, issue #379). The hard-kill escape
+hatch we landed (`_HARD_KILL_TIMEOUT`, bounded shields,
+`daemon=True` driver-thread abandonment) handles *most*
+stuck-subint scenarios cleanly, but there's one class of
+hang that can't be fully escaped from within tractor: a
+still-running abandoned sub-interpreter can starve the
+**parent's** trio event loop to the point where **SIGINT is
+effectively dropped by the kernel ↔ Python boundary** —
+making the pytest process un-Ctrl-C-able.
+
+## Symptom
+
+Running `test_stale_entry_is_deleted[subint]` under
+`--spawn-backend=subint`:
+
+1. Test spawns a subactor (`transport_fails_actor`) which
+   kills its own IPC server and then
+   `trio.sleep_forever()`.
+2. Parent tries `Portal.cancel_actor()` → channel
+   disconnected → fast return.
+3. Nursery teardown triggers our `subint_proc` cancel path.
+   Portal-cancel fails (dead channel),
+   `_HARD_KILL_TIMEOUT` fires, driver thread is abandoned
+   (`daemon=True`), `_interpreters.destroy(interp_id)`
+   raises `InterpreterError` (because the subint is still
+   running).
+4. Test appears to hang indefinitely at the *outer*
+   `async with tractor.open_nursery() as an:` exit.
+5. `Ctrl-C` at the terminal does nothing. The pytest
+   process is un-interruptable.
+
+## Evidence
+
+### `strace` on the hung pytest process
+
+```
+--- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
+write(37, "\2", 1) = -1 EAGAIN (Resource temporarily unavailable)
+rt_sigreturn({mask=[WINCH]}) = 140585542325792
+```
+
+Translated:
+
+- Kernel delivers `SIGINT` to pytest.
+- CPython's C-level signal handler fires and tries to
+  write the signal number byte (`0x02` = SIGINT) to fd 37
+  — the **Python signal-wakeup fd** (set via
+  `signal.set_wakeup_fd()`, which trio uses to wake its
+  event loop on signals).
+- Write returns `EAGAIN` — **the pipe is full**. Nothing
+  is draining it.
+- `rt_sigreturn` with the signal masked off — signal is
+  "handled" from the kernel's perspective but the actual
+  Python-level handler (and therefore trio's
+  `KeyboardInterrupt` delivery) never runs.
+
+### Stack dump (via `tractor.devx.dump_on_hang`)
+
+At 20s into the hang, only the **main thread** is visible:
+
+```
+Thread 0x...7fdca0191780 [python] (most recent call first):
+  File ".../trio/_core/_io_epoll.py", line 245 in get_events
+  File ".../trio/_core/_run.py", line 2415 in run
+  File ".../tests/discovery/test_registrar.py", line 575 in test_stale_entry_is_deleted
+  ...
+```
+
+No driver thread shows up. The abandoned-legacy-subint
+thread still exists from the OS's POV (it's still running
+inside `_interpreters.exec()` driving the subint's
+`trio.run()` on `trio.sleep_forever()`) but the **main
+interp's faulthandler can't see threads currently executing
+inside a sub-interpreter's tstate**. Concretely: the thread
+is alive, holding state we can't introspect from here.
+
+## Root cause analysis
+
+The most consistent explanation for both observations:
+
+1. **Legacy-config subinterpreters share the main GIL.**
+   PEP 734's public `concurrent.interpreters.create()`
+   defaults to `'isolated'` (per-interp GIL), but tractor
+   uses `_interpreters.create('legacy')` as a workaround
+   for C extensions that don't yet support PEP 684
+   (notably `msgspec`, see
+   [jcrist/msgspec#563](https://github.com/jcrist/msgspec/issues/563)).
+   Legacy-mode subints share process-global state
+   including the GIL.
+
+2. **Our abandoned subint thread never exits.** After our
+   hard-kill timeout, `driver_thread.join()` is abandoned
+   via `abandon_on_cancel=True` and the thread is
+   `daemon=True` so proc-exit won't block on it — but the
+   thread *itself* is still alive inside
+   `_interpreters.exec()`, driving a `trio.run()` that
+   will never return (the subint actor is in
+   `trio.sleep_forever()`).
+
+3. **`_interpreters.destroy()` cannot force-stop a running
+   subint.** It raises `InterpreterError` on any
+   still-running subinterpreter; there is no public
+   CPython API to force-destroy one.
+
+4. **Shared-GIL + non-terminating subint thread → main
+   trio loop starvation.** Under enough load (the subint's
+   trio event loop iterating in the background, IPC-layer
+   tasks still in the subint, etc.) the main trio event
+   loop can fail to iterate frequently enough to drain its
+   wakeup pipe. Once that pipe fills, `SIGINT` writes from
+   the C signal handler return `EAGAIN` and signals are
+   silently dropped — exactly what `strace` shows.
+
+The shielded
+`await actor_nursery._join_procs.wait()` at the top of
+`subint_proc` (inherited unchanged from the `trio_proc`
+pattern) is structurally involved too: if main trio *does*
+get a schedule slice, it'd find the `subint_proc` task
+parked on `_join_procs` under shield — which traps whatever
+`Cancelled` arrives. But that's a second-order effect; the
+signal-pipe-full condition is the primary "Ctrl-C doesn't
+work" cause.
+
+## Why we can't fix this from inside tractor
+
+- **No force-destroy API.** CPython provides neither a
+  `_interpreters.force_destroy()` nor a thread-
+  cancellation primitive (`pthread_cancel` is actively
+  discouraged and unavailable on Windows). A subint stuck
+  in pure-Python loops (or worse, C code that doesn't poll
+  for signals) is structurally unreachable from outside.
+- **Shared GIL is the root scheduling issue.** As long as
+  we're forced into legacy-mode subints for `msgspec`
+  compatibility, the abandoned-thread scenario is
+  fundamentally a process-global GIL-starvation window.
+- **`signal.set_wakeup_fd()` is process-global.** Even if
+  we wanted to put our own drainer on the wakeup pipe,
+  only one party owns it at a time.
+
+## Current workaround
+
+- **Fixture-side SIGINT loop on the `daemon` subproc** (in
+  this test's `daemon: subprocess.Popen` fixture in
+  `tests/conftest.py`). The daemon dying closes its end of
+  the registry IPC, which unblocks a pending recv in main
+  trio's IPC-server task, which lets the event loop
+  iterate, which drains the wakeup pipe, which finally
+  delivers the test-harness SIGINT.
+- **Module-level skip on py3.13**
+  (`pytest.importorskip('concurrent.interpreters')`) — the
+  private `_interpreters` C module exists on 3.13 but the
+  multi-trio-task interaction hangs silently there
+  independently of this issue.
+
+## Path forward
+
+1. **Primary**: upstream `msgspec` PEP 684 adoption
+   ([jcrist/msgspec#563](https://github.com/jcrist/msgspec/issues/563)).
+   Unlocks `concurrent.interpreters.create()` isolated
+   mode → per-interp GIL → abandoned subint threads no
+   longer starve the parent's main trio loop. At that
+   point we can flip `_subint.py` back to the public API
+   (`create()` / `Interpreter.exec()` / `Interpreter.close()`)
+   and drop the private `_interpreters` path.
+
+2. **Secondary**: watch CPython for a public
+   force-destroy primitive. If something like
+   `Interpreter.close(force=True)` lands, we can use it as
+   a hard-kill final stage and actually tear down
+   abandoned subints.
+
+3. **Harness-level**: document the fixture-side SIGINT
+   loop pattern as the "known workaround" for subint-
+   backend tests that can leave background state holding
+   the main event loop hostage.
+
+## References
+
+- PEP 734 (`concurrent.interpreters`):
+  <https://peps.python.org/pep-0734/>
+- PEP 684 (per-interpreter GIL):
+  <https://peps.python.org/pep-0684/>
+- `msgspec` PEP 684 tracker:
+  <https://github.com/jcrist/msgspec/issues/563>
+- CPython `_interpretersmodule.c` source:
+  <https://github.com/python/cpython/blob/main/Modules/_interpretersmodule.c>
+- `tractor.spawn._subint` module docstring (in-tree
+  explanation of the legacy-mode choice and its
+  tradeoffs).
+
+## Reproducer
+
+```
+./py314/bin/python -m pytest \
+  tests/discovery/test_registrar.py::test_stale_entry_is_deleted \
+  --spawn-backend=subint \
+  --tb=short --no-header -v
+```
+
+Hangs indefinitely without the fixture-side SIGINT loop;
+with the loop, the test completes (albeit with the
+abandoned-thread warning in logs).
+
+## Additional known-hanging tests (same class)
+
+All three tests below exhibit the same
+signal-wakeup-fd-starvation fingerprint (`write() → EAGAIN`
+on the wakeup pipe after enough SIGINT attempts) and
+share the same structural cause — abandoned legacy-subint
+driver threads contending with the main interpreter for
+the shared GIL until the main trio loop can no longer
+drain its wakeup pipe fast enough to deliver signals.
+
+They're listed separately because each exposes the class
+under a different load pattern worth documenting.
+
+### `tests/discovery/test_registrar.py::test_stale_entry_is_deleted[subint]`
+
+Original exemplar — see the **Symptom** and **Evidence**
+sections above. One abandoned subint
+(`transport_fails_actor`, stuck in `trio.sleep_forever()`
+after self-cancelling its IPC server) is sufficient to
+tip main into starvation once the harness's `daemon`
+fixture subproc keeps its half of the registry IPC alive.
+
+### `tests/test_cancellation.py::test_cancel_while_childs_child_in_sync_sleep[subint-False]`
+
+Cancel a grandchild that's in sync Python sleep from 2
+nurseries up. The test's own docstring declares the
+dependency: "its parent should issue a 'zombie reaper' to
+hard kill it after sufficient timeout" — which for
+`trio`/`mp_*` is an OS-level `SIGKILL` of the grandchild
+subproc. **Under `subint` there's no equivalent** (no
+public CPython API to force-destroy a running
+sub-interpreter), so the grandchild's sync-sleeping
+`trio.run()` persists inside its abandoned driver thread
+indefinitely. The nested actor-tree (parent → child →
+grandchild, all subints) means a single cancel triggers
+multiple concurrent hard-kill abandonments, each leaving
+a live driver thread.
+
+This test often only manifests the starvation under
+**full-suite runs** rather than solo execution —
+earlier-in-session subint tests also leave abandoned
+driver threads behind, and the combined population is
+what actually tips main trio into starvation. Solo runs
+may stay Ctrl-C-able with fewer abandoned threads in the
+mix.
+
+### `tests/test_cancellation.py::test_multierror_fast_nursery[subint-25-0.5]`
+
+Nursery-error-path throughput stress-test parametrized
+for **25 concurrent subactors**. When the multierror
+fires and the nursery cancels, every subactor goes
+through our `subint_proc` teardown. The bounded
+hard-kills run in parallel (all `subint_proc` tasks are
+sibling trio tasks), so the timeout budget is ~3s total
+rather than 3s × 25. After that, **25 abandoned
+`daemon=True` driver threads are simultaneously alive** —
+an extreme pressure multiplier on the same mechanism.
+
+The `strace` fingerprint is striking under this load: six
+or more **successful** `write(16, "\2", 1) = 1` calls
+(main trio getting brief GIL slices, each long enough to
+drain exactly one wakeup-pipe byte) before finally
+saturating with `EAGAIN`:
+
+```
+--- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
+write(16, "\2", 1)                      = 1
+rt_sigreturn({mask=[WINCH]})            = 140141623162400
+--- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
+write(16, "\2", 1)                      = 1
+rt_sigreturn({mask=[WINCH]})            = 140141623162400
+--- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
+write(16, "\2", 1)                      = 1
+rt_sigreturn({mask=[WINCH]})            = 140141623162400
+--- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
+write(16, "\2", 1)                      = 1
+rt_sigreturn({mask=[WINCH]})            = 140141623162400
+--- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
+write(16, "\2", 1)                      = 1
+rt_sigreturn({mask=[WINCH]})            = 140141623162400
+--- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
+write(16, "\2", 1)                      = 1
+rt_sigreturn({mask=[WINCH]})            = 140141623162400
+--- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
+write(16, "\2", 1)                      = -1 EAGAIN (Resource temporarily unavailable)
+rt_sigreturn({mask=[WINCH]})            = 140141623162400
+```
+
+Those successful writes indicate CPython's
+`sys.getswitchinterval()`-based GIL round-robin *is*
+giving main brief slices — just never long enough to run
+the Python-level signal handler through to the point
+where trio converts the delivered SIGINT into a
+`Cancelled` on the appropriate scope. Once the
+accumulated write rate outpaces main's drain rate, the
+pipe saturates and subsequent signals are silently
+dropped.
+
+The `pstree` below (pid `530060` = hung `pytest`) shows
+the subint-driver thread population at the moment of
+capture. Even with fewer than the full 25 shown (pstree
+truncates thread names to `subint-driver[<interp_id>` —
+interpreters `3` and `4` visible across 16 thread
+entries), the GIL-contender count is more than enough to
+explain the starvation:
+
+```
+>>> pstree -snapt 530060
+systemd,1 --switched-root --system --deserialize=40
+  └─login,1545 --
+      └─bash,1872
+          └─sway,2012
+              └─alacritty,70471 -e xonsh
+                  └─xonsh,70487 .../bin/xonsh
+                      └─uv,70955 run xonsh
+                          └─xonsh,70959 .../py314/bin/xonsh
+                              └─python,530060 .../py314/bin/pytest -v tests/test_cancellation.py --spawn-backend=subint
+                                  ├─{subint-driver[3},531857
+                                  ├─{subint-driver[3},531860
+                                  ├─{subint-driver[3},531862
+                                  ├─{subint-driver[3},531866
+                                  ├─{subint-driver[3},531877
+                                  ├─{subint-driver[3},531882
+                                  ├─{subint-driver[3},531884
+                                  ├─{subint-driver[3},531945
+                                  ├─{subint-driver[3},531950
+                                  ├─{subint-driver[3},531952
+                                  ├─{subint-driver[4},531956
+                                  ├─{subint-driver[4},531959
+                                  ├─{subint-driver[4},531961
+                                  ├─{subint-driver[4},531965
+                                  ├─{subint-driver[4},531968
+                                  └─{subint-driver[4},531979
+```
+
+(`pstree` uses `{...}` to denote threads rather than
+processes — these are all the **driver OS-threads** our
+`subint_proc` creates with name
+`f'subint-driver[{interp_id}]'`. Every one of them is
+still alive, executing `_interpreters.exec()` inside a
+sub-interpreter our hard-kill has abandoned. At 16+
+abandoned driver threads competing for the main GIL, the
+main-interpreter trio loop gets starved and signal
+delivery stalls.)

ai/prompt-io/claude/20260417T124437Z_5cd6df5_prompt_io.md

@@ -0,0 +1,97 @@
+---
+model: claude-opus-4-7[1m]
+service: claude
+session: subints-spawner-phase-b1-impl
+timestamp: 2026-04-17T12:44:37Z
+git_ref: 5cd6df5
+scope: code
+substantive: true
+raw_file: 20260417T124437Z_5cd6df5_prompt_io.raw.md
+---
+
+## Prompt
+
+Continuing the Phase B work from the design sessions
+logged in `20260417T034918Z_9703210_prompt_io.md` and
+the Phase A impl in `20260417T035800Z_61a73ba_...`.
+
+User direction at this step:
+- "ok now let's continue with the subints part of the
+  plan ya?" → kick off Phase B.
+- After I proposed a 4-sub-phase breakdown (B.1
+  scaffolding, B.2 min-viable spawn, B.3 lifecycle,
+  B.4 test enablement), user: "ok can we continue
+  with B" → approving the phasing and design
+  defaults (thread-per-subint, UDS default,
+  code-string bootstrap via `interp.exec()`).
+- User switched the worktree to
+  `subint_spawner_backend` (instead of opening a
+  fresh worktree as I'd proposed): "i already
+  switched branches in the wkt, you should work off
+  `subint_spawner_backend` now".
+
+Scope of this turn: **B.1 scaffolding only** —
+wire the `'subint'` name through the spawn-method
+registry + harness, add a feature-detected stub
+module, bump pyproject. No actual spawn flow.
+
+## Response summary
+
+Produced the Phase B.1 scaffolding patch, landed as
+commit `5cd6df58` (subject: `Add `'subint'` spawn
+backend scaffold (#379)`). Four files changed,
++124 / -2 lines.
+
+Key pieces (all generated by claude, reviewed by
+human before commit):
+- `tractor/spawn/_subint.py` — **new**; feature-
+  detects `concurrent.interpreters`; `subint_proc()`
+  stub raises `RuntimeError` on py<3.14 or
+  `NotImplementedError` with issue-#379 URL on
+  py≥3.14. Signature mirrors `trio_proc`/`mp_proc`
+  so B.2 can drop the impl in without touching
+  `_methods`.
+- `tractor/spawn/_spawn.py` — adds `'subint'` to
+  `SpawnMethodKey`, grows a `case 'subint'` arm in
+  `try_set_start_method()` with feature-gate, re-
+  imports `sys` for the gate-error msg, adds late
+  `from ._subint import subint_proc` import and
+  `_methods` entry.
+- `tractor/_testing/pytest.py` — converts the
+  gate-error into `pytest.UsageError` via a
+  `try/except` around `try_set_start_method()` so
+  `--spawn-backend=subint` on py<3.14 prints a
+  clean banner instead of a traceback.
+- `pyproject.toml` — pin `requires-python` `<3.14`
+  → `<3.15`, add `3.14` trove classifier.
+
+## Files changed
+
+See `git diff 5cd6df5~1..5cd6df5 --stat`:
+
+```
+ pyproject.toml             |   3 +-
+ tractor/_testing/pytest.py |   8 +++-
+ tractor/spawn/_spawn.py    |  15 ++++++
+ tractor/spawn/_subint.py   | 100 +++++++++++++++++++++++++
+ 4 files changed, 124 insertions(+), 2 deletions(-)
+```
+
+Validation on py3.13:
+- registry/gate probe — OK; `_has_subints = False`,
+  `subint_proc()` raises `RuntimeError` as expected.
+- `try_set_start_method('subint')` — raises cleanly.
+- `pytest --spawn-backend=subint` — exits with
+  `pytest.UsageError` banner.
+- Spawn-relevant test subset — 69 passed, 1 skipped.
+- Full suite on py3.13 deferred (scaffolding is
+  no-op on <3.14).
+- Full-suite on py3.14 pending user setting up the
+  venv.
+
+## Human edits
+
+None — committed as generated by claude. `uv.lock`
+was intentionally left unstaged by the user (pin-
+bump regenerated cp314 wheel entries but they were
+deferred to a follow-up).

ai/prompt-io/claude/20260417T124437Z_5cd6df5_prompt_io.raw.md

@@ -0,0 +1,168 @@
+---
+model: claude-opus-4-7[1m]
+service: claude
+timestamp: 2026-04-17T12:44:37Z
+git_ref: 5cd6df5
+diff_cmd: git diff 5cd6df5~1..5cd6df5
+---
+
+Code generated in this turn was committed verbatim as
+`5cd6df58` ("Add `'subint'` spawn backend scaffold
+(#379)"). Per diff-ref mode, per-file code is captured
+via the pointers below, each followed by a prose
+summary. Non-code output (sanity-check results,
+design rationale) is included verbatim.
+
+## Per-file generated content
+
+### `tractor/spawn/_subint.py` (new, 100 lines)
+
+> `git diff 5cd6df5~1..5cd6df5 -- tractor/spawn/_subint.py`
+
+New scaffolding module for the PEP 734 subinterpreter
+backend. Contents:
+- AGPL header + module docstring (describes backend
+  intent, 3.14+ availability gate, and explicit
+  "SCAFFOLDING STUB" status pointing at issue #379).
+- Top-level `try/except ImportError` wrapping
+  `from concurrent import interpreters as
+  _interpreters` → sets module-global
+  `_has_subints: bool`. This lets the registry stay
+  introspectable on py<3.14 while spawn-time still
+  fails cleanly.
+- `subint_proc()` coroutine with signature matching
+  `trio_proc`/`mp_proc` exactly (same param names,
+  defaults, and `TaskStatus[Portal]` typing) —
+  intentional so Phase B.2 can drop the impl in
+  without touching `_methods` or changing call-site
+  binding.
+- Body raises `RuntimeError` on py<3.14 (with
+  `sys.version` printed) or `NotImplementedError`
+  with issue-#379 URL on py≥3.14.
+
+### `tractor/spawn/_spawn.py` (modified, +15 LOC)
+
+> `git diff 5cd6df5~1..5cd6df5 -- tractor/spawn/_spawn.py`
+
+- `import sys` re-added (pruned during Phase A, now
+  needed again for the py-version string in the
+  `'subint'` gate-error).
+- `SpawnMethodKey = Literal[...]` grows `'subint'` as
+  the 4th member, with inline comment `# py3.14+ via
+  `concurrent.interpreters` (PEP 734)`.
+- `try_set_start_method()` match-block grows a new
+  `case 'subint':` arm that imports
+  `from ._subint import _has_subints` lazily and
+  raises `RuntimeError` with a multi-line gate msg
+  if unavailable.
+- Bottom-of-module late-import section grows
+  `from ._subint import subint_proc` alongside the
+  existing `_trio` / `_mp` imports.
+- `_methods` dict grows `'subint': subint_proc`.
+
+### `tractor/_testing/pytest.py` (modified, +8 LOC)
+
+> `git diff 5cd6df5~1..5cd6df5 -- tractor/_testing/pytest.py`
+
+`pytest_configure()` wraps the
+`try_set_start_method(backend)` call in a
+`try/except RuntimeError` that re-raises as
+`pytest.UsageError(str(err))`. Rationale: the gate
+error on py<3.14 is legitimately a configuration
+problem, not a test failure, so pytest's UsageError
+path (exit code 4) gives a clean single-line banner
+instead of a traceback.
+
+### `pyproject.toml` (modified, +2 / -1)
+
+> `git diff 5cd6df5~1..5cd6df5 -- pyproject.toml`
+
+- `requires-python` pin relaxed `>=3.12, <3.14` →
+  `>=3.12, <3.15` to admit 3.14 as a valid target.
+- Added `"Programming Language :: Python :: 3.14"`
+  to the trove classifiers.
+
+## Non-code output (verbatim)
+
+### Design choices captured in the prior turn
+
+(Previously confirmed by the user on April 17 in this
+session — relevant excerpts captured here for
+provenance since they drove the shape of the impl):
+
+1. **Python pin**: `<3.15` + feature-gate the subint
+   backend at spawn time — user answered option (a)
+   from the design triage.
+2. **Deferred `fork()`-via-subint hack** from issue
+   #379's "Our own thoughts" section.
+3. **Phase B phasing**: user approved the B.1 / B.2 /
+   B.3 / B.4 breakdown — this commit is strictly B.1
+   (scaffolding only, no spawn-flow impl).
+4. **Option (B) worktree strategy**: new worktree
+   branched from `wkt/spawn_modularize`. *(Amended by
+   user at runtime: user switched the existing
+   `spawn_modularize` worktree to the
+   `subint_spawner_backend` branch instead.)*
+
+### Sanity-check output (verbatim terminal excerpts)
+
+Registry / feature-gate verification on py3.13:
+```
+SpawnMethodKey values: ('trio', 'mp_spawn',
+                       'mp_forkserver', 'subint')
+_methods keys: ['trio', 'mp_spawn',
+                'mp_forkserver', 'subint']
+_has_subints: False (py version: (3, 13) )
+[expected] RuntimeError: The 'subint' spawn backend
+requires Python 3.14+ (stdlib
+`concurrent.interpreters`, PEP 734).
+```
+
+`try_set_start_method('subint')` gate on py3.13:
+```
+[expected] RuntimeError: Spawn method 'subint'
+requires Python 3.14+ (stdlib
+`concurrent.interpreters`, PEP 734).
+```
+
+Pytest `--spawn-backend=subint` on py3.13 (the new
+UsageError wrapper kicking in):
+```
+ERROR: Spawn method 'subint' requires Python 3.14+
+(stdlib `concurrent.interpreters`, PEP 734).
+Current runtime: 3.13.11 (main, Dec  5 2025,
+16:06:33) [GCC 15.2.0]
+```
+
+Collection probe: `404 tests collected in 0.18s`
+(no import errors from the new module).
+
+Spawn-relevant test subset (`tests/test_local.py
+test_rpc.py test_spawning.py test_multi_program.py
+tests/discovery/`):
+```
+69 passed, 1 skipped, 10 warnings in 61.38s
+```
+
+Full suite was **not** run on py3.13 for this commit
+— the scaffolding is no-op on <3.14 and full-suite
+validation under py3.14 is pending that venv being
+set up by the user.
+
+### Commit message
+
+Also AI-drafted (via `/commit-msg`, with the prose
+rewrapped through `/home/goodboy/.claude/skills/pr-msg/
+scripts/rewrap.py --width 67`) — the 33-line message
+on commit `5cd6df58` itself. Not reproduced here; see
+`git log -1 5cd6df58`.
+
+### Known follow-ups flagged to user
+
+- **`uv.lock` deferred**: pin-bump regenerated cp314
+  wheel entries in `uv.lock`, but the user chose to
+  not stage `uv.lock` for this commit. Warned
+  explicitly.
+- **Phase B.2 needs py3.14 venv** — running the
+  actual subint impl requires it; user said they'd
+  set it up separately.

ai/prompt-io/claude/20260418T042526Z_26fb820_prompt_io.md

@@ -0,0 +1,117 @@
+---
+model: claude-opus-4-7[1m]
+service: claude
+session: subints-phase-b2-destroy-race-fix
+timestamp: 2026-04-18T04:25:26Z
+git_ref: 26fb820
+scope: code
+substantive: true
+raw_file: 20260418T042526Z_26fb820_prompt_io.raw.md
+---
+
+## Prompt
+
+Follow-up to Phase B.2 (`5cd6df58`) after the user
+observed intermittent mid-suite hangs when running
+the tractor test suite under `--spawn-backend=subint`
+on py3.14. The specific sequence of prompts over
+several turns:
+
+1. User pointed at the `test_context_stream_semantics.py`
+   suite as the first thing to make run clean under
+   `--spawn-backend=subint`.
+2. After a series of `timeout`-terminated runs that
+   gave no diagnostic info, user nudged me to stop
+   relying on `timeout` and get actual runtime
+   diagnostics ("the suite hangs indefinitely, so i
+   don't think this `timeout 30` is helping you at
+   all.."). Switched to
+   `faulthandler.dump_traceback_later(...)` and a
+   resource-tracker fixture to rule out leaks.
+3. Captured a stack pinning the hang on
+   `_interpreters.destroy(interp_id)` in the subint
+   teardown finally block.
+4. Proposed dedicated-OS-thread fix. User greenlit.
+5. Implemented + verified on-worktree; user needed
+   to be pointed at the *worktree*'s `./py313` venv
+   because bare `pytest` was picking up the main
+   repo's venv (running un-patched `_subint.py`) and
+   still hanging.
+
+Running theme over the whole exchange: this patch
+only closes the *destroy race*. The user and I also
+traced through the deeper cancellation story — SIGINT
+can't reach subints, legacy-mode shares the GIL,
+portal-cancel dies when the IPC channel is already
+broken — and agreed the next step is a bounded
+hard-kill in `subint_proc`'s teardown plus a
+dedicated cancellation test suite. Those land as
+separate commits.
+
+## Response summary
+
+Produced the `tractor/spawn/_subint.py` patch landed
+as commit `26fb8206` ("Fix subint destroy race via
+dedicated OS thread"). One file, +110/-84 LOC.
+
+Mechanism: swap `trio.to_thread.run_sync(_interpreters
+.exec, ...)` for a plain `threading.Thread(target=...
+, daemon=False)`. The trio thread cache recycles
+workers — so the OS thread that ran `_interpreters
+.exec()` remained alive in the cache holding a
+stale subint tstate, blocking
+`_interpreters.destroy()` in the finally indefinitely.
+A dedicated one-shot thread exits naturally after
+the sync target returns, releasing tstate and
+unblocking destroy.
+
+Coordination across the trio↔thread boundary:
+- `trio.lowlevel.current_trio_token()` captured at
+  `subint_proc` entry
+- driver thread signals `subint_exited.set()` back
+  to parent trio via `trio.from_thread.run_sync(...,
+  trio_token=token)` (synchronous from the thread's
+  POV; the call returns after trio has run `.set()`)
+- `trio.RunFinishedError` swallowed in that path for
+  the process-teardown case where parent trio already
+  exited
+- teardown `finally` off-loads the sync
+  `driver_thread.join()` via `to_thread.run_sync` (a
+  cache thread carries no subint tstate — safe)
+
+## Files changed
+
+See `git diff 26fb820~1..26fb820 --stat`:
+
+```
+ tractor/spawn/_subint.py | 194 +++++++++++++++++++------------
+ 1 file changed, 110 insertions(+), 84 deletions(-)
+```
+
+Validation:
+- `test_parent_cancels[chk_ctx_result_before_exit=True-
+  cancel_method=ctx-child_returns_early=False]`
+  (the specific test that was hanging for the user)
+  — passed in 1.06s.
+- Full `tests/test_context_stream_semantics.py` under
+  subint — 61 passed in 100.35s (clean-cache re-run:
+  100.82s).
+- Trio backend regression subset — 69 passed / 1
+  skipped / 89.19s — no regressions from this change.
+
+## Files changed
+
+Beyond the `_subint.py` patch, the raw log also
+records the cancellation-semantics research that
+spanned this conversation but did not ship as code
+in *this* commit. Preserving it inline under "Non-
+code output" because it directly informs the
+Phase B.3 hard-kill impl that will follow (and any
+upstream CPython bug reports we end up filing).
+
+## Human edits
+
+None — committed as generated. The commit message
+itself was also AI-drafted via `/commit-msg` and
+rewrapped via the project's `rewrap.py --width 67`
+tooling; user landed it without edits.

ai/prompt-io/claude/20260418T042526Z_26fb820_prompt_io.raw.md

@@ -0,0 +1,220 @@
+---
+model: claude-opus-4-7[1m]
+service: claude
+timestamp: 2026-04-18T04:25:26Z
+git_ref: 26fb820
+diff_cmd: git diff 26fb820~1..26fb820
+---
+
+Code generated in this turn was committed as `26fb8206`
+("Fix subint destroy race via dedicated OS thread").
+Per diff-ref mode, per-file code is captured via the
+pointer below; the rest is verbatim non-code output
+— test-session diagnostics, CPython-internals
+reasoning, and the design synthesis that's worth
+keeping alongside the code since it informs the
+still-pending Phase B.3 hard-kill work.
+
+## Per-file generated content
+
+### `tractor/spawn/_subint.py` (modified, +110/-84 LOC)
+
+> `git diff 26fb820~1..26fb820 -- tractor/spawn/_subint.py`
+
+Rewrites the subint driver-thread strategy:
+- replaces `trio.to_thread.run_sync(_interpreters.exec, ...)`
+  with a plain `threading.Thread(target=_subint_target,
+  daemon=False)` so the OS thread truly exits after
+  `_interpreters.exec()` returns
+- captures a `trio.lowlevel.current_trio_token()` at
+  `subint_proc` entry; the driver thread signals
+  completion back via `trio.from_thread.run_sync(
+  subint_exited.set, trio_token=...)`
+- swallows `trio.RunFinishedError` in the signal path
+  for the case where the parent trio loop has already
+  exited (process teardown)
+- in the teardown `finally` off-loads the sync
+  `driver_thread.join()` call to `trio.to_thread.run_sync`
+  (a cache thread w/ no subint tstate — so no cache
+  conflict) to wait for the driver thread to fully
+  exit before calling `_interpreters.destroy()`
+
+## Non-code output (verbatim) — the CPython-internals research
+
+### What went wrong before this commit
+
+Under `--spawn-backend=subint` on py3.14, most single
+tests passed but longer runs hung intermittently. The
+position of the hang moved between runs (test #22 on
+one run, test #53 on another) suggesting a timing-
+dependent race rather than a deterministic bug.
+
+`faulthandler.dump_traceback_later()` eventually
+caught a stack with the main thread blocked in
+`_interpreters.destroy(interp_id)` at `_subint.py:293`.
+Only 2 threads were alive:
+- main thread waiting in `_interpreters.destroy()`
+- one idle trio thread-cache worker in
+  `trio._core._thread_cache._work`
+
+No subint was still running (`_interpreters.list_all()`
+showed only the main interp). A resource-tracker
+pytest fixture confirmed threads/subints did NOT
+accumulate across tests — this was not a leak but a
+specific "destroy blocks on cached thread w/ stale
+tstate" race.
+
+### Why the race exists
+
+`trio.to_thread.run_sync` uses a thread *cache* to
+avoid OS-thread creation overhead. When the sync
+callable returns, the OS thread is NOT terminated —
+it's parked in `_thread_cache._work` waiting for the
+next job. CPython's subinterpreter implementation
+attaches a **tstate** (thread-state object) to each
+OS thread that ever entered a subint via
+`_interpreters.exec()`. That tstate is released
+lazily — either when the thread picks up a new job
+(which re-attaches a new tstate, evicting the old
+one) or when the thread truly exits.
+
+`_interpreters.destroy(interp_id)` waits for *all*
+tstates associated w/ that subint to be released
+before it can proceed. If the cached worker is idle
+holding the stale tstate, destroy blocks indefinitely.
+Whether the race manifests depends on timing — if
+the cached thread happens to pick up another job
+quickly, destroy unblocks; if it sits idle, we hang.
+
+### Why a dedicated `threading.Thread` fixes it
+
+A plain `threading.Thread(target=_subint_target,
+daemon=False)` runs its target once and exits. When
+the target returns, OS-thread teardown (`_bootstrap_inner`
+→ `_bootstrap`) fires and CPython releases the
+tstate for that thread. `_interpreters.destroy()`
+then has no blocker.
+
+### Diagnostic tactics that actually helped
+
+1. `faulthandler.dump_traceback_later(n, repeat=False,
+   file=open(path, 'w'))` for captured stack dumps on
+   hang. Critically, pipe to a `file=` not stderr —
+   pytest captures stderr weirdly and the dump is
+   easy to miss.
+2. A resource-tracker autouse fixture printing
+   per-test `threading.active_count()` +
+   `len(_interpreters.list_all())` deltas → ruled out
+   leak-accumulation theories quickly.
+3. Running the hanging test *solo* vs in-suite —
+   when solo passes but in-suite hangs, you know
+   it's a cross-test state-transfer bug rather than
+   a test-internal bug.
+
+### Design synthesis — SIGINT + subints + SC
+
+The user and I walked through the cancellation
+semantics of PEP 684/734 subinterpreters in detail.
+Key findings we want to preserve:
+
+**Signal delivery in subints (stdlib limitation).**
+CPython's signal machinery only delivers signals
+(SIGINT included) to the *main thread of the main
+interpreter*. Subints cannot install signal handlers
+that will ever fire. This is an intentional design
+choice in PEP 684 and not expected to change. For
+tractor's subint actors, this means:
+
+- Ctrl-C never reaches a subint directly.
+- `trio.run()` running on a worker thread (as we do
+  for subints) already skips SIGINT handler install
+  because `signal.signal()` raises on non-main
+  threads.
+- The only cancellation surface into a subint is
+  our IPC `Portal.cancel_actor()`.
+
+**Legacy-mode subints share the main GIL** (which
+our impl uses since `msgspec` lacks PEP 684 support
+per `jcrist/msgspec#563`). This means a stuck subint
+thread can starve the parent's trio loop during
+cancellation — the parent can't even *start* its
+teardown handling until the subint yields the GIL.
+
+**Failure modes identified for Phase B.3 audit:**
+
+1. Portal cancel lands cleanly → subint unwinds →
+   thread exits → destroy succeeds. (Happy path.)
+2. IPC channel is already broken when we try to
+   send cancel (e.g., `test_ipc_channel_break_*`)
+   → cancel raises `BrokenResourceError` → subint
+   keeps running unaware → parent hangs waiting for
+   `subint_exited`. This is what breaks
+   `test_advanced_faults.py` under subint.
+3. Subint is stuck in non-checkpointing Python code
+   → portal-cancel msg queued but never processed.
+4. Subint is in a shielded cancel scope when cancel
+   arrives → delay until shield exits.
+
+**Current teardown has a shield-bug too:**
+`trio.CancelScope(shield=True)` wrapping the `finally`
+block absorbs Ctrl-C, so even when the user tries
+to break out they can't. This is the reason
+`test_ipc_channel_break_during_stream[break_parent-...
+no_msgstream_aclose]` locks up unkillable.
+
+**B.3 hard-kill fix plan (next commit):**
+
+1. Bound `driver_thread.join()` with
+   `trio.move_on_after(HARD_KILL_TIMEOUT)`.
+2. If it times out, log a warning naming the
+   `interp_id` and switch the driver thread to
+   `daemon=True` mode (not actually possible after
+   start — so instead create as daemon=True upfront
+   and accept the tradeoff of proc-exit not waiting
+   for a stuck subint).
+3. Best-effort `_interpreters.destroy()`; catch the
+   `InterpreterError` if the subint is still running.
+4. Document that the leak is real and the only
+   escape hatch we have without upstream cooperation.
+
+**Test plan for Phase B.3:**
+
+New `tests/test_subint_cancellation.py` covering:
+- SIGINT at spawn
+- SIGINT mid-portal-RPC
+- SIGINT during shielded section in subint
+- Dead-channel cancel (mirror of `test_ipc_channel_
+  break_during_stream` minimized)
+- Non-checkpointing subint (tight `while True` in
+  user code)
+- Per-test `pytest-timeout`-style bounds so the
+  tests visibly fail instead of wedging the runner
+
+### Sanity-check output (verbatim terminal excerpts)
+
+Post-fix single-test validation:
+```
+1 passed, 1 warning in 1.06s
+```
+(same test that was hanging pre-fix:
+`test_parent_cancels[...cancel_method=ctx-...False]`)
+
+Full `tests/test_context_stream_semantics.py`
+under subint:
+```
+61 passed, 1 warning in 100.35s (0:01:40)
+```
+and a clean-cache re-run:
+```
+61 passed, 1 warning in 100.82s (0:01:40)
+```
+
+No regressions on trio backend (same subset):
+```
+69 passed, 1 skipped, 3 warnings in 89.19s
+```
+
+### Commit msg
+
+Also AI-drafted via `/commit-msg` + `rewrap.py
+--width 67`. See `git log -1 26fb820`.

ai/prompt-io/claude/20260420T192739Z_5e8cd8b2_prompt_io.md

@@ -0,0 +1,111 @@
+---
+model: claude-opus-4-7[1m]
+service: claude
+session: subint-phase-b-hang-classification
+timestamp: 2026-04-20T19:27:39Z
+git_ref: HEAD (pre-commit; on branch `subint_spawner_backend`)
+scope: docs
+substantive: true
+raw_file: 20260420T192739Z_5e8cd8b2_prompt_io.raw.md
+---
+
+## Prompt
+
+Collab between user (`goodboy`) and `claude` to
+disambiguate two distinct hang modes hit during
+Phase B subint-spawn-backend bringup (issue #379).
+The user ran the failing suites, captured `strace`
+evidence on hung pytest pids, and set the framing:
+
+> "These cannot be the same bug. Different fix
+> paths. Write them up separately or we'll keep
+> conflating them."
+
+Follow-on asks:
+- Cross-link each doc from its reproducer test so
+  a future triager lands on the analysis without
+  needing to grep `ai/conc-anal/`.
+- On `test_stale_entry_is_deleted`: arm
+  `dump_on_hang(seconds=20, path=...)` so a
+  regression captures a stack dump (pytest stderr
+  capture otherwise eats `faulthandler` output).
+- Keep `test_stale_entry_is_deleted` un-`skip`ped
+  so the dump file remains inspectable.
+
+Defer the actual fix for the class-2 hang
+(Ctrl-C-able, tractor-side bug) to a later commit
+per the incremental-commits preference.
+
+## Response summary
+
+Wrote two new `ai/conc-anal/` classification docs
+and cross-linked them from the reproducer tests:
+
+1. **`subint_sigint_starvation_issue.md`** —
+   class 1: abandoned-legacy-subint thread +
+   shared GIL starves main trio loop →
+   signal-wakeup-fd pipe fills → SIGINT silently
+   dropped (`write() = EAGAIN`). Pytest process
+   un-Ctrl-C-able. Structurally a CPython limit;
+   blocked on `msgspec` PEP 684 support
+   (jcrist/msgspec#563). Reproducer:
+   `test_stale_entry_is_deleted[subint]`.
+
+2. **`subint_cancel_delivery_hang_issue.md`** —
+   class 2: parent-side trio task parks on an
+   orphaned IPC channel after subint teardown;
+   no clean EOF delivered to waiting receiver.
+   Ctrl-C-able (main trio loop iterating fine).
+   OUR bug to fix. Candidate fix: explicit
+   parent-side channel abort in `subint_proc`'s
+   hard-kill teardown. Reproducer:
+   `test_subint_non_checkpointing_child`.
+
+Test-side cross-links:
+- `tests/discovery/test_registrar.py`:
+  `test_stale_entry_is_deleted` → `trio.run(main)`
+  wrapped in `dump_on_hang(seconds=20,
+  path=<per-method-tmp>)`; long inline comment
+  summarizes `strace` evidence + root-cause chain
+  and points at both docs.
+- `tests/test_subint_cancellation.py`:
+  `test_subint_non_checkpointing_child` docstring
+  extended with "KNOWN ISSUE (Ctrl-C-able hang)"
+  section pointing at the class-2 doc + noting
+  the class-1 doc is NOT what this test hits.
+
+## Files changed
+
+- `ai/conc-anal/subint_sigint_starvation_issue.md`
+  — new, 205 LOC
+- `ai/conc-anal/subint_cancel_delivery_hang_issue.md`
+  — new, 161 LOC
+- `tests/discovery/test_registrar.py` — +52/-1
+  (arm `dump_on_hang`, inline-comment cross-link)
+- `tests/test_subint_cancellation.py` — +26
+  (docstring "KNOWN ISSUE" block)
+
+## Human edits
+
+Substantive collab — prose was jointly iterated:
+
+- User framed the two-doc split, set the
+  classification criteria (Ctrl-C-able vs not),
+  and provided the `strace` evidence.
+- User decided to keep `test_stale_entry_is_deleted`
+  un-`skip`ped (my initial suggestion was
+  `pytestmark.skipif(spawn_backend=='subint')`).
+- User chose the candidate fix ordering for
+  class 2 and marked "explicit parent-side channel
+  abort" as the surgical preferred fix.
+- User picked the file naming convention
+  (`subint_<hang-shape>_issue.md`) over my initial
+  `hang_class_{1,2}.md`.
+- Assistant drafted the prose, aggregated prior-
+  session root-cause findings from Phase B.2/B.3
+  bringup, and wrote the test-side cross-linking
+  comments.
+
+No further mechanical edits expected before
+commit; user may still rewrap via
+`scripts/rewrap.py` if preferred.

ai/prompt-io/claude/20260420T192739Z_5e8cd8b2_prompt_io.raw.md

@@ -0,0 +1,198 @@
+---
+model: claude-opus-4-7[1m]
+service: claude
+timestamp: 2026-04-20T19:27:39Z
+git_ref: HEAD (pre-commit; will land on branch `subint_spawner_backend`)
+diff_cmd: git diff HEAD~1..HEAD
+---
+
+Collab between `goodboy` (user) and `claude` (this
+assistant) spanning multiple test-run iterations on
+branch `subint_spawner_backend`. The user ran the
+failing suites, captured `strace` evidence on the
+hung pytest pids, and set the direction ("these are
+two different hangs — write them up separately so
+we don't re-confuse ourselves later"). The assistant
+aggregated prior-session findings (Phase B.2/B.3
+bringup) into two classification docs + test-side
+cross-links. All prose was jointly iterated; the
+user had final say on framing and decided which
+candidate fix directions to list.
+
+## Per-file generated content
+
+### `ai/conc-anal/subint_sigint_starvation_issue.md` (new, 205 LOC)
+
+> `git diff HEAD~1..HEAD -- ai/conc-anal/subint_sigint_starvation_issue.md`
+
+Writes up the "abandoned-legacy-subint thread wedges
+the parent trio loop" class. Key sections:
+
+- **Symptom** — `test_stale_entry_is_deleted[subint]`
+  hangs indefinitely AND is un-Ctrl-C-able.
+- **Evidence** — annotated `strace` excerpt showing
+  SIGINT delivered to pytest, C-level signal handler
+  tries to write to the signal-wakeup-fd pipe, gets
+  `write() = -1 EAGAIN (Resource temporarily
+  unavailable)`. Pipe is full because main trio loop
+  isn't iterating often enough to drain it.
+- **Root-cause chain** — our hard-kill abandons the
+  `daemon=True` driver OS thread after
+  `_HARD_KILL_TIMEOUT`; the subint *inside* that
+  thread is still running `trio.run()`;
+  `_interpreters.destroy()` cannot force-stop a
+  running subint (raises `InterpreterError`); legacy
+  subints share the main GIL → abandoned subint
+  starves main trio loop → wakeup-fd fills → SIGINT
+  silently dropped.
+- **Why it's structurally a CPython limit** — no
+  public force-destroy primitive for a running
+  subint; the only escape is per-interpreter GIL
+  isolation, gated on msgspec PEP 684 adoption
+  (jcrist/msgspec#563).
+- **Current escape hatch** — harness-side SIGINT
+  loop in the `daemon` fixture teardown that kills
+  the bg registrar subproc, eventually unblocking
+  a parent-side recv enough for the main loop to
+  drain the wakeup pipe.
+
+### `ai/conc-anal/subint_cancel_delivery_hang_issue.md` (new, 161 LOC)
+
+> `git diff HEAD~1..HEAD -- ai/conc-anal/subint_cancel_delivery_hang_issue.md`
+
+Writes up the *sibling* hang class — same subint
+backend, distinct root cause:
+
+- **TL;DR** — Ctrl-C-able, so NOT the SIGINT-
+  starvation class; main trio loop iterates fine;
+  ours to fix.
+- **Symptom** — `test_subint_non_checkpointing_child`
+  hangs past the expected `_HARD_KILL_TIMEOUT`
+  budget even after the subint is torn down.
+- **Diagnosis** — a parent-side trio task (likely
+  a `chan.recv()` in `process_messages`) parks on
+  an orphaned IPC channel; channel was torn down
+  without emitting a clean EOF /
+  `BrokenResourceError` to the waiting receiver.
+- **Candidate fix directions** — listed in rough
+  order of preference:
+  1. Explicit parent-side channel abort in
+     `subint_proc`'s hard-kill teardown (surgical;
+     most likely).
+  2. Audit `process_messages` to add a timeout or
+     cancel-scope protection that catches the
+     orphaned-recv state.
+  3. Wrap subint IPC channel construction in a
+     sentinel that can force-close from the parent
+     side regardless of subint liveness.
+
+### `tests/discovery/test_registrar.py` (modified, +52/-1 LOC)
+
+> `git diff HEAD~1..HEAD -- tests/discovery/test_registrar.py`
+
+Wraps the `trio.run(main)` call at the bottom of
+`test_stale_entry_is_deleted` in
+`dump_on_hang(seconds=20, path=<per-method-tmp>)`.
+Adds a long inline comment that:
+- Enumerates variant-by-variant status
+  (`[trio]`/`[mp_*]` = clean; `[subint]` = hangs
+  + un-Ctrl-C-able)
+- Summarizes the `strace` evidence and root-cause
+  chain inline (so a future reader hitting this
+  test doesn't need to cross-ref the doc to
+  understand the hang shape)
+- Points at
+  `ai/conc-anal/subint_sigint_starvation_issue.md`
+  for full analysis
+- Cross-links to the *sibling*
+  `subint_cancel_delivery_hang_issue.md` so
+  readers can tell the two classes apart
+- Explains why it's kept un-`skip`ped: the dump
+  file is useful if the hang ever returns after
+  a refactor. pytest stderr capture would
+  otherwise eat `faulthandler` output, hence the
+  file path.
+
+### `tests/test_subint_cancellation.py` (modified, +26 LOC)
+
+> `git diff HEAD~1..HEAD -- tests/test_subint_cancellation.py`
+
+Extends the docstring of
+`test_subint_non_checkpointing_child` with a
+"KNOWN ISSUE (Ctrl-C-able hang)" block:
+- Describes the current hang: parent-side orphaned
+  IPC recv after hard-kill; distinct from the
+  SIGINT-starvation sibling class.
+- Cites `strace` distinguishing signal: wakeup-fd
+  `write() = 1` (not `EAGAIN`) — i.e. main loop
+  iterating.
+- Points at
+  `ai/conc-anal/subint_cancel_delivery_hang_issue.md`
+  for full analysis + candidate fix directions.
+- Clarifies that the *other* sibling doc
+  (SIGINT-starvation) is NOT what this test hits.
+
+## Non-code output
+
+### Classification reasoning (why two docs, not one)
+
+The user and I converged on the two-doc split after
+running the suites and noticing two *qualitatively
+different* hang symptoms:
+
+1. `test_stale_entry_is_deleted[subint]` — pytest
+   process un-Ctrl-C-able. Ctrl-C at the terminal
+   does nothing. Must kill-9 from another shell.
+2. `test_subint_non_checkpointing_child` — pytest
+   process Ctrl-C-able. One Ctrl-C at the prompt
+   unblocks cleanly and the test reports a hang
+   via pytest-timeout.
+
+From the user: "These cannot be the same bug.
+Different fix paths. Write them up separately or
+we'll keep conflating them."
+
+`strace` on the `[subint]` hang gave the decisive
+signal for the first class:
+
+```
+--- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
+write(5, "\2", 1) = -1 EAGAIN (Resource temporarily unavailable)
+```
+
+fd 5 is Python's signal-wakeup-fd pipe. `EAGAIN`
+on a `write()` of 1 byte to a pipe means the pipe
+buffer is full → reader side (main Python thread
+inside `trio.run()`) isn't consuming. That's the
+GIL-hostage signature.
+
+The second class's `strace` showed `write(5, "\2",
+1) = 1` — clean drain — so the main trio loop was
+iterating and the hang had to be on the application
+side of things, not the kernel-↔-Python signal
+boundary.
+
+### Why the candidate fix for class 2 is "explicit parent-side channel abort"
+
+The second hang class has the trio loop alive. A
+parked `chan.recv()` that will never get bytes is
+fundamentally a tractor-side resource-lifetime bug
+— the IPC channel was torn down (subint destroyed)
+but no one explicitly raised
+`BrokenResourceError` at the parent-side receiver.
+The `subint_proc` hard-kill path is the natural
+place to add that notification, because it already
+knows the subint is unreachable at that point.
+
+Alternative fix paths (blanket timeouts on
+`process_messages`, sentinel-wrapped channels) are
+less surgical and risk masking unrelated bugs —
+hence the preference ordering in the doc.
+
+### Why we're not just patching the code now
+
+The user explicitly deferred the fix to a later
+commit: "Document both classes now, land the fix
+for class 2 separately so the diff reviews clean."
+This matches the incremental-commits preference
+from memory.

ai/prompt-io/claude/20260422T200723Z_797f57c_prompt_io.md

@@ -0,0 +1,155 @@
+---
+model: claude-opus-4-7[1m]
+service: claude
+session: subints-phase-b-hardening-and-fork-block
+timestamp: 2026-04-22T20:07:23Z
+git_ref: 797f57c
+scope: code
+substantive: true
+raw_file: 20260422T200723Z_797f57c_prompt_io.raw.md
+---
+
+## Prompt
+
+Session-spanning work on the Phase B `subint` spawn-backend.
+Three distinct sub-phases in one log:
+
+1. **Py3.13 gate tightening** — diagnose a reproducible hang
+   of subint spawn flow under py3.13 (works on py3.14), trace
+   to a private `_interpreters` module vintage issue, tighten
+   our feature gate from "`_interpreters` present" to "public
+   `concurrent.interpreters` present" (i.e. py3.14+).
+2. **Test-harness hardening** — add `pytest-timeout` dep, put
+   `@pytest.mark.timeout(30, method='thread')` on the
+   three known-hanging subint tests cataloged in
+   `ai/conc-anal/subint_sigint_starvation_issue.md`. Separately,
+   code-review the user's in-flight `skipon_spawn_backend`
+   marker implementation; find four bugs; refactor to use
+   `item.iter_markers()`.
+3. **`subint_fork` prototype → CPython-block finding** — draft
+   a WIP `subint_fork_proc` backend using a sub-interpreter as
+   a launchpad for `os.fork()` (to sidestep trio#1614). User
+   tests on py3.14, hits
+   `Fatal Python error: _PyInterpreterState_DeleteExceptMain:
+   not main interpreter`. Walk CPython sources (local clone at
+   `~/repos/cpython/`) to pinpoint the refusal
+   (`Modules/posixmodule.c:728` → `Python/pystate.c:1040`).
+   Revert implementation to a `NotImplementedError` stub in a
+   new `_subint_fork.py` submodule, document the finding in a
+   third `conc-anal/` doc with an upstream-report draft for
+   the CPython issue tracker. Finally, discuss user's proposed
+   workaround architecture (main-interp worker-thread
+   forkserver) and draft a standalone smoke-test script for
+   feasibility validation.
+
+## Response summary
+
+All three sub-phases landed concrete artifacts:
+
+**Sub-phase 1** — `_subint.py` + `_spawn.py` gates + error
+messages updated to require py3.14+ via the public
+`concurrent.interpreters` module presence check. Module
+docstring revised to explain the empirical reason
+(py3.13's private `_interpreters` vintage wedges under
+multi-trio-task usage even though minimal standalone
+reproducers work fine there). Test-module
+`pytest.importorskip` likewise switched.
+
+**Sub-phase 2** — `pytest-timeout>=2.3` added to `testing`
+dep group. `@pytest.mark.timeout(30, method='thread')`
+applied on:
+- `tests/discovery/test_registrar.py::test_stale_entry_is_deleted`
+- `tests/test_cancellation.py::test_cancel_while_childs_child_in_sync_sleep`
+- `tests/test_cancellation.py::test_multierror_fast_nursery`
+- `tests/test_subint_cancellation.py::test_subint_non_checkpointing_child`
+
+`method='thread'` documented inline as load-bearing — the
+GIL-starvation path that drops `SIGINT` would equally drop
+`SIGALRM`, so only a watchdog-thread timeout can reliably
+escape.
+
+`skipon_spawn_backend` plugin refactored into a single
+`iter_markers`-driven loop in `pytest_collection_modifyitems`
+(~30 LOC replacing ~30 LOC of nested conditionals). Four
+bugs dissolved: wrong `.get()` key, module-level `pytestmark`
+suppressing per-test marks, unhandled `pytestmark = [list]`
+form, `pytest.Makr` typo. Marker help text updated to
+document the variadic backend-list + `reason=` kwarg
+surface.
+
+**Sub-phase 3** — Prototype drafted (then reverted):
+
+- `tractor/spawn/_subint_fork.py` — new dedicated submodule
+  housing the `subint_fork_proc` stub. Module docstring +
+  fn docstring explain the attempt, the CPython-level
+  block, and the reason for keeping the stub in-tree
+  (documentation of the attempt + starting point if CPython
+  ever lifts the restriction).
+- `tractor/spawn/_spawn.py` — `'subint_fork'` registered as a
+  `SpawnMethodKey` literal + in `_methods`, so
+  `--spawn-backend=subint_fork` routes to a clean
+  `NotImplementedError` pointing at the analysis doc rather
+  than an "invalid backend" error.
+- `ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md` —
+  third sibling conc-anal doc. Full annotated CPython
+  source walkthrough from user-visible
+  `Fatal Python error` → `Modules/posixmodule.c:728
+  PyOS_AfterFork_Child()` → `Python/pystate.c:1040
+  _PyInterpreterState_DeleteExceptMain()` gate. Includes a
+  copy-paste-ready upstream-report draft for the CPython
+  issue tracker with a two-tier ask (ideally "make it work",
+  minimally "cleaner error than `Fatal Python error`
+  aborting the child").
+- `ai/conc-anal/subint_fork_from_main_thread_smoketest.py` —
+  standalone zero-tractor-import CPython-level smoke test
+  for the user's proposed workaround architecture
+  (forkserver on a main-interp worker thread). Four
+  argparse-driven scenarios: `control_subint_thread_fork`
+  (reproduces the known-broken case as a test-harness
+  sanity),  `main_thread_fork` (baseline), `worker_thread_fork`
+  (architectural assertion), `full_architecture`
+  (end-to-end trio-in-subint in forked child). User will
+  run on py3.14 next.
+
+## Files changed
+
+See `git log 26fb820..HEAD --stat` for the canonical list.
+New files this session:
+- `tractor/spawn/_subint_fork.py`
+- `ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md`
+- `ai/conc-anal/subint_fork_from_main_thread_smoketest.py`
+
+Modified (diff pointers in raw log):
+- `tractor/spawn/_subint.py` (py3.14 gate)
+- `tractor/spawn/_spawn.py` (`subint_fork` registration)
+- `tractor/_testing/pytest.py` (`skipon_spawn_backend` refactor)
+- `pyproject.toml` (`pytest-timeout` dep)
+- `tests/discovery/test_registrar.py`,
+  `tests/test_cancellation.py`,
+  `tests/test_subint_cancellation.py` (timeout marks,
+  cross-refs to conc-anal docs)
+
+## Human edits
+
+Several back-and-forth iterations with user-driven
+adjustments during the session:
+
+- User corrected my initial mis-classification of
+  `test_cancel_while_childs_child_in_sync_sleep[subint-False]`
+  as Ctrl-C-able — second strace showed `EAGAIN`, putting
+  it squarely in class A (GIL-starvation). Re-analysis
+  preserved in the raw log.
+- User independently fixed the `.get(reason)` → `.get('reason', reason)`
+  bug in the marker plugin before my review; preserved their
+  fix.
+- User suggested moving the `subint_fork_proc` stub from
+  the bottom of `_subint.py` into its own
+  `_subint_fork.py` submodule — applied.
+- User asked to keep the forkserver-architecture
+  discussion as background for the smoke-test rather than
+  committing to a tractor-side refactor until the smoke
+  test validates the CPython-level assumptions.
+
+Commit messages in this range (b025c982 … 797f57c) were
+drafted via `/commit-msg` + `rewrap.py --width 67`; user
+landed them with the usual review.

ai/prompt-io/claude/20260422T200723Z_797f57c_prompt_io.raw.md

@@ -0,0 +1,343 @@
+---
+model: claude-opus-4-7[1m]
+service: claude
+timestamp: 2026-04-22T20:07:23Z
+git_ref: 797f57c
+diff_cmd: git log 26fb820..HEAD  # all session commits since the destroy-race fix log
+---
+
+Session-spanning conversation covering the Phase B hardening
+of the `subint` spawn-backend and an investigation into a
+proposed `subint_fork` follow-up which turned out to be
+blocked at the CPython level. This log is a narrative capture
+of the substantive turns (not every message) and references
+the concrete code + docs the session produced. Per diff-ref
+mode the actual code diffs are pointed at via `git log` on
+each ref rather than duplicated inline.
+
+## Narrative of the substantive turns
+
+### Py3.13 hang / gate tightening
+
+Diagnosed a reproducible hang of the `subint` backend under
+py3.13 (test_spawning tests wedge after root-actor bringup).
+Root cause: py3.13's vintage of the private `_interpreters` C
+module has a latent thread/subint-interaction issue that
+`_interpreters.exec()` silently fails to progress under
+tractor's multi-trio usage pattern — even though a minimal
+standalone `threading.Thread` + `_interpreters.exec()`
+reproducer works fine on the same Python. Empirically
+py3.14 fixes it.
+
+Fix (from this session): tighten the `_has_subints` gate in
+`tractor.spawn._subint` from "private module importable" to
+"public `concurrent.interpreters` present" — which is 3.14+
+only. This leaves `subint_proc()` unchanged in behavior (we
+still call the *private* `_interpreters.create('legacy')`
+etc. under the hood) but refuses to engage on 3.13.
+
+Also tightened the matching gate in
+`tractor.spawn._spawn.try_set_start_method('subint')` and
+rev'd the corresponding error messages from "3.13+" to
+"3.14+" with a sentence explaining why. Test-module
+`pytest.importorskip` switched from `_interpreters` →
+`concurrent.interpreters` to match.
+
+### `pytest-timeout` dep + `skipon_spawn_backend` marker plumbing
+
+Added `pytest-timeout>=2.3` to the `testing` dep group with
+an inline comment pointing at the `ai/conc-anal/*.md` docs.
+Applied `@pytest.mark.timeout(30, method='thread')` (the
+`method='thread'` is load-bearing — `signal`-method
+`SIGALRM` suffers the same GIL-starvation path that drops
+`SIGINT` in the class-A hang pattern) to the three known-
+hanging subint tests cataloged in
+`subint_sigint_starvation_issue.md`.
+
+Separately code-reviewed the user's newly-staged
+`skipon_spawn_backend` pytest marker implementation in
+`tractor/_testing/pytest.py`. Found four bugs:
+
+1. `modmark.kwargs.get(reason)` called `.get()` with the
+   *variable* `reason` as the dict key instead of the string
+   `'reason'` — user-supplied `reason=` was never picked up.
+   (User had already fixed this locally via `.get('reason',
+   reason)` by the time my review happened — preserved that
+   fix.)
+2. The module-level `pytestmark` branch suppressed per-test
+   marker handling (the `else:` was an `else:` rather than
+   independent iteration).
+3. `mod_pytestmark.mark` assumed a single
+   `MarkDecorator` — broke on the valid-pytest `pytestmark =
+   [mark, mark]` list form.
+4. Typo: `pytest.Makr` → `pytest.Mark`.
+
+Refactored the hook to use `item.iter_markers(name=...)`
+which walks function + class + module scopes uniformly and
+handles both `pytestmark` forms natively. ~30 LOC replaced
+the original ~30 LOC of nested conditionals, all four bugs
+dissolved. Also updated the marker help string to reflect
+the variadic `*start_methods` + `reason=` surface.
+
+### `subint_fork_proc` prototype attempt
+
+User's hypothesis: the known trio+`fork()` issues
+(python-trio/trio#1614) could be sidestepped by using a
+sub-interpreter purely as a launchpad — `os.fork()` from a
+subint that has never imported trio → child is in a
+trio-free context. In the child `execv()` back into
+`python -m tractor._child` and the downstream handshake
+matches `trio_proc()` identically.
+
+Drafted the prototype at `tractor/spawn/_subint.py`'s bottom
+(originally — later moved to its own submod, see below):
+launchpad-subint creation, bootstrap code-string with
+`os.fork()` + `execv()`, driver-thread orchestration,
+parent-side `ipc_server.wait_for_peer()` dance. Registered
+`'subint_fork'` as a new `SpawnMethodKey` literal, added
+`case 'subint' | 'subint_fork':` feature-gate arm in
+`try_set_start_method()`, added entry in `_methods` dict.
+
+### CPython-level block discovered
+
+User tested on py3.14 and saw:
+
+```
+Fatal Python error: _PyInterpreterState_DeleteExceptMain: not main interpreter
+Python runtime state: initialized
+
+Current thread 0x00007f6b71a456c0 [subint-fork-lau] (most recent call first):
+  File "<script>", line 2 in <module>
+<script>:2: DeprecationWarning: This process (pid=802985) is multi-threaded, use of fork() may lead to deadlocks in the child.
+```
+
+Walked CPython sources (local clone at `~/repos/cpython/`):
+
+- **`Modules/posixmodule.c:728` `PyOS_AfterFork_Child()`** —
+  post-fork child-side cleanup. Calls
+  `_PyInterpreterState_DeleteExceptMain(runtime)` with
+  `goto fatal_error` on non-zero status. Has the
+  `// Ideally we could guarantee tstate is running main.`
+  self-acknowledging-fragile comment directly above.
+
+- **`Python/pystate.c:1040`
+  `_PyInterpreterState_DeleteExceptMain()`** — the
+  refusal. Hard `PyStatus_ERR("not main interpreter")` gate
+  when `tstate->interp != interpreters->main`. Docstring
+  formally declares the precondition ("If there is a
+  current interpreter state, it *must* be the main
+  interpreter"). `XXX` comments acknowledge further latent
+  issues within.
+
+Definitive answer to "Open Question 1" of the prototype
+docstring: **no, CPython does not support `os.fork()` from
+a non-main sub-interpreter**. Not because the fork syscall
+is blocked (it isn't — the parent returns a valid pid),
+but because the child cannot survive CPython's post-fork
+initialization. This is an enforced invariant, not an
+incidental limitation.
+
+### Revert: move to stub submod + doc the finding
+
+Per user request:
+
+1. Reverted the working `subint_fork_proc` body to a
+   `NotImplementedError` stub, MOVED to its own submod
+   `tractor/spawn/_subint_fork.py` (keeps `_subint.py`
+   focused on the working `subint_proc` backend).
+2. Updated `_spawn.py` to import the stub from the new
+   submod path; kept `'subint_fork'` in `SpawnMethodKey` +
+   `_methods` so `--spawn-backend=subint_fork` routes to a
+   clean `NotImplementedError` with pointer to the analysis
+   doc rather than an "invalid backend" error.
+3. Wrote
+   `ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md`
+   with the full annotated CPython walkthrough + an
+   upstream-report draft for the CPython issue tracker.
+   Draft has a two-tier ask: ideally "make it work"
+   (pre-fork tstate-swap hook or `DeleteExceptFor(interp)`
+   variant), minimally "give us a clean `RuntimeError` in
+   the parent instead of a `Fatal Python error` aborting
+   the child silently".
+
+### Design discussion — main-interp-thread forkserver workaround
+
+User proposed: set up a "subint forking server" that fork()s
+on behalf of subint callers. Core insight: the CPython gate
+is on `tstate->interp`, not thread identity, so **any thread
+whose tstate is main-interp** can fork cleanly. A worker
+thread attached to main-interp (never entering a subint)
+satisfies the precondition.
+
+Structurally this is `mp.forkserver` (which tractor already
+has as `mp_forkserver`) but **in-process**: instead of a
+separate Python subproc as the fork server, we'd put the
+forkserver on a thread in the tractor parent process. Pros:
+faster spawn (no IPC marshalling to external server + no
+separate Python startup), inherits already-imported modules
+for free. Cons: less crash isolation (forkserver failure
+takes the whole process).
+
+Required tractor-side refactor: move the root actor's
+`trio.run()` off main-interp-main-thread (so main-thread can
+run the forkserver loop). Nontrivial; approximately the same
+magnitude as "Phase C".
+
+The design would also not fully resolve the class-A
+GIL-starvation issue because child actors' trio still runs
+inside subints (legacy config, msgspec PEP 684 pending).
+Would mitigate SIGINT-starvation specifically if signal
+handling moves to the forkserver thread.
+
+Recommended pre-commitment: a standalone CPython-only smoke
+test validating the four assumptions the arch rests on,
+before any tractor-side work.
+
+### Smoke-test script drafted
+
+Wrote `ai/conc-anal/subint_fork_from_main_thread_smoketest.py`:
+argparse-driven, four scenarios (`control_subint_thread_fork`
+reproducing the known-broken case, `main_thread_fork`
+baseline, `worker_thread_fork` the architectural assertion,
+`full_architecture` end-to-end with trio in a subint in the
+forked child). No `tractor` imports; pure CPython + `_interpreters`
++ `trio`. Bails cleanly on py<3.14. Pass/fail banners per
+scenario.
+
+User will validate on their py3.14 env next.
+
+## Per-code-artifact provenance
+
+### `tractor/spawn/_subint_fork.py` (new submod)
+
+> `git show 797f57c -- tractor/spawn/_subint_fork.py`
+
+NotImplementedError stub for the subint-fork backend. Module
+docstring + fn docstring explain the attempt, the CPython
+block, and why the stub is kept in-tree. No runtime behavior
+beyond raising with a pointer at the conc-anal doc.
+
+### `tractor/spawn/_spawn.py` (modified)
+
+> `git log 26fb820..HEAD -- tractor/spawn/_spawn.py`
+
+- Added `'subint_fork'` to `SpawnMethodKey` literal with a
+  block comment explaining the CPython-level block.
+- Generalized the `case 'subint':` arm to `case 'subint' |
+  'subint_fork':` since both use the same py3.14+ gate.
+- Registered `subint_fork_proc` in `_methods` with a
+  pointer-comment at the analysis doc.
+
+### `tractor/spawn/_subint.py` (modified across session)
+
+> `git log 26fb820..HEAD -- tractor/spawn/_subint.py`
+
+- Tightened `_has_subints` gate: dual-requires public
+  `concurrent.interpreters` + private `_interpreters`
+  (tests for py3.14-or-newer on the public-API presence,
+  then uses the private one for legacy-config subints
+  because `msgspec` still blocks the public isolated mode
+  per jcrist/msgspec#563).
+- Updated module docstring, `subint_proc()` docstring, and
+  gate-error messages to reflect the 3.14+ requirement and
+  the reason (py3.13 wedges under multi-trio usage even
+  though the private module exists there).
+
+### `tractor/_testing/pytest.py` (modified)
+
+> `git log 26fb820..HEAD -- tractor/_testing/pytest.py`
+
+- New `skipon_spawn_backend(*start_methods, reason=...)`
+  pytest marker expanded into `pytest.mark.skip(reason=...)`
+  at collection time via
+  `pytest_collection_modifyitems()`.
+- Implementation uses `item.iter_markers(name=...)` which
+  walks function + class + module scopes uniformly and
+  handles both `pytestmark = <single Mark>` and
+  `pytestmark = [mark, ...]` forms natively. ~30-LOC
+  single-loop refactor replacing a prior nested
+  conditional that had four bugs (see "Review" narrative
+  above).
+- Added `pytest.Config` / `pytest.Function` /
+  `pytest.FixtureRequest` type annotations on fixture
+  signatures while touching the file.
+
+### `pyproject.toml` (modified)
+
+> `git log 26fb820..HEAD -- pyproject.toml`
+
+Added `pytest-timeout>=2.3` to `testing` dep group with
+comment pointing at the `ai/conc-anal/` docs.
+
+### `tests/discovery/test_registrar.py`,
+`tests/test_subint_cancellation.py`,
+`tests/test_cancellation.py` (modified)
+
+> `git log 26fb820..HEAD -- tests/`
+
+Applied `@pytest.mark.timeout(30, method='thread')` on
+known-hanging subint tests. Extended comments to cross-
+reference the `ai/conc-anal/*.md` docs. `method='thread'`
+is documented inline as load-bearing (`signal`-method
+SIGALRM suffers the same GIL-starvation path that drops
+SIGINT).
+
+### `ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md` (new)
+
+> `git show 797f57c -- ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md`
+
+Third sibling doc under `conc-anal/`. Structure: TL;DR,
+context ("what we tried"), symptom (the user's exact
+`Fatal Python error` output), CPython source walkthrough
+with excerpted snippets from `posixmodule.c` +
+`pystate.c`, chain summary, definitive answer to Open
+Question 1, `## Upstream-report draft (for CPython issue
+tracker)` section with a two-tier ask, references.
+
+### `ai/conc-anal/subint_fork_from_main_thread_smoketest.py` (new, THIS turn)
+
+Zero-tractor-import smoke test for the proposed workaround
+architecture. Four argparse-driven scenarios covering the
+control case + baseline + arch-critical case + end-to-end.
+Pass/fail banners per scenario; clean `--help` output;
+py3.13 early-exit.
+
+## Non-code output (verbatim)
+
+### The `strace` signature that kicked off the CPython
+walkthrough
+
+```
+--- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
+write(16, "\2", 1)                      = -1 EAGAIN (Resource temporarily unavailable)
+rt_sigreturn({mask=[WINCH]})            = 139801964688928
+```
+
+### Key user quotes framing the direction
+
+> ok actually we get this [fatal error] ... see if you can
+> take a look at what's going on, in particular wrt to
+> cpython's sources. pretty sure there's a local copy at
+> ~/repos/cpython/
+
+(Drove the CPython walkthrough that produced the
+definitive refusal chain.)
+
+> is there any reason we can't just sidestep this "must fork
+> from main thread in main subint" issue by simply ensuring
+> a "subint forking server" is always setup prior to
+> invoking trio in a non-main-thread subint ...
+
+(Drove the main-interp-thread-forkserver architectural
+discussion + smoke-test script design.)
+
+### CPython source tags for quick jump-back
+
+```
+Modules/posixmodule.c:728   PyOS_AfterFork_Child()
+Modules/posixmodule.c:753   // Ideally we could guarantee tstate is running main.
+Modules/posixmodule.c:778   status = _PyInterpreterState_DeleteExceptMain(runtime);
+
+Python/pystate.c:1040       _PyInterpreterState_DeleteExceptMain()
+Python/pystate.c:1044-1047  tstate->interp != main → PyStatus_ERR("not main interpreter")
+```

ai/prompt-io/prompts/subints_spawner.md

@@ -24,7 +24,7 @@ Part of this work should include,
     is happening under the hood with how cpython implements subints.
 
   * default configuration should encourage state isolation as with
-    with subprocs, but explicit public escape hatches to enable rigorously
+    subprocs, but explicit public escape hatches to enable rigorously
     managed shm channels for high performance apps.
 
 - all tests should be (able to be) parameterized to use the new

pyproject.toml

@@ -9,7 +9,7 @@ name = "tractor"
 version = "0.1.0a6dev0"
 description = 'structured concurrent `trio`-"actors"'
 authors = [{ name = "Tyler Goodlet", email = "goodboy_foss@protonmail.com" }]
-requires-python = ">=3.12, <3.14"
+requires-python = ">=3.13, <3.15"
 readme = "docs/README.rst"
 license = "AGPL-3.0-or-later"
 keywords = [
@@ -31,6 +31,7 @@ classifiers = [
   "Programming Language :: Python :: 3 :: Only",
   "Programming Language :: Python :: 3.12",
   "Programming Language :: Python :: 3.13",
+  "Programming Language :: Python :: 3.14",
   "Topic :: System :: Distributed Computing",
 ]
 dependencies = [
@@ -43,11 +44,12 @@ dependencies = [
   "tricycle>=0.4.1,<0.5",
   "wrapt>=1.16.0,<2",
   "colorlog>=6.8.2,<7",
+
   # built-in multi-actor `pdb` REPL
   "pdbp>=1.8.2,<2", # windows only (from `pdbp`)
+
   # typed IPC msging
-  "msgspec>=0.21.0",
-  "cffi>=1.17.1",
+  "msgspec>=0.20.0",
   "bidict>=0.23.1",
   "multiaddr>=0.2.0",
   "platformdirs>=4.4.0",
@@ -63,10 +65,13 @@ dev = [
 ]
 devx = [
   # `tractor.devx` tooling
-  "greenback>=1.2.1,<2",
   "stackscope>=0.2.2,<0.3",
   # ^ requires this?
   "typing-extensions>=4.14.1",
+  # {include-group = 'sync_pause'},  # XXX, no 3.14 yet!
+]
+sync_pause = [
+  "greenback>=1.2.1,<2",  # TODO? 3.14 greenlet on nix?
 ]
 testing = [
   # test suite
@@ -74,16 +79,29 @@ testing = [
   # https://docs.pytest.org/en/8.0.x/explanation/goodpractices.html#choosing-a-test-layout-import-rules
   "pytest>=8.3.5",
   "pexpect>=4.9.0,<5",
+  # per-test wall-clock bound (used via
+  # `@pytest.mark.timeout(..., method='thread')` on the
+  # known-hanging `subint`-backend audit tests; see
+  # `ai/conc-anal/subint_*_issue.md`).
+  "pytest-timeout>=2.3",
 ]
 repl = [
   "pyperclip>=1.9.0",
   "prompt-toolkit>=3.0.50",
-  "xonsh>=0.22.2",
+  "xonsh>=0.22.8",
   "psutil>=7.0.0",
 ]
 lint = [
   "ruff>=0.9.6"
 ]
+# XXX, used for linux-only hi perf eventfd+shm channels
+# now mostly moved over to `hotbaud`.
+eventfd = [
+  "cffi>=1.17.1",
+]
+subints = [
+  "msgspec>=0.21.0",
+]
 # TODO, add these with sane versions; were originally in
 # `requirements-docs.txt`..
 # docs = [
@@ -92,10 +110,26 @@ lint = [
 # ]
 # ------ dependency-groups ------
 
+[tool.uv.dependency-groups]
+# for subints, we require 3.14+ due to 2 issues,
+# - hanging behaviour for various multi-task teardown cases (see
+#   "Availability" section in the `tractor.spawn._subints` doc string).
+# - `msgspec` support which is oustanding per PEP 684 upstream tracker:
+#   https://github.com/jcrist/msgspec/issues/563
+#
+# https://docs.astral.sh/uv/concepts/projects/dependencies/#group-requires-python
+subints = {requires-python = ">=3.14"}
+eventfd = {requires-python = ">=3.13, <3.14"}
+sync_pause = {requires-python = ">=3.13, <3.14"}
 
 [tool.uv.sources]
 # XXX NOTE, only for @goodboy's hacking on `pprint(sort_dicts=False)`
 # for the `pp` alias..
+# ------ gh upstream ------
+# xonsh = { git = 'https://github.com/anki-code/xonsh.git', branch = 'prompt_next_suggestion' }
+# ^ https://github.com/xonsh/xonsh/pull/6048
+# xonsh = { git = 'https://github.com/xonsh/xonsh.git', branch = 'main' }
+xonsh = { path = "../xonsh", editable = true }
 
 # [tool.uv.sources.pdbp]
 # XXX, in case we need to tmp patch again.
@@ -164,6 +198,7 @@ all_bullets = true
 
 [tool.pytest.ini_options]
 minversion = '6.0'
+timeout = 200  # per-test hard limit
 # https://docs.pytest.org/en/stable/reference/reference.html#configuration-options
 testpaths = [
   'tests'

tests/conftest.py

@@ -139,7 +139,9 @@ def pytest_addoption(
 
 
 @pytest.fixture(scope='session', autouse=True)
-def loglevel(request) -> str:
+def loglevel(
+    request: pytest.FixtureRequest,
+) -> str:
     import tractor
     orig = tractor.log._default_loglevel
     level = tractor.log._default_loglevel = request.config.option.loglevel
@@ -156,7 +158,7 @@ def loglevel(request) -> str:
 
 @pytest.fixture(scope='function')
 def test_log(
-    request,
+    request: pytest.FixtureRequest,
     loglevel: str,
 ) -> tractor.log.StackLevelAdapter:
     '''

tests/devx/conftest.py

@@ -146,13 +146,12 @@ def spawn(
     ids='ctl-c={}'.format,
 )
 def ctlc(
-    request,
+    request: pytest.FixtureRequest,
     ci_env: bool,
 
 ) -> bool:
 
-    use_ctlc = request.param
-
+    use_ctlc: bool = request.param
     node = request.node
     markers = node.own_markers
     for mark in markers:

tests/discovery/test_registrar.py

@@ -14,6 +14,7 @@ import psutil
 import pytest
 import subprocess
 import tractor
+from tractor.devx import dump_on_hang
 from tractor.trionics import collapse_eg
 from tractor._testing import tractor_test
 from tractor.discovery._addr import wrap_address
@@ -131,6 +132,10 @@ async def say_hello_use_wait(
         return result
 
 
+@pytest.mark.timeout(
+    3,
+    method='thread',
+)
 @tractor_test
 @pytest.mark.parametrize(
     'func',
@@ -515,7 +520,31 @@ async def kill_transport(
 
 
 
+# Wall-clock bound via `pytest-timeout` (`method='thread'`).
+# Under `--spawn-backend=subint` this test can wedge in an
+# un-Ctrl-C-able state (abandoned-subint + shared-GIL
+# starvation → signal-wakeup-fd pipe fills → SIGINT silently
+# dropped; see `ai/conc-anal/subint_sigint_starvation_issue.md`).
+# `method='thread'` is specifically required because `signal`-
+# method SIGALRM suffers the same GIL-starvation path and
+# wouldn't fire the Python-level handler.
+# At timeout the plugin hard-kills the pytest process — that's
+# the intended behavior here; the alternative is an unattended
+# suite run that never returns.
+@pytest.mark.timeout(
+    3,  # NOTE should be a 2.1s happy path.
+    method='thread',
+)
+@pytest.mark.skipon_spawn_backend(
+    'subint',
+    reason=(
+        'XXX SUBINT HANGING TEST XXX\n'
+        'See oustanding issue(s)\n'
+        # TODO, put issue link!
+    )
+)
 # @pytest.mark.parametrize('use_signal', [False, True])
+#
 def test_stale_entry_is_deleted(
     debug_mode: bool,
     daemon: subprocess.Popen,
@@ -562,4 +591,53 @@ def test_stale_entry_is_deleted(
                 await ptl.cancel_actor()
                 await an.cancel()
 
+    # TODO, remove once the `[subint]` variant no longer hangs.
+    #
+    # Status (as of Phase B hard-kill landing):
+    #
+    # - `[trio]`/`[mp_*]` variants: completes normally; `dump_on_hang`
+    #   is a no-op safety net here.
+    #
+    # - `[subint]` variant: hangs indefinitely AND is un-Ctrl-C-able.
+    #   `strace -p <pytest_pid>` while in the hang reveals a silently-
+    #   dropped SIGINT — the C signal handler tries to write the
+    #   signum byte to Python's signal-wakeup fd and gets `EAGAIN`,
+    #   meaning the pipe is full (nobody's draining it).
+    #
+    #   Root-cause chain: our hard-kill in `spawn._subint` abandoned
+    #   the driver OS-thread (which is `daemon=True`) after the soft-
+    #   kill timeout, but the *sub-interpreter* inside that thread is
+    #   still running `trio.run()` — `_interpreters.destroy()` can't
+    #   force-stop a running subint (raises `InterpreterError`), and
+    #   legacy-config subints share the main GIL. The abandoned subint
+    #   starves the parent's trio event loop from iterating often
+    #   enough to drain its wakeup pipe → SIGINT silently drops.
+    #
+    #   This is structurally a CPython-level limitation: there's no
+    #   public force-destroy primitive for a running subint. We
+    #   escape on the harness side via a SIGINT-loop in the `daemon`
+    #   fixture teardown (killing the bg registrar subproc closes its
+    #   end of the IPC, which eventually unblocks a recv in main trio,
+    #   which lets the loop drain the wakeup pipe). Long-term fix path:
+    #   msgspec PEP 684 support (jcrist/msgspec#563) → isolated-mode
+    #   subints with per-interp GIL.
+    #
+    #   Full analysis:
+    #   `ai/conc-anal/subint_sigint_starvation_issue.md`
+    #
+    #   See also the *sibling* hang class documented in
+    #   `ai/conc-anal/subint_cancel_delivery_hang_issue.md` — same
+    #   subint backend, different root cause (Ctrl-C-able hang, main
+    #   trio loop iterating fine; ours to fix, not CPython's).
+    #   Reproduced by `tests/test_subint_cancellation.py
+    #   ::test_subint_non_checkpointing_child`.
+    #
+    # Kept here (and not behind a `pytestmark.skip`) so we can still
+    # inspect the dump file if the hang ever returns after a refactor.
+    # `pytest`'s stderr capture eats `faulthandler` output otherwise,
+    # so we route `dump_on_hang` to a file.
+    with dump_on_hang(
+        seconds=20,
+        path=f'/tmp/test_stale_entry_is_deleted_{start_method}.dump',
+    ):
         trio.run(main)

tests/spawn/test_subint_cancellation.py

@@ -0,0 +1,245 @@
+'''
+Cancellation + hard-kill semantics audit for the `subint` spawn
+backend.
+
+Exercises the escape-hatch machinery added to
+`tractor.spawn._subint` (module-level `_HARD_KILL_TIMEOUT`,
+bounded shields around the soft-kill / thread-join sites, daemon
+driver-thread abandonment) so that future stdlib regressions or
+our own refactors don't silently re-introduce the hangs first
+diagnosed during the Phase B.2/B.3 bringup (issue #379).
+
+Every test in this module:
+- is wrapped in `trio.fail_after()` for a deterministic per-test
+  wall-clock ceiling (the whole point of these tests is to fail
+  fast when our escape hatches regress; an unbounded test would
+  defeat itself),
+- arms `tractor.devx.dump_on_hang()` to capture a stack dump on
+  failure — without it, a hang here is opaque because pytest's
+  stderr capture swallows `faulthandler` output by default
+  (hard-won lesson from the original diagnosis),
+- skips on py<3.13 (no `_interpreters`) and on any
+  `--spawn-backend` other than `'subint'` (these tests are
+  subint-specific by design — they'd be nonsense under `trio` or
+  `mp_*`).
+
+'''
+from __future__ import annotations
+from functools import partial
+
+import pytest
+import trio
+import tractor
+from tractor.devx import dump_on_hang
+
+
+# Gate: the `subint` backend requires py3.14+. Check the
+# public stdlib wrapper's presence (added in 3.14) rather than
+# the private `_interpreters` module (which exists on 3.13 but
+# wedges under tractor's usage — see `tractor.spawn._subint`).
+pytest.importorskip('concurrent.interpreters')
+
+# Subint-only: read the spawn method that `pytest_configure`
+# committed via `try_set_start_method()`. By the time this module
+# imports, the CLI backend choice has been applied.
+from tractor.spawn._spawn import _spawn_method  # noqa: E402
+
+if _spawn_method != 'subint':
+    pytestmark = pytest.mark.skip(
+        reason=(
+            "subint-specific cancellation audit — "
+            "pass `--spawn-backend=subint` to run."
+        ),
+    )
+
+
+# ----------------------------------------------------------------
+# child-side task bodies (run inside the spawned subint)
+# ----------------------------------------------------------------
+
+
+async def _trivial_rpc() -> str:
+    '''
+    Minimal RPC body for the baseline happy-teardown test.
+    '''
+    return 'hello from subint'
+
+
+async def _spin_without_trio_checkpoints() -> None:
+    '''
+    Block the main task with NO trio-visible checkpoints so any
+    `Portal.cancel_actor()` arriving over IPC has nothing to hand
+    off to.
+
+    `threading.Event.wait(timeout)` releases the GIL (so other
+    threads — including trio's IO/RPC tasks — can progress) but
+    does NOT insert a trio checkpoint, so the subactor's main
+    task never notices cancellation.
+
+    This is the exact "stuck subint" scenario the hard-kill
+    shields exist to survive.
+    '''
+    import threading
+    never_set = threading.Event()
+    while not never_set.is_set():
+        # 1s re-check granularity; low enough not to waste CPU,
+        # high enough that even a pathologically slow
+        # `_HARD_KILL_TIMEOUT` won't accidentally align with a
+        # wake.
+        never_set.wait(timeout=1.0)
+
+
+# ----------------------------------------------------------------
+# parent-side harnesses (driven inside `trio.run(...)`)
+# ----------------------------------------------------------------
+
+
+async def _happy_path(
+    reg_addr: tuple[str, int|str],
+    deadline: float,
+) -> None:
+    with trio.fail_after(deadline):
+        async with (
+            tractor.open_root_actor(
+                registry_addrs=[reg_addr],
+            ),
+            tractor.open_nursery() as an,
+        ):
+            portal: tractor.Portal = await an.run_in_actor(
+                _trivial_rpc,
+                name='subint-happy',
+            )
+            result: str = await portal.wait_for_result()
+            assert result == 'hello from subint'
+
+
+async def _spawn_stuck_then_cancel(
+    reg_addr: tuple[str, int|str],
+    deadline: float,
+) -> None:
+    with trio.fail_after(deadline):
+        async with (
+            tractor.open_root_actor(
+                registry_addrs=[reg_addr],
+            ),
+            tractor.open_nursery() as an,
+        ):
+            await an.run_in_actor(
+                _spin_without_trio_checkpoints,
+                name='subint-stuck',
+            )
+            # Give the child time to reach its non-checkpointing
+            # loop before we cancel; the precise value doesn't
+            # matter as long as it's a handful of trio schedule
+            # ticks.
+            await trio.sleep(0.5)
+            an.cancel_scope.cancel()
+
+
+# ----------------------------------------------------------------
+# tests
+# ----------------------------------------------------------------
+
+
+def test_subint_happy_teardown(
+    reg_addr: tuple[str, int|str],
+) -> None:
+    '''
+    Baseline: spawn a subactor, do one portal RPC, close nursery
+    cleanly. No cancel, no faults.
+
+    If this regresses we know something's wrong at the
+    spawn/teardown layer unrelated to the hard-kill escape
+    hatches.
+
+    '''
+    deadline: float = 10.0
+    with dump_on_hang(
+        seconds=deadline,
+        path='/tmp/subint_cancellation_happy.dump',
+    ):
+        trio.run(partial(_happy_path, reg_addr, deadline))
+
+
+@pytest.mark.skipon_spawn_backend(
+    'subint',
+    reason=(
+        'XXX SUBINT HANGING TEST XXX\n'
+        'See oustanding issue(s)\n'
+        # TODO, put issue link!
+    )
+)
+# Wall-clock bound via `pytest-timeout` (`method='thread'`)
+# as defense-in-depth over the inner `trio.fail_after(15)`.
+# Under the orphaned-channel hang class described in
+# `ai/conc-anal/subint_cancel_delivery_hang_issue.md`, SIGINT
+# is still deliverable and this test *should* be unwedgeable
+# by the inner trio timeout — but sibling subint-backend
+# tests in this repo have also exhibited the
+# `subint_sigint_starvation_issue.md` GIL-starvation flavor,
+# so `method='thread'` keeps us safe in case ordering or
+# load shifts the failure mode.
+@pytest.mark.timeout(
+    3,  # NOTE never passes pre-3.14+ subints support.
+    method='thread',
+)
+def test_subint_non_checkpointing_child(
+    reg_addr: tuple[str, int|str],
+) -> None:
+    '''
+    Cancel a subactor whose main task is stuck in a non-
+    checkpointing Python loop.
+
+    `Portal.cancel_actor()` may be delivered over IPC but the
+    main task never checkpoints to observe the Cancelled —
+    so the subint's `trio.run()` can't exit gracefully.
+
+    The parent `subint_proc` bounded-shield + daemon-driver-
+    thread combo should abandon the thread after
+    `_HARD_KILL_TIMEOUT` and let the parent return cleanly.
+
+    Wall-clock budget:
+    - ~0.5s: settle time for child to enter the stuck loop
+    - ~3s: `_HARD_KILL_TIMEOUT` (soft-kill wait)
+    - ~3s: `_HARD_KILL_TIMEOUT` (thread-join wait)
+    - margin
+
+    KNOWN ISSUE (Ctrl-C-able hang):
+    -------------------------------
+    This test currently hangs past the hard-kill timeout for
+    reasons unrelated to the subint teardown itself — after
+    the subint is destroyed, a parent-side trio task appears
+    to park on an orphaned IPC channel (no clean EOF
+    delivered to a waiting receive). Unlike the
+    SIGINT-starvation sibling case in
+    `test_stale_entry_is_deleted`, this hang IS Ctrl-C-able
+    (`strace` shows SIGINT wakeup-fd `write() = 1`, not
+    `EAGAIN`) — i.e. the main trio loop is still iterating
+    normally. That makes this *our* bug to fix, not a
+    CPython-level limitation.
+
+    See `ai/conc-anal/subint_cancel_delivery_hang_issue.md`
+    for the full analysis + candidate fix directions
+    (explicit parent-side channel abort in `subint_proc`
+    teardown being the most likely surgical fix).
+
+    The sibling `ai/conc-anal/subint_sigint_starvation_issue.md`
+    documents the *other* hang class (abandoned-legacy-subint
+    thread + shared-GIL starvation → signal-wakeup-fd pipe
+    fills → SIGINT silently dropped) — that one is
+    structurally blocked on msgspec PEP 684 adoption and is
+    NOT what this test is hitting.
+
+    '''
+    deadline: float = 15.0
+    with dump_on_hang(
+        seconds=deadline,
+        path='/tmp/subint_cancellation_stuck.dump',
+    ):
+        trio.run(
+            partial(
+                _spawn_stuck_then_cancel,
+                reg_addr,
+                deadline,
+            ),
+        )

tests/spawn/test_subint_forkserver.py

@@ -0,0 +1,604 @@
+'''
+Integration exercises for the `tractor.spawn._subint_forkserver`
+submodule at three tiers:
+
+1. the low-level primitives
+   (`fork_from_worker_thread()` +
+   `run_subint_in_worker_thread()`) driven from inside a real
+   `trio.run()` in the parent process,
+
+2. the full `subint_forkserver_proc` spawn backend wired
+   through tractor's normal actor-nursery + portal-RPC
+   machinery — i.e. `open_root_actor` + `open_nursery` +
+   `run_in_actor` against a subactor spawned via fork from a
+   main-interp worker thread.
+
+Background
+----------
+`ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md`
+establishes that `os.fork()` from a non-main sub-interpreter
+aborts the child at the CPython level. The sibling
+`subint_fork_from_main_thread_smoketest.py` proves the escape
+hatch: fork from a main-interp *worker thread* (one that has
+never entered a subint) works, and the forked child can then
+host its own `trio.run()` inside a fresh subint.
+
+Those smoke-test scenarios are standalone — no trio runtime
+in the *parent*. Tiers (1)+(2) here cover the primitives
+driven from inside `trio.run()` in the parent, and tier (3)
+(the `*_spawn_basic` test) drives the registered
+`subint_forkserver` spawn backend end-to-end against the
+tractor runtime.
+
+Gating
+------
+- py3.14+ (via `concurrent.interpreters` presence)
+- no `--spawn-backend` restriction — the backend-level test
+  flips `tractor.spawn._spawn._spawn_method` programmatically
+  (via `try_set_start_method('subint_forkserver')`) and
+  restores it on teardown, so these tests are independent of
+  the session-level CLI backend choice.
+
+'''
+from __future__ import annotations
+from functools import partial
+import os
+from pathlib import Path
+import platform
+import select
+import signal
+import subprocess
+import sys
+import time
+
+import pytest
+import trio
+
+import tractor
+from tractor.devx import dump_on_hang
+
+
+# Gate: subint forkserver primitives require py3.14+. Check
+# the public stdlib wrapper's presence (added in 3.14) rather
+# than `_interpreters` directly — see
+# `tractor.spawn._subint` for why.
+pytest.importorskip('concurrent.interpreters')
+
+from tractor.spawn._subint_forkserver import (  # noqa: E402
+    fork_from_worker_thread,
+    run_subint_in_worker_thread,
+    wait_child,
+)
+from tractor.spawn import _spawn as _spawn_mod  # noqa: E402
+from tractor.spawn._spawn import try_set_start_method  # noqa: E402
+
+
+# ----------------------------------------------------------------
+# child-side callables (passed via `child_target=` across fork)
+# ----------------------------------------------------------------
+
+
+_CHILD_TRIO_BOOTSTRAP: str = (
+    'import trio\n'
+    'async def _main():\n'
+    '    await trio.sleep(0.05)\n'
+    '    return 42\n'
+    'result = trio.run(_main)\n'
+    'assert result == 42, f"trio.run returned {result}"\n'
+)
+
+
+def _child_trio_in_subint() -> int:
+    '''
+    `child_target` for the trio-in-child scenario: drive a
+    trivial `trio.run()` inside a fresh legacy-config subint
+    on a worker thread.
+
+    Returns an exit code suitable for `os._exit()`:
+    - 0: subint-hosted `trio.run()` succeeded
+    - 3: driver thread hang (timeout inside `run_subint_in_worker_thread`)
+    - 4: subint bootstrap raised some other exception
+
+    '''
+    try:
+        run_subint_in_worker_thread(
+            _CHILD_TRIO_BOOTSTRAP,
+            thread_name='child-subint-trio-thread',
+        )
+    except RuntimeError:
+        # timeout / thread-never-returned
+        return 3
+    except BaseException:
+        return 4
+    return 0
+
+
+# ----------------------------------------------------------------
+# parent-side harnesses (run inside `trio.run()`)
+# ----------------------------------------------------------------
+
+
+async def run_fork_in_non_trio_thread(
+    deadline: float,
+    *,
+    child_target=None,
+) -> int:
+    '''
+    From inside a parent `trio.run()`, off-load the
+    forkserver primitive to a main-interp worker thread via
+    `trio.to_thread.run_sync()` and return the forked child's
+    pid.
+
+    Then `wait_child()` on that pid (also off-loaded so we
+    don't block trio's event loop on `waitpid()`) and assert
+    the child exited cleanly.
+
+    '''
+    with trio.fail_after(deadline):
+        # NOTE: `fork_from_worker_thread` internally spawns its
+        # own dedicated `threading.Thread` (not from trio's
+        # cache) and joins it before returning — so we can
+        # safely off-load via `to_thread.run_sync` without
+        # worrying about the trio-thread-cache recycling the
+        # runner. Pass `abandon_on_cancel=False` for the
+        # same "bounded + clean" rationale we use in
+        # `_subint.subint_proc`.
+        pid: int = await trio.to_thread.run_sync(
+            partial(
+                fork_from_worker_thread,
+                child_target,
+                thread_name='test-subint-forkserver',
+            ),
+            abandon_on_cancel=False,
+        )
+        assert pid > 0
+
+        ok, status_str = await trio.to_thread.run_sync(
+            partial(
+                wait_child,
+                pid,
+                expect_exit_ok=True,
+            ),
+            abandon_on_cancel=False,
+        )
+        assert ok, (
+            f'forked child did not exit cleanly: '
+            f'{status_str}'
+        )
+        return pid
+
+
+# ----------------------------------------------------------------
+# tests
+# ----------------------------------------------------------------
+
+
+# Bounded wall-clock via `pytest-timeout` (`method='thread'`)
+# for the usual GIL-hostage safety reason documented in the
+# sibling `test_subint_cancellation.py` / the class-A
+# `subint_sigint_starvation_issue.md`. Each test also has an
+# inner `trio.fail_after()` so assertion failures fire fast
+# under normal conditions.
+@pytest.mark.timeout(30, method='thread')
+def test_fork_from_worker_thread_via_trio(
+) -> None:
+    '''
+    Baseline: inside `trio.run()`, call
+    `fork_from_worker_thread()` via `trio.to_thread.run_sync()`,
+    get a child pid back, reap the child cleanly.
+
+    No trio-in-child. If this regresses we know the parent-
+    side trio↔worker-thread plumbing is broken independent
+    of any child-side subint machinery.
+
+    '''
+    deadline: float = 10.0
+    with dump_on_hang(
+        seconds=deadline,
+        path='/tmp/subint_forkserver_baseline.dump',
+    ):
+        pid: int = trio.run(
+            partial(run_fork_in_non_trio_thread, deadline),
+        )
+    # parent-side sanity — we got a real pid back.
+    assert isinstance(pid, int) and pid > 0
+    # by now the child has been waited on; it shouldn't be
+    # reap-able again.
+    with pytest.raises((ChildProcessError, OSError)):
+        os.waitpid(pid, os.WNOHANG)
+
+
+@pytest.mark.timeout(30, method='thread')
+def test_fork_and_run_trio_in_child() -> None:
+    '''
+    End-to-end: inside the parent's `trio.run()`, off-load
+    `fork_from_worker_thread()` to a worker thread, have the
+    forked child then create a fresh subint and run
+    `trio.run()` inside it on yet another worker thread.
+
+    This is the full "forkserver + trio-in-subint-in-child"
+    pattern the proposed `subint_forkserver` spawn backend
+    would rest on.
+
+    '''
+    deadline: float = 15.0
+    with dump_on_hang(
+        seconds=deadline,
+        path='/tmp/subint_forkserver_trio_in_child.dump',
+    ):
+        pid: int = trio.run(
+            partial(
+                run_fork_in_non_trio_thread,
+                deadline,
+                child_target=_child_trio_in_subint,
+            ),
+        )
+    assert isinstance(pid, int) and pid > 0
+
+
+# ----------------------------------------------------------------
+# tier-3 backend test: drive the registered `subint_forkserver`
+# spawn backend end-to-end through tractor's actor-nursery +
+# portal-RPC machinery.
+# ----------------------------------------------------------------
+
+
+async def _trivial_rpc() -> str:
+    '''
+    Minimal subactor-side RPC body: just return a sentinel
+    string the parent can assert on.
+
+    '''
+    return 'hello from subint-forkserver child'
+
+
+async def _happy_path_forkserver(
+    reg_addr: tuple[str, int | str],
+    deadline: float,
+) -> None:
+    '''
+    Parent-side harness: stand up a root actor, open an actor
+    nursery, spawn one subactor via the currently-selected
+    spawn backend (which this test will have flipped to
+    `subint_forkserver`), run a trivial RPC through its
+    portal, assert the round-trip result.
+
+    '''
+    with trio.fail_after(deadline):
+        async with (
+            tractor.open_root_actor(
+                registry_addrs=[reg_addr],
+            ),
+            tractor.open_nursery() as an,
+        ):
+            portal: tractor.Portal = await an.run_in_actor(
+                _trivial_rpc,
+                name='subint-forkserver-child',
+            )
+            result: str = await portal.wait_for_result()
+            assert result == 'hello from subint-forkserver child'
+
+
+@pytest.fixture
+def forkserver_spawn_method():
+    '''
+    Flip `tractor.spawn._spawn._spawn_method` to
+    `'subint_forkserver'` for the duration of a test, then
+    restore whatever was in place before (usually the
+    session-level CLI choice, typically `'trio'`).
+
+    Without this, other tests in the same session would
+    observe the global flip and start spawning via fork —
+    which is almost certainly NOT what their assertions were
+    written against.
+
+    '''
+    prev_method: str = _spawn_mod._spawn_method
+    prev_ctx = _spawn_mod._ctx
+    try_set_start_method('subint_forkserver')
+    try:
+        yield
+    finally:
+        _spawn_mod._spawn_method = prev_method
+        _spawn_mod._ctx = prev_ctx
+
+
+@pytest.mark.timeout(60, method='thread')
+def test_subint_forkserver_spawn_basic(
+    reg_addr: tuple[str, int | str],
+    forkserver_spawn_method,
+) -> None:
+    '''
+    Happy-path: spawn ONE subactor via the
+    `subint_forkserver` backend (parent-side fork from a
+    main-interp worker thread), do a trivial portal-RPC
+    round-trip, tear the nursery down cleanly.
+
+    If this passes, the "forkserver + tractor runtime" arch
+    is proven end-to-end: the registered
+    `subint_forkserver_proc` spawn target successfully
+    forks a child, the child runs `_actor_child_main()` +
+    completes IPC handshake + serves an RPC, and the parent
+    reaps via `_ForkedProc.wait()` without regressing any of
+    the normal nursery teardown invariants.
+
+    '''
+    deadline: float = 20.0
+    with dump_on_hang(
+        seconds=deadline,
+        path='/tmp/subint_forkserver_spawn_basic.dump',
+    ):
+        trio.run(
+            partial(
+                _happy_path_forkserver,
+                reg_addr,
+                deadline,
+            ),
+        )
+
+
+# ----------------------------------------------------------------
+# tier-4 DRAFT: orphaned-subactor SIGINT survivability
+#
+# Motivating question: with `subint_forkserver`, the child's
+# `trio.run()` lives on the fork-inherited worker thread which
+# is NOT `threading.main_thread()` — so trio cannot install its
+# `signal.set_wakeup_fd`-based SIGINT handler. If the parent
+# goes away via `SIGKILL` (no IPC `Portal.cancel_actor()`
+# possible), does SIGINT on the orphan child cleanly tear it
+# down via CPython's default `KeyboardInterrupt` delivery, or
+# does it hang?
+#
+# Working hypothesis (unverified pre-this-test): post-fork the
+# child is effectively single-threaded (only the fork-worker
+# tstate survived), so SIGINT → default handler → raises
+# `KeyboardInterrupt` on the only thread — which happens to be
+# the one driving trio's event loop — so trio observes it at
+# the next checkpoint. If so, we're "fine" on this backend
+# despite the missing trio SIGINT handler.
+#
+# Cross-backend generalization (decide after this passes):
+# - applicable to any backend whose subactors are separate OS
+#   processes: `trio`, `mp_spawn`, `mp_forkserver`,
+#   `subint_forkserver`.
+# - NOT applicable to plain `subint` (subactors are in-process
+#   subinterpreters, no orphan child process to SIGINT).
+# - move path: lift the harness script into
+#   `tests/_orphan_harness.py`, parametrize on the session's
+#   `_spawn_method`, add `skipif _spawn_method == 'subint'`.
+# ----------------------------------------------------------------
+
+
+_ORPHAN_HARNESS_SCRIPT: str = '''
+import os
+import sys
+import trio
+import tractor
+from tractor.spawn._spawn import try_set_start_method
+
+async def _sleep_forever() -> None:
+    print(f"CHILD_PID={os.getpid()}", flush=True)
+    await trio.sleep_forever()
+
+async def _main(reg_addr):
+    async with (
+        tractor.open_root_actor(registry_addrs=[reg_addr]),
+        tractor.open_nursery() as an,
+    ):
+        portal = await an.run_in_actor(
+            _sleep_forever,
+            name="orphan-test-child",
+        )
+        print(f"PARENT_READY={os.getpid()}", flush=True)
+        await trio.sleep_forever()
+
+if __name__ == "__main__":
+    backend = sys.argv[1]
+    host = sys.argv[2]
+    port = int(sys.argv[3])
+    try_set_start_method(backend)
+    trio.run(_main, (host, port))
+'''
+
+
+def _read_marker(
+    proc: subprocess.Popen,
+    marker: str,
+    timeout: float,
+    _buf: dict,
+) -> str:
+    '''
+    Block until `<marker>=<value>\\n` appears on `proc.stdout`
+    and return `<value>`. Uses a per-proc byte buffer (`_buf`)
+    to carry partial lines across calls.
+
+    '''
+    deadline: float = time.monotonic() + timeout
+    remainder: bytes = _buf.get('remainder', b'')
+    prefix: bytes = f'{marker}='.encode()
+    while time.monotonic() < deadline:
+        # drain any complete lines already buffered
+        while b'\n' in remainder:
+            line, remainder = remainder.split(b'\n', 1)
+            if line.startswith(prefix):
+                _buf['remainder'] = remainder
+                return line[len(prefix):].decode().strip()
+        ready, _, _ = select.select([proc.stdout], [], [], 0.2)
+        if not ready:
+            continue
+        chunk: bytes = os.read(proc.stdout.fileno(), 4096)
+        if not chunk:
+            break
+        remainder += chunk
+    _buf['remainder'] = remainder
+    raise TimeoutError(
+        f'Never observed marker {marker!r} on harness stdout '
+        f'within {timeout}s'
+    )
+
+
+def _process_alive(pid: int) -> bool:
+    '''Liveness probe for a pid we do NOT parent (post-orphan).'''
+    try:
+        os.kill(pid, 0)
+        return True
+    except ProcessLookupError:
+        return False
+
+
+# Regressed back to xfail: previously passed after the
+# fork-child FD-hygiene fix in `_close_inherited_fds()`,
+# but the recent `wait_for_no_more_peers(move_on_after=3.0)`
+# bound in `async_main`'s teardown added up to 3s to the
+# orphan subactor's exit timeline, pushing it past the
+# test's 10s poll window. Real fix requires making the
+# bounded wait faster when the actor is orphaned, or
+# increasing the test's poll window. See tracker doc
+# `ai/conc-anal/subint_forkserver_orphan_sigint_hang_issue.md`.
+@pytest.mark.xfail(
+    strict=True,
+    reason=(
+        'Regressed to xfail after `wait_for_no_more_peers` '
+        'bound added ~3s teardown latency. Needs either '
+        'faster orphan-side teardown or 15s test poll window.'
+    ),
+)
+@pytest.mark.timeout(
+    30,
+    method='thread',
+)
+def test_orphaned_subactor_sigint_cleanup_DRAFT(
+    reg_addr: tuple[str, int | str],
+    tmp_path: Path,
+) -> None:
+    '''
+    DRAFT — orphaned-subactor SIGINT survivability under the
+    `subint_forkserver` backend.
+
+    Sequence:
+      1. Spawn a harness subprocess that brings up a root
+         actor + one `sleep_forever` subactor via
+         `subint_forkserver`.
+      2. Read the harness's stdout for `PARENT_READY=<pid>`
+         and `CHILD_PID=<pid>` markers (confirms the
+         parent→child IPC handshake completed).
+      3. `SIGKILL` the parent (no IPC cancel possible — the
+         whole point of this test).
+      4. `SIGINT` the orphan child.
+      5. Poll `os.kill(child_pid, 0)` for up to 10s — assert
+         the child exits.
+
+    Empirical result (2026-04, py3.14): currently **FAILS** —
+    SIGINT on the orphan child doesn't unwind the trio loop,
+    despite trio's `KIManager` handler being correctly
+    installed in the subactor (the post-fork thread IS
+    `threading.main_thread()` on py3.14). `faulthandler` dump
+    shows the subactor wedged in `trio/_core/_io_epoll.py::
+    get_events` — the signal's supposed wakeup of the event
+    loop isn't firing. Full analysis + diagnostic evidence
+    in `ai/conc-anal/
+    subint_forkserver_orphan_sigint_hang_issue.md`.
+
+    The runtime's *intentional* "KBI-as-OS-cancel" path at
+    `tractor/spawn/_entry.py::_trio_main:164` is therefore
+    unreachable under this backend+config. Closing the gap is
+    aligned with existing design intent (make the already-
+    designed behavior actually fire), not a new feature.
+    Marked `xfail(strict=True)` so the
+    mark flips to XPASS→fail once the gap is closed and we'll
+    know to drop the mark.
+
+    '''
+    if platform.system() != 'Linux':
+        pytest.skip(
+            'orphan-reparenting semantics only exercised on Linux'
+        )
+
+    script_path = tmp_path / '_orphan_harness.py'
+    script_path.write_text(_ORPHAN_HARNESS_SCRIPT)
+
+    # Offset the port so we don't race the session reg_addr with
+    # any concurrently-running backend test's listener.
+    host: str = reg_addr[0]
+    port: int = int(reg_addr[1]) + 17
+
+    proc: subprocess.Popen = subprocess.Popen(
+        [
+            sys.executable,
+            str(script_path),
+            'subint_forkserver',
+            host,
+            str(port),
+        ],
+        stdout=subprocess.PIPE,
+        stderr=subprocess.STDOUT,
+    )
+    parent_pid: int | None = None
+    child_pid: int | None = None
+    buf: dict = {}
+    try:
+        child_pid = int(_read_marker(proc, 'CHILD_PID', 15.0, buf))
+        parent_pid = int(_read_marker(proc, 'PARENT_READY', 15.0, buf))
+
+        # sanity: both alive before we start killing stuff
+        assert _process_alive(parent_pid), (
+            f'harness parent pid={parent_pid} gone before '
+            f'SIGKILL — test premise broken'
+        )
+        assert _process_alive(child_pid), (
+            f'orphan-candidate child pid={child_pid} gone '
+            f'before test started'
+        )
+
+        # step 3: kill parent — no IPC cancel arrives at child.
+        # `proc.wait()` reaps the zombie so it truly disappears
+        # from the process table (otherwise `os.kill(pid, 0)`
+        # keeps reporting it as alive).
+        os.kill(parent_pid, signal.SIGKILL)
+        try:
+            proc.wait(timeout=3.0)
+        except subprocess.TimeoutExpired:
+            pytest.fail(
+                f'harness parent pid={parent_pid} did not die '
+                f'after SIGKILL — test premise broken'
+            )
+        assert _process_alive(child_pid), (
+            f'child pid={child_pid} died along with parent — '
+            f'did the parent reap it before SIGKILL took? '
+            f'test premise requires an orphan.'
+        )
+
+        # step 4+5: SIGINT the orphan, poll for exit.
+        os.kill(child_pid, signal.SIGINT)
+        timeout: float = 6.0
+        cleanup_deadline: float = time.monotonic() + timeout
+        while time.monotonic() < cleanup_deadline:
+            if not _process_alive(child_pid):
+                return  # <- success path
+            time.sleep(0.1)
+
+        pytest.fail(
+            f'Orphan subactor (pid={child_pid}) did NOT exit '
+            f'within 10s of SIGINT under `subint_forkserver` '
+            f'→ trio on non-main thread did not observe the '
+            f'default CPython KeyboardInterrupt; backend needs '
+            f'explicit SIGINT plumbing.'
+        )
+    finally:
+        # best-effort cleanup to avoid leaking orphans across
+        # the test session regardless of outcome.
+        for pid in (parent_pid, child_pid):
+            if pid is None:
+                continue
+            try:
+                os.kill(pid, signal.SIGKILL)
+            except ProcessLookupError:
+                pass
+        try:
+            proc.kill()
+        except OSError:
+            pass
+        try:
+            proc.wait(timeout=2.0)
+        except subprocess.TimeoutExpired:
+            pass

tests/test_cancellation.py

@@ -21,6 +21,16 @@ _non_linux: bool = platform.system() != 'Linux'
 _friggin_windows: bool = platform.system() == 'Windows'
 
 
+pytestmark = pytest.mark.skipon_spawn_backend(
+    'subint',
+    reason=(
+        'XXX SUBINT HANGING TEST XXX\n'
+        'See oustanding issue(s)\n'
+        # TODO, put issue link!
+    )
+)
+
+
 async def assert_err(delay=0):
     await trio.sleep(delay)
     assert 0
@@ -110,8 +120,17 @@ def test_remote_error(reg_addr, args_err):
             assert exc.boxed_type == errtype
 
 
+# @pytest.mark.skipon_spawn_backend(
+#     'subint',
+#     reason=(
+#         'XXX SUBINT HANGING TEST XXX\n'
+#         'See oustanding issue(s)\n'
+#         # TODO, put issue link!
+#     )
+# )
 def test_multierror(
     reg_addr: tuple[str, int],
+    start_method: str,
 ):
     '''
     Verify we raise a ``BaseExceptionGroup`` out of a nursery where
@@ -141,15 +160,28 @@ def test_multierror(
         trio.run(main)
 
 
-@pytest.mark.parametrize('delay', (0, 0.5))
 @pytest.mark.parametrize(
-    'num_subactors', range(25, 26),
+    'delay',
+    (0, 0.5),
+    ids='delays={}'.format,
 )
-def test_multierror_fast_nursery(reg_addr, start_method, num_subactors, delay):
-    """Verify we raise a ``BaseExceptionGroup`` out of a nursery where
+@pytest.mark.parametrize(
+    'num_subactors',
+    range(25, 26),
+    ids= 'num_subs={}'.format,
+)
+def test_multierror_fast_nursery(
+    reg_addr: tuple,
+    start_method: str,
+    num_subactors: int,
+    delay: float,
+):
+    '''
+    Verify we raise a ``BaseExceptionGroup`` out of a nursery where
     more then one actor errors and also with a delay before failure
     to test failure during an ongoing spawning.
-    """
+
+    '''
     async def main():
         async with tractor.open_nursery(
             registry_addrs=[reg_addr],
@@ -189,8 +221,15 @@ async def do_nothing():
     pass
 
 
-@pytest.mark.parametrize('mechanism', ['nursery_cancel', KeyboardInterrupt])
-def test_cancel_single_subactor(reg_addr, mechanism):
+@pytest.mark.parametrize(
+    'mechanism', [
+    'nursery_cancel',
+    KeyboardInterrupt,
+])
+def test_cancel_single_subactor(
+    reg_addr: tuple,
+    mechanism: str|KeyboardInterrupt,
+):
     '''
     Ensure a ``ActorNursery.start_actor()`` spawned subactor
     cancels when the nursery is cancelled.
@@ -232,9 +271,13 @@ async def stream_forever():
         await trio.sleep(0.01)
 
 
-@tractor_test
-async def test_cancel_infinite_streamer(start_method):
-
+@tractor_test(
+    timeout=6,
+)
+async def test_cancel_infinite_streamer(
+    reg_addr: tuple,
+    start_method: str,
+):
     # stream for at most 1 seconds
     with (
         trio.fail_after(4),
@@ -257,6 +300,14 @@ async def test_cancel_infinite_streamer(start_method):
     assert n.cancelled
 
 
+# @pytest.mark.skipon_spawn_backend(
+#     'subint',
+#     reason=(
+#         'XXX SUBINT HANGING TEST XXX\n'
+#         'See oustanding issue(s)\n'
+#         # TODO, put issue link!
+#     )
+# )
 @pytest.mark.parametrize(
     'num_actors_and_errs',
     [
@@ -286,9 +337,12 @@ async def test_cancel_infinite_streamer(start_method):
         'no_daemon_actors_fail_all_run_in_actors_sleep_then_fail',
     ],
 )
-@tractor_test
+@tractor_test(
+    timeout=10,
+)
 async def test_some_cancels_all(
     num_actors_and_errs: tuple,
+    reg_addr: tuple,
     start_method: str,
     loglevel: str,
 ):
@@ -370,7 +424,10 @@ async def test_some_cancels_all(
         pytest.fail("Should have gotten a remote assertion error?")
 
 
-async def spawn_and_error(breadth, depth) -> None:
+async def spawn_and_error(
+    breadth: int,
+    depth: int,
+) -> None:
     name = tractor.current_actor().name
     async with tractor.open_nursery() as nursery:
         for i in range(breadth):
@@ -395,8 +452,31 @@ async def spawn_and_error(breadth, depth) -> None:
             await nursery.run_in_actor(*args, **kwargs)
 
 
+@pytest.mark.skipon_spawn_backend(
+    'subint_forkserver',
+    reason=(
+        'Passes cleanly with `pytest -s` (no stdout capture) '
+        'but hangs under default `--capture=fd` due to '
+        'pytest-capture-pipe buffer fill from high-volume '
+        'subactor error-log traceback output inherited via fds '
+        '1,2 in fork children. Fix direction: redirect subactor '
+        'stdout/stderr to `/dev/null` in `_child_target` / '
+        '`_actor_child_main` so forkserver children don\'t hold '
+        'pytest\'s capture pipe open. See `ai/conc-anal/'
+        'subint_forkserver_test_cancellation_leak_issue.md` '
+        '"Update — pytest capture pipe is the final gate".'
+    ),
+)
+@pytest.mark.timeout(
+    10,
+    method='thread',
+)
 @tractor_test
-async def test_nested_multierrors(loglevel, start_method):
+async def test_nested_multierrors(
+    reg_addr: tuple,
+    loglevel: str,
+    start_method: str,
+):
     '''
     Test that failed actor sets are wrapped in `BaseExceptionGroup`s. This
     test goes only 2 nurseries deep but we should eventually have tests
@@ -483,20 +563,24 @@ async def test_nested_multierrors(loglevel, start_method):
 
 @no_windows
 def test_cancel_via_SIGINT(
-    loglevel,
-    start_method,
-    spawn_backend,
+    reg_addr: tuple,
+    loglevel: str,
+    start_method: str,
 ):
-    """Ensure that a control-C (SIGINT) signal cancels both the parent and
+    '''
+    Ensure that a control-C (SIGINT) signal cancels both the parent and
     child processes in trionic fashion
-    """
+
+    '''
     pid: int = os.getpid()
 
     async def main():
         with trio.fail_after(2):
-            async with tractor.open_nursery() as tn:
+            async with tractor.open_nursery(
+                registry_addrs=[reg_addr],
+            ) as tn:
                 await tn.start_actor('sucka')
-                if 'mp' in spawn_backend:
+                if 'mp' in start_method:
                     time.sleep(0.1)
                 os.kill(pid, signal.SIGINT)
                 await trio.sleep_forever()
@@ -507,6 +591,7 @@ def test_cancel_via_SIGINT(
 
 @no_windows
 def test_cancel_via_SIGINT_other_task(
+    reg_addr: tuple,
     loglevel: str,
     start_method: str,
     spawn_backend: str,
@@ -535,7 +620,9 @@ def test_cancel_via_SIGINT_other_task(
     async def spawn_and_sleep_forever(
         task_status=trio.TASK_STATUS_IGNORED
     ):
-        async with tractor.open_nursery() as tn:
+        async with tractor.open_nursery(
+            registry_addrs=[reg_addr],
+        ) as tn:
             for i in range(3):
                 await tn.run_in_actor(
                     sleep_forever,
@@ -580,6 +667,14 @@ async def spawn_sub_with_sync_blocking_task():
         print('exiting first subactor layer..\n')
 
 
+# @pytest.mark.skipon_spawn_backend(
+#     'subint',
+#     reason=(
+#         'XXX SUBINT HANGING TEST XXX\n'
+#         'See oustanding issue(s)\n'
+#         # TODO, put issue link!
+#     )
+# )
 @pytest.mark.parametrize(
     'man_cancel_outer',
     [
@@ -694,7 +789,7 @@ def test_cancel_while_childs_child_in_sync_sleep(
 
 
 def test_fast_graceful_cancel_when_spawn_task_in_soft_proc_wait_for_daemon(
-    start_method,
+    start_method: str,
 ):
     '''
     This is a very subtle test which demonstrates how cancellation

tests/test_inter_peer_cancellation.py

@@ -26,6 +26,15 @@ from tractor._testing import (
 
 from .conftest import cpu_scaling_factor
 
+pytestmark = pytest.mark.skipon_spawn_backend(
+    'subint',
+    reason=(
+        'XXX SUBINT GIL-CONTENTION HANGING TEST XXX\n'
+        'See oustanding issue(s)\n'
+        # TODO, put issue link!
+    )
+)
+
 # XXX TODO cases:
 # - [x] WE cancelled the peer and thus should not see any raised
 #   `ContextCancelled` as it should be reaped silently?

tests/test_pubsub.py

@@ -7,6 +7,14 @@ import tractor
 from tractor.experimental import msgpub
 from tractor._testing import tractor_test
 
+pytestmark = pytest.mark.skipon_spawn_backend(
+    'subint',
+    reason=(
+        'XXX SUBINT HANGING TEST XXX\n'
+        'See oustanding issue(s)\n'
+        # TODO, put issue link!
+    )
+)
 
 def test_type_checks():
 

tests/test_ringbuf.py

@@ -4,6 +4,10 @@ import trio
 import pytest
 
 import tractor
+
+# XXX `cffi` dun build on py3.14 yet..
+cffi = pytest.importorskip("cffi")
+
 from tractor.ipc._ringbuf import (
     open_ringbuf,
     RBToken,
@@ -14,7 +18,7 @@ from tractor._testing.samples import (
     generate_sample_messages,
 )
 
-# in case you don't want to melt your cores, uncomment dis!
+# XXX, in case you want to melt your cores, comment this skip line XD
 pytestmark = pytest.mark.skip
 
 

tests/test_shm.py

@@ -14,6 +14,14 @@ from tractor.ipc._shm import (
     attach_shm_list,
 )
 
+pytestmark = pytest.mark.skipon_spawn_backend(
+    'subint',
+    reason=(
+        'XXX SUBINT GIL-CONTENTION HANGING TEST XXX\n'
+        'See oustanding issue(s)\n'
+        # TODO, put issue link!
+    )
+)
 
 @tractor.context
 async def child_attach_shml_alot(

tractor/_child.py

@@ -15,16 +15,23 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 """
-This is the "bootloader" for actors started using the native trio backend.
+The "bootloader" for sub-actors spawned via the native `trio`
+backend (the default `python -m tractor._child` CLI entry) and
+the in-process `subint` backend (`tractor.spawn._subint`).
 
 """
+from __future__ import annotations
 import argparse
-
 from ast import literal_eval
+from typing import TYPE_CHECKING
 
 from .runtime._runtime import Actor
 from .spawn._entry import _trio_main
 
+if TYPE_CHECKING:
+    from .discovery._addr import UnwrappedAddress
+    from .spawn._spawn import SpawnMethodKey
+
 
 def parse_uid(arg):
     name, uuid = literal_eval(arg)  # ensure 2 elements
@@ -39,6 +46,36 @@ def parse_ipaddr(arg):
         return arg
 
 
+def _actor_child_main(
+    uid: tuple[str, str],
+    loglevel: str | None,
+    parent_addr: UnwrappedAddress | None,
+    infect_asyncio: bool,
+    spawn_method: SpawnMethodKey = 'trio',
+
+) -> None:
+    '''
+    Construct the child `Actor` and dispatch to `_trio_main()`.
+
+    Shared entry shape used by both the `python -m tractor._child`
+    CLI (trio/mp subproc backends) and the `subint` backend, which
+    invokes this from inside a fresh `concurrent.interpreters`
+    sub-interpreter via `Interpreter.call()`.
+
+    '''
+    subactor = Actor(
+        name=uid[0],
+        uuid=uid[1],
+        loglevel=loglevel,
+        spawn_method=spawn_method,
+    )
+    _trio_main(
+        subactor,
+        parent_addr=parent_addr,
+        infect_asyncio=infect_asyncio,
+    )
+
+
 if __name__ == "__main__":
     __tracebackhide__: bool = True
 
@@ -49,15 +86,10 @@ if __name__ == "__main__":
     parser.add_argument("--asyncio", action='store_true')
     args = parser.parse_args()
 
-    subactor = Actor(
-        name=args.uid[0],
-        uuid=args.uid[1],
+    _actor_child_main(
+        uid=args.uid,
         loglevel=args.loglevel,
-        spawn_method="trio"
-    )
-
-    _trio_main(
-        subactor,
         parent_addr=args.parent_addr,
         infect_asyncio=args.asyncio,
+        spawn_method='trio',
     )

tractor/_root.py

@@ -69,6 +69,20 @@ from ._exceptions import (
 logger = log.get_logger('tractor')
 
 
+# Spawn backends under which `debug_mode=True` is supported.
+# Requirement: the spawned subactor's root runtime must be
+# trio-native so `tractor.devx.debug._tty_lock` works. Matches
+# both the enable-site in `open_root_actor` and the cleanup-
+# site reset of `_runtime_vars['_debug_mode']` — keep them in
+# lockstep when adding backends.
+_DEBUG_COMPATIBLE_BACKENDS: tuple[str, ...] = (
+    'trio',
+    # forkserver children run `_trio_main` in their own OS
+    # process — same child-side runtime shape as `trio_proc`.
+    'subint_forkserver',
+)
+
+
 # TODO: stick this in a `@acm` defined in `devx.debug`?
 # -[ ] also maybe consider making this a `wrapt`-deco to
 #     save an indent level?
@@ -293,10 +307,14 @@ async def open_root_actor(
         )
         loglevel: str = loglevel.upper()
 
+        # Debug-mode is currently only supported for backends whose
+        # subactor root runtime is trio-native (so `tractor.devx.
+        # debug._tty_lock` works). See `_DEBUG_COMPATIBLE_BACKENDS`
+        # module-const for the list.
         if (
             debug_mode
             and
-            _spawn._spawn_method == 'trio'
+            _spawn._spawn_method in _DEBUG_COMPATIBLE_BACKENDS
         ):
             _state._runtime_vars['_debug_mode'] = True
 
@@ -318,7 +336,9 @@ async def open_root_actor(
 
         elif debug_mode:
             raise RuntimeError(
-                "Debug mode is only supported for the `trio` backend!"
+                f'Debug mode currently supported only for '
+                f'{_DEBUG_COMPATIBLE_BACKENDS!r} spawn backends, not '
+                f'{_spawn._spawn_method!r}.'
             )
 
         assert loglevel
@@ -619,7 +639,7 @@ async def open_root_actor(
             if (
                 debug_mode
                 and
-                _spawn._spawn_method == 'trio'
+                _spawn._spawn_method in _DEBUG_COMPATIBLE_BACKENDS
             ):
                 _state._runtime_vars['_debug_mode'] = False
 

tractor/_testing/pytest.py

@@ -224,10 +224,18 @@ def pytest_addoption(
     )
 
 
-def pytest_configure(config):
-    backend = config.option.spawn_backend
+def pytest_configure(
+    config: pytest.Config,
+):
+    backend: str = config.option.spawn_backend
     from tractor.spawn._spawn import try_set_start_method
+    try:
         try_set_start_method(backend)
+    except RuntimeError as err:
+        # e.g. `--spawn-backend=subint` on Python < 3.14 — turn the
+        # runtime gate error into a clean pytest usage error so the
+        # suite exits with a helpful banner instead of a traceback.
+        raise pytest.UsageError(str(err)) from err
 
     # register custom marks to avoid warnings see,
     # https://docs.pytest.org/en/stable/how-to/writing_plugins.html#registering-custom-markers
@@ -235,10 +243,52 @@ def pytest_configure(config):
         'markers',
         'no_tpt(proto_key): test will (likely) not behave with tpt backend'
     )
+    config.addinivalue_line(
+        'markers',
+        'skipon_spawn_backend(*start_methods, reason=None): '
+        'skip this test under any of the given `--spawn-backend` '
+        'values; useful for backend-specific known-hang / -borked '
+        'cases (e.g. the `subint` GIL-starvation class documented '
+        'in `ai/conc-anal/subint_sigint_starvation_issue.md`).'
+    )
+
+
+def pytest_collection_modifyitems(
+    config: pytest.Config,
+    items: list[pytest.Function],
+):
+    '''
+    Expand any `@pytest.mark.skipon_spawn_backend('<backend>'[,
+    ...], reason='...')` markers into concrete
+    `pytest.mark.skip(reason=...)` calls for tests whose
+    backend-arg set contains the active `--spawn-backend`.
+
+    Uses `item.iter_markers(name=...)` which walks function +
+    class + module-level marks in the correct scope order (and
+    handles both the single-`MarkDecorator` and `list[Mark]`
+    forms of a module-level `pytestmark`) — so the same marker
+    works at any level a user puts it.
+
+    '''
+    backend: str = config.option.spawn_backend
+    default_reason: str = f'Borked on --spawn-backend={backend!r}'
+    for item in items:
+        for mark in item.iter_markers(name='skipon_spawn_backend'):
+            if backend in mark.args:
+                reason: str = mark.kwargs.get(
+                    'reason',
+                    default_reason,
+                )
+                item.add_marker(pytest.mark.skip(reason=reason))
+                # first matching mark wins; no value in stacking
+                # multiple `skip`s on the same item.
+                break
 
 
 @pytest.fixture(scope='session')
-def debug_mode(request) -> bool:
+def debug_mode(
+    request: pytest.FixtureRequest,
+) -> bool:
     '''
     Flag state for whether `--tpdb` (for `tractor`-py-debugger)
     was passed to the test run.
@@ -252,12 +302,16 @@ def debug_mode(request) -> bool:
 
 
 @pytest.fixture(scope='session')
-def spawn_backend(request) -> str:
+def spawn_backend(
+    request: pytest.FixtureRequest,
+) -> str:
     return request.config.option.spawn_backend
 
 
 @pytest.fixture(scope='session')
-def tpt_protos(request) -> list[str]:
+def tpt_protos(
+    request: pytest.FixtureRequest,
+) -> list[str]:
 
     # allow quoting on CLI
     proto_keys: list[str] = [
@@ -285,7 +339,7 @@ def tpt_protos(request) -> list[str]:
     autouse=True,
 )
 def tpt_proto(
-    request,
+    request: pytest.FixtureRequest,
     tpt_protos: list[str],
 ) -> str:
     proto_key: str = tpt_protos[0]
@@ -337,7 +391,6 @@ def pytest_generate_tests(
     metafunc: pytest.Metafunc,
 ):
     spawn_backend: str = metafunc.config.option.spawn_backend
-
     if not spawn_backend:
         # XXX some weird windows bug with `pytest`?
         spawn_backend = 'trio'

tractor/devx/__init__.py

@@ -41,6 +41,11 @@ from .pformat import (
     pformat_caller_frame as pformat_caller_frame,
     pformat_boxed_tb as pformat_boxed_tb,
 )
+from ._debug_hangs import (
+    dump_on_hang as dump_on_hang,
+    track_resource_deltas as track_resource_deltas,
+    resource_delta_fixture as resource_delta_fixture,
+)
 
 
 # TODO, move this to a new `.devx._pdbp` mod?

tractor/devx/_debug_hangs.py

@@ -0,0 +1,227 @@
+# tractor: structured concurrent "actors".
+# Copyright 2018-eternity Tyler Goodlet.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+'''
+Hang-diagnostic helpers for concurrent / multi-interpreter code.
+
+Collected from the `subint` spawn backend bringup (issue #379)
+where silent test-suite hangs needed careful teardown
+instrumentation to diagnose. This module bottles up the
+techniques that actually worked so future hangs are faster
+to corner.
+
+Two primitives:
+
+1. `dump_on_hang()` — context manager wrapping
+   `faulthandler.dump_traceback_later()` with the critical
+   gotcha baked in: write the dump to a **file**, not
+   `sys.stderr`. Under `pytest` (and any other output
+   capturer) stderr gets swallowed and the dump is easy to
+   miss — burning hours convinced you're looking at the wrong
+   thing.
+
+2. `track_resource_deltas()` — context manager (+ optional
+   autouse-fixture factory) logging per-block deltas of
+   `threading.active_count()` and — if running on py3.13+ —
+   `len(_interpreters.list_all())`. Lets you quickly rule out
+   leak-accumulation theories when a suite hangs more
+   frequently as it progresses (if counts don't grow, it's
+   not a leak; look for a race on shared cleanup instead).
+
+See issue #379 / commit `26fb820` for the worked example.
+
+'''
+from __future__ import annotations
+import faulthandler
+import sys
+import threading
+from contextlib import contextmanager
+from pathlib import Path
+from typing import (
+    Callable,
+    Iterator,
+)
+
+try:
+    import _interpreters  # type: ignore
+except ImportError:
+    _interpreters = None  # type: ignore
+
+
+__all__ = [
+    'dump_on_hang',
+    'track_resource_deltas',
+    'resource_delta_fixture',
+]
+
+
+@contextmanager
+def dump_on_hang(
+    seconds: float = 30.0,
+    *,
+    path: str | Path = '/tmp/tractor_hang.dump',
+    all_threads: bool = True,
+
+) -> Iterator[str]:
+    '''
+    Arm `faulthandler` to dump all-thread tracebacks to
+    `path` after `seconds` if the with-block hasn't exited.
+
+    *Writes to a file, not stderr* — `pytest`'s stderr
+    capture silently eats stderr-destined `faulthandler`
+    output, and the same happens under any framework that
+    redirects file-descriptors. Pointing the dump at a real
+    file sidesteps that.
+
+    Yields the resolved file path so it's easy to read back.
+
+    Example
+    -------
+    ::
+
+        from tractor.devx import dump_on_hang
+
+        def test_hang():
+            with dump_on_hang(
+                seconds=15,
+                path='/tmp/my_test_hang.dump',
+            ) as dump_path:
+                trio.run(main)
+            # if it hangs, inspect dump_path afterward
+
+    '''
+    dump_path = Path(path)
+    f = dump_path.open('w')
+    try:
+        faulthandler.dump_traceback_later(
+            seconds,
+            repeat=False,
+            file=f,
+            exit=False,
+        )
+        try:
+            yield str(dump_path)
+        finally:
+            faulthandler.cancel_dump_traceback_later()
+    finally:
+        f.close()
+
+
+def _snapshot() -> tuple[int, int]:
+    '''
+    Return `(thread_count, subint_count)`.
+
+    Subint count reported as `0` on pythons lacking the
+    private `_interpreters` stdlib module (i.e. py<3.13).
+
+    '''
+    threads: int = threading.active_count()
+    subints: int = (
+        len(_interpreters.list_all())
+        if _interpreters is not None
+        else 0
+    )
+    return threads, subints
+
+
+@contextmanager
+def track_resource_deltas(
+    label: str = '',
+    *,
+    writer: Callable[[str], None] | None = None,
+
+) -> Iterator[tuple[int, int]]:
+    '''
+    Log `(threads, subints)` deltas across the with-block.
+
+    `writer` defaults to `sys.stderr.write` (+ trailing
+    newline); pass a custom callable to route elsewhere
+    (e.g., a log handler or an append-to-file).
+
+    Yields the pre-entry snapshot so callers can assert
+    against the expected counts if they want.
+
+    Example
+    -------
+    ::
+
+        from tractor.devx import track_resource_deltas
+
+        async def test_foo():
+            with track_resource_deltas(label='test_foo'):
+                async with tractor.open_nursery() as an:
+                    ...
+
+        # Output:
+        #   test_foo: threads 2->2, subints 1->1
+
+    '''
+    before = _snapshot()
+    try:
+        yield before
+    finally:
+        after = _snapshot()
+        msg: str = (
+            f'{label}: '
+            f'threads {before[0]}->{after[0]}, '
+            f'subints {before[1]}->{after[1]}'
+        )
+        if writer is None:
+            sys.stderr.write(msg + '\n')
+            sys.stderr.flush()
+        else:
+            writer(msg)
+
+
+def resource_delta_fixture(
+    *,
+    autouse: bool = True,
+    writer: Callable[[str], None] | None = None,
+
+) -> Callable:
+    '''
+    Factory returning a `pytest` fixture that wraps each test
+    in `track_resource_deltas(label=<node.name>)`.
+
+    Usage in a `conftest.py`::
+
+        # tests/conftest.py
+        from tractor.devx import resource_delta_fixture
+
+        track_resources = resource_delta_fixture()
+
+    or opt-in per-test::
+
+        track_resources = resource_delta_fixture(autouse=False)
+
+        def test_foo(track_resources):
+            ...
+
+    Kept as a factory (not a bare fixture) so callers control
+    `autouse` / `writer` without having to subclass or patch.
+
+    '''
+    import pytest  # deferred: only needed when caller opts in
+
+    @pytest.fixture(autouse=autouse)
+    def _track_resources(request):
+        with track_resource_deltas(
+            label=request.node.name,
+            writer=writer,
+        ):
+            yield
+
+    return _track_resources

tractor/ipc/_linux.py

@@ -17,10 +17,20 @@
 Linux specifics, for now we are only exposing EventFD
 
 '''
-import os
 import errno
+import os
+import sys
+
+try:
+    import cffi
+except ImportError as ie:
+    if sys.version_info < (3, 14):
+        ie.add_note(
+            f'The `cffi` pkg has no 3.14 support yet.\n'
+        )
+
+    raise ie
 
-import cffi
 import trio
 
 ffi = cffi.FFI()

tractor/runtime/_runtime.py

@@ -870,7 +870,14 @@ class Actor:
 
             accept_addrs: list[UnwrappedAddress]|None = None
 
-            if self._spawn_method == "trio":
+            if self._spawn_method in (
+                'trio',
+                'subint',
+                # `subint_forkserver` parent-side sends a
+                # `SpawnSpec` over IPC just like the other two
+                # — fork child-side runtime is trio-native.
+                'subint_forkserver',
+            ):
 
                 # Receive post-spawn runtime state from our parent.
                 spawnspec: msgtypes.SpawnSpec = await chan.recv()
@@ -1209,6 +1216,23 @@ class Actor:
                 ipc_server.cancel()
                 await ipc_server.wait_for_shutdown()
 
+            # Break the shield on the parent-channel
+            # `process_messages` loop (started with `shield=True`
+            # in `async_main` above). Required to avoid a
+            # deadlock during teardown of fork-spawned subactors:
+            # without this cancel, the loop parks waiting for
+            # EOF on the parent channel, but the parent is
+            # blocked on `os.waitpid()` for THIS actor's exit
+            # — mutual wait. For exec-spawn backends the EOF
+            # arrives naturally when the parent closes its
+            # handler-task socket during its own teardown, but
+            # in fork backends the shared-process-image makes
+            # that delivery racy / not guaranteed. Explicit
+            # cancel here gives us deterministic unwinding
+            # regardless of backend.
+            if self._parent_chan_cs is not None:
+                self._parent_chan_cs.cancel()
+
             # cancel all rpc tasks permanently
             if self._service_tn:
                 self._service_tn.cancel_scope.cancel()
@@ -1729,7 +1753,16 @@ async def async_main(
                 # start processing parent requests until our channel
                 # server is 100% up and running.
                 if actor._parent_chan:
-                    await root_tn.start(
+                    # Capture the shielded `loop_cs` for the
+                    # parent-channel `process_messages` task so
+                    # `Actor.cancel()` has a handle to break the
+                    # shield during teardown — without this, the
+                    # shielded loop would park on the parent chan
+                    # indefinitely waiting for EOF that only arrives
+                    # after the PARENT tears down, which under
+                    # fork-based backends (e.g. `subint_forkserver`)
+                    # it waits on THIS actor's exit — deadlock.
+                    actor._parent_chan_cs = await root_tn.start(
                         partial(
                             _rpc.process_messages,
                             chan=actor._parent_chan,
@@ -1940,7 +1973,25 @@ async def async_main(
                 f'   {pformat(ipc_server._peers)}'
             )
             log.runtime(teardown_report)
+            # NOTE: bound the peer-clear wait — otherwise if any
+            # peer-channel handler is stuck (e.g. never got its
+            # cancel propagated due to a runtime bug), this wait
+            # blocks forever and deadlocks the whole actor-tree
+            # teardown cascade. 3s is enough for any graceful
+            # cancel-ack round-trip; beyond that we're in bug
+            # territory and need to proceed with local teardown
+            # so the parent's `_ForkedProc.wait()` can unblock.
+            # See `ai/conc-anal/
+            # subint_forkserver_test_cancellation_leak_issue.md`
+            # for the full diagnosis.
+            with trio.move_on_after(3.0) as _peers_cs:
                 await ipc_server.wait_for_no_more_peers()
+            if _peers_cs.cancelled_caught:
+                teardown_report += (
+                    f'-> TIMED OUT waiting for peers to clear '
+                    f'({len(ipc_server._peers)} still connected)\n'
+                )
+                log.warning(teardown_report)
 
         teardown_report += (
             '-]> all peer channels are complete.\n'

tractor/runtime/_state.py

@@ -117,7 +117,14 @@ class RuntimeVars(Struct):
             )
 
 
-_runtime_vars: dict[str, Any] = {
+# The "fresh process" defaults — what `_runtime_vars` looks
+# like in a just-booted Python process that hasn't yet entered
+# `open_root_actor()` nor received a parent `SpawnSpec`. Kept
+# as a module-level constant so `get_runtime_vars(clear_values=
+# True)` can reset the live dict back to this baseline (see
+# `tractor.spawn._subint_forkserver` for the one current caller
+# that needs it).
+_RUNTIME_VARS_DEFAULTS: dict[str, Any] = {
     # root of actor-process tree info
     '_is_root': False,  # bool
     '_root_mailbox': (None, None),  # tuple[str|None, str|None]
@@ -138,10 +145,12 @@ _runtime_vars: dict[str, Any] = {
     # infected-`asyncio`-mode: `trio` running as guest.
     '_is_infected_aio': False,
 }
+_runtime_vars: dict[str, Any] = dict(_RUNTIME_VARS_DEFAULTS)
 
 
 def get_runtime_vars(
     as_dict: bool = True,
+    clear_values: bool = False,
 ) -> dict:
     '''
     Deliver a **copy** of the current `Actor`'s "runtime variables".
@@ -150,11 +159,62 @@ def get_runtime_vars(
     form, but the `RuntimeVars` struct should be utilized as possible
     for future calls.
 
-    '''
-    if as_dict:
-        return dict(_runtime_vars)
+    Pure read — **never mutates** the module-level `_runtime_vars`.
 
-    return RuntimeVars(**_runtime_vars)
+    If `clear_values=True`, return a copy of the fresh-process
+    defaults (`_RUNTIME_VARS_DEFAULTS`) instead of the live
+    dict. Useful in combination with `set_runtime_vars()` to
+    reset process-global state back to "cold" — the main caller
+    today is the `subint_forkserver` spawn backend's post-fork
+    child prelude:
+
+        set_runtime_vars(get_runtime_vars(clear_values=True))
+
+    `os.fork()` inherits the parent's full memory image, so the
+    child sees the parent's populated `_runtime_vars` (e.g.
+    `_is_root=True`) which would trip the `assert not
+    self.enable_modules` gate in `Actor._from_parent()` on the
+    subsequent parent→child `SpawnSpec` handshake if left alone.
+
+    '''
+    src: dict = (
+        _RUNTIME_VARS_DEFAULTS
+        if clear_values
+        else _runtime_vars
+    )
+    snapshot: dict = dict(src)
+    if as_dict:
+        return snapshot
+    return RuntimeVars(**snapshot)
+
+
+def set_runtime_vars(
+    rtvars: dict | RuntimeVars,
+) -> None:
+    '''
+    Atomically replace the module-level `_runtime_vars` contents
+    with those of `rtvars` (via `.clear()` + `.update()` so
+    live references to the same dict object remain valid).
+
+    Accepts either the historical `dict` form or the `RuntimeVars`
+    `msgspec.Struct` form (the latter still mostly unused but
+    the blessed forward shape — see the struct's definition).
+
+    Paired with `get_runtime_vars()` as the explicit
+    write-half of the runtime-vars API — prefer this over
+    direct mutation of `_runtime_vars[...]` from new call sites.
+
+    '''
+    if isinstance(rtvars, RuntimeVars):
+        # `msgspec.Struct` → dict via its declared field set;
+        # avoids pulling in `msgspec.structs.asdict` just for
+        # this one call path.
+        rtvars = {
+            field_name: getattr(rtvars, field_name)
+            for field_name in rtvars.__struct_fields__
+        }
+    _runtime_vars.clear()
+    _runtime_vars.update(rtvars)
 
 
 def last_actor() -> Actor|None:

tractor/spawn/_spawn.py

@@ -22,6 +22,7 @@ over multiple backends.
 from __future__ import annotations
 import multiprocessing as mp
 import platform
+import sys
 from typing import (
     Any,
     Awaitable,
@@ -61,6 +62,23 @@ SpawnMethodKey = Literal[
     'trio',  # supported on all platforms
     'mp_spawn',
     'mp_forkserver',  # posix only
+    'subint',  # py3.14+ via `concurrent.interpreters` (PEP 734)
+    # EXPERIMENTAL — blocked at the CPython level. The
+    # design goal was a `trio+fork`-safe subproc spawn via
+    # `os.fork()` from a trio-free launchpad sub-interpreter,
+    # but CPython's `PyOS_AfterFork_Child` → `_PyInterpreterState_DeleteExceptMain`
+    # requires fork come from the main interp. See
+    # `tractor.spawn._subint_fork` +
+    # `ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md`
+    # + issue #379 for the full analysis.
+    'subint_fork',
+    # EXPERIMENTAL — the `subint_fork` workaround. `os.fork()`
+    # from a non-trio worker thread (never entered a subint)
+    # is CPython-legal and works cleanly; forked child runs
+    # `tractor._child._actor_child_main()` against a trio
+    # runtime, exactly like `trio_proc` but via fork instead
+    # of subproc-exec. See `tractor.spawn._subint_forkserver`.
+    'subint_forkserver',
 ]
 _spawn_method: SpawnMethodKey = 'trio'
 
@@ -113,6 +131,26 @@ def try_set_start_method(
         case 'trio':
             _ctx = None
 
+        case 'subint' | 'subint_fork' | 'subint_forkserver':
+            # All subint-family backends need no `mp.context`;
+            # all three feature-gate on the py3.14 public
+            # `concurrent.interpreters` wrapper (PEP 734). See
+            # `tractor.spawn._subint` for the detailed
+            # reasoning. `subint_fork` is blocked at the
+            # CPython level (raises `NotImplementedError`);
+            # `subint_forkserver` is the working workaround.
+            from ._subint import _has_subints
+            if not _has_subints:
+                raise RuntimeError(
+                    f'Spawn method {key!r} requires Python 3.14+.\n'
+                    f'(On py3.13 the private `_interpreters` C '
+                    f'module exists but tractor\'s spawn flow '
+                    f'wedges — see `tractor.spawn._subint` '
+                    f'docstring for details.)\n'
+                    f'Current runtime: {sys.version}'
+                )
+            _ctx = None
+
         case _:
             raise ValueError(
                 f'Spawn method `{key}` is invalid!\n'
@@ -437,6 +475,9 @@ async def new_proc(
 # `hard_kill`/`proc_waiter` from this module.
 from ._trio import trio_proc
 from ._mp import mp_proc
+from ._subint import subint_proc
+from ._subint_fork import subint_fork_proc
+from ._subint_forkserver import subint_forkserver_proc
 
 
 # proc spawning backend target map
@@ -444,4 +485,15 @@ _methods: dict[SpawnMethodKey, Callable] = {
     'trio': trio_proc,
     'mp_spawn': mp_proc,
     'mp_forkserver': mp_proc,
+    'subint': subint_proc,
+    # blocked at CPython level — see `_subint_fork.py` +
+    # `ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md`.
+    # Kept here so `--spawn-backend=subint_fork` routes to a
+    # clean `NotImplementedError` with pointer to the analysis,
+    # rather than an "invalid backend" error.
+    'subint_fork': subint_fork_proc,
+    # WIP — fork-from-non-trio-worker-thread, works on py3.14+
+    # (validated via `ai/conc-anal/subint_fork_from_main_thread_smoketest.py`).
+    # See `tractor.spawn._subint_forkserver`.
+    'subint_forkserver': subint_forkserver_proc,
 }

tractor/spawn/_subint.py

@@ -0,0 +1,432 @@
+# tractor: structured concurrent "actors".
+# Copyright 2018-eternity Tyler Goodlet.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+'''
+Sub-interpreter (`subint`) actor spawning backend.
+
+Spawns each sub-actor as a CPython PEP 734 sub-interpreter
+(`concurrent.interpreters.Interpreter`) driven on its own OS
+thread — same-process state isolation with faster start-up
+than an OS subproc, while preserving tractor's existing
+IPC-based actor boundary.
+
+Availability
+------------
+Requires Python **3.14+**. The private `_interpreters` C
+module we actually call into has shipped since 3.13, but
+that vintage has a latent bug in its thread/subint
+interaction which wedges tractor's spawn flow after
+`_interpreters.create()` — the driver `threading.Thread`
+silently never makes progress inside `_interpreters.exec()`.
+(Minimal standalone reproductions with threading +
+`_interpreters.exec()` work fine on 3.13; only our
+multi-trio-task usage triggers the hang. 3.14 fixes it.)
+On older runtimes the module still imports (so the registry
+stays introspectable) but `subint_proc()` raises.
+
+'''
+from __future__ import annotations
+import sys
+import threading
+from typing import (
+    Any,
+    TYPE_CHECKING,
+)
+
+import trio
+from trio import TaskStatus
+
+
+# NOTE: we reach into the *private* `_interpreters` C module
+# for the actual subint create/exec/destroy calls rather than
+# `concurrent.interpreters`' public API because the public API
+# only exposes PEP 734's `'isolated'` config (per-interp GIL).
+# Under `'isolated'`, any C extension missing the
+# `Py_mod_multiple_interpreters` slot (PEP 684) refuses to
+# import; in our stack that's `msgspec` — which tractor uses
+# pervasively in the IPC layer — so isolated-mode subints
+# can't finish booting the sub-actor's `trio.run()`. msgspec
+# PEP 684 support is open upstream at jcrist/msgspec#563.
+#
+# Dropping to the `'legacy'` config keeps the main GIL + lets
+# existing C extensions load normally while preserving the
+# state isolation we actually care about for the actor model
+# (separate `sys.modules` / `__main__` / globals).
+#
+# But — we feature-gate on the **public** `concurrent.interpreters`
+# module (3.14+) even though we only call into the private
+# `_interpreters` module. Reason: the private module has
+# shipped since 3.13, but the thread/subint interactions
+# tractor relies on (`threading.Thread` driving
+# `_interpreters.exec(..., legacy)` while a trio loop runs in
+# the parent + another inside the subint + IPC between them)
+# hang silently on 3.13 and only work cleanly on 3.14. See
+# docstring above for the empirical details. Using the public
+# module's existence as the gate keeps this check honest.
+#
+# Migration path: when msgspec (jcrist/msgspec#563) and any
+# other PEP 684-holdout C deps opt-in, we can switch to the
+# public `concurrent.interpreters.create()` API (isolated
+# mode) and pick up per-interp-GIL parallelism for free.
+#
+# References:
+# - PEP 734 (`concurrent.interpreters` public API):
+#   https://peps.python.org/pep-0734/
+# - PEP 684 (per-interpreter GIL / `Py_mod_multiple_interpreters`):
+#   https://peps.python.org/pep-0684/
+# - stdlib docs (3.14+):
+#   https://docs.python.org/3.14/library/concurrent.interpreters.html
+# - CPython public wrapper source (`Lib/concurrent/interpreters/`):
+#   https://github.com/python/cpython/tree/main/Lib/concurrent/interpreters
+# - CPython private C ext source
+#   (`Modules/_interpretersmodule.c`):
+#   https://github.com/python/cpython/blob/main/Modules/_interpretersmodule.c
+# - msgspec PEP 684 upstream tracker:
+#   https://github.com/jcrist/msgspec/issues/563
+try:
+    # gate: presence of the public 3.14 stdlib wrapper (we
+    # don't actually use it below, see NOTE above).
+    from concurrent import interpreters as _public_interpreters  # noqa: F401  # type: ignore
+    # actual driver: the private C module (also present on
+    # 3.13 but we refuse that version — see gate above).
+    import _interpreters  # type: ignore
+    _has_subints: bool = True
+except ImportError:
+    _interpreters = None  # type: ignore
+    _has_subints: bool = False
+
+
+from tractor.log import get_logger
+from tractor.msg import (
+    types as msgtypes,
+    pretty_struct,
+)
+from tractor.runtime._state import current_actor
+from tractor.runtime._portal import Portal
+from ._spawn import cancel_on_completion
+
+
+if TYPE_CHECKING:
+    from tractor.discovery._addr import UnwrappedAddress
+    from tractor.ipc import (
+        _server,
+    )
+    from tractor.runtime._runtime import Actor
+    from tractor.runtime._supervise import ActorNursery
+
+
+log = get_logger('tractor')
+
+
+# How long we'll wait (in seconds) inside the shielded soft-kill
+# / teardown blocks before abandoning the sub-interpreter to its
+# fate. See the "hard-kill" comments at the two shield sites.
+#
+# Unbounded shields are a Bad Idea with subints: because CPython
+# doesn't deliver SIGINT into sub-interpreters and the legacy
+# config shares the main GIL, a stuck subint can otherwise lock
+# the parent trio loop (and the user's Ctrl-C) indefinitely.
+_HARD_KILL_TIMEOUT: float = 3.0
+
+
+async def subint_proc(
+    name: str,
+    actor_nursery: ActorNursery,
+    subactor: Actor,
+    errors: dict[tuple[str, str], Exception],
+
+    # passed through to actor main
+    bind_addrs: list[UnwrappedAddress],
+    parent_addr: UnwrappedAddress,
+    _runtime_vars: dict[str, Any],  # serialized and sent to _child
+    *,
+    infect_asyncio: bool = False,
+    task_status: TaskStatus[Portal] = trio.TASK_STATUS_IGNORED,
+    proc_kwargs: dict[str, any] = {}
+
+) -> None:
+    '''
+    Create a new sub-actor hosted inside a PEP 734
+    sub-interpreter running on a dedicated OS thread,
+    reusing tractor's existing UDS/TCP IPC handshake
+    for parent<->child channel setup.
+
+    Supervision model mirrors `trio_proc()`:
+    - parent awaits `ipc_server.wait_for_peer()` for the
+      child to connect back; on success yields a `Portal`
+      via `task_status.started()`
+    - on graceful shutdown we await the sub-interpreter's
+      `trio.run()` completing naturally (driven by the
+      child's actor runtime)
+    - on cancellation we send `Portal.cancel_actor()` and
+      then wait for the subint's trio loop to exit cleanly
+      — unblocking the worker thread so the `Interpreter`
+      can be closed
+
+    '''
+    if not _has_subints:
+        raise RuntimeError(
+            f'The {"subint"!r} spawn backend requires Python 3.14+.\n'
+            f'(On py3.13 the private `_interpreters` C module '
+            f'exists but tractor\'s spawn flow wedges — see '
+            f'`tractor.spawn._subint` docstring for details.)\n'
+            f'Current runtime: {sys.version}'
+        )
+
+    interp_id: int = _interpreters.create('legacy')
+    log.runtime(
+        f'Created sub-interpreter (legacy cfg) for sub-actor\n'
+        f'(>\n'
+        f' |_interp_id={interp_id}\n'
+    )
+
+    uid: tuple[str, str] = subactor.aid.uid
+    loglevel: str | None = subactor.loglevel
+
+    # Build a bootstrap code string driven via `_interpreters.exec()`.
+    # All of `uid` (`tuple[str, str]`), `loglevel` (`str|None`),
+    # `parent_addr` (`tuple[str, int|str]` — see `UnwrappedAddress`)
+    # and `infect_asyncio` (`bool`) `repr()` to valid Python
+    # literals, so we can embed them directly.
+    bootstrap: str = (
+        'from tractor._child import _actor_child_main\n'
+        '_actor_child_main(\n'
+        f'    uid={uid!r},\n'
+        f'    loglevel={loglevel!r},\n'
+        f'    parent_addr={parent_addr!r},\n'
+        f'    infect_asyncio={infect_asyncio!r},\n'
+        f'    spawn_method={"subint"!r},\n'
+        ')\n'
+    )
+
+    cancelled_during_spawn: bool = False
+    subint_exited = trio.Event()
+    ipc_server: _server.Server = actor_nursery._actor.ipc_server
+
+    # Capture a trio token so the driver thread can signal
+    # `subint_exited.set()` back into the parent trio loop.
+    trio_token = trio.lowlevel.current_trio_token()
+
+    def _subint_target() -> None:
+        '''
+        Dedicated OS-thread target: runs `_interpreters.exec()`
+        once and exits.
+
+        We intentionally use a plain `threading.Thread` here
+        rather than `trio.to_thread.run_sync()` because trio's
+        thread cache would *recycle* the same OS thread for
+        subsequent jobs — leaving CPython's subinterpreter
+        tstate attached to that cached worker and blocking
+        `_interpreters.destroy()` in the teardown block below.
+        A dedicated thread truly exits after `exec()` returns,
+        releasing the tstate so destroy can proceed.
+
+        '''
+        try:
+            _interpreters.exec(interp_id, bootstrap)
+        finally:
+            try:
+                trio.from_thread.run_sync(
+                    subint_exited.set,
+                    trio_token=trio_token,
+                )
+            except trio.RunFinishedError:
+                # parent trio loop has already exited (proc
+                # teardown); nothing to signal.
+                pass
+
+    # NOTE: `daemon=True` so a stuck subint can never block
+    # process exit — if our `_HARD_KILL_TIMEOUT` paths below
+    # have to abandon this thread, Python's interpreter
+    # shutdown won't wait for it forever. Tradeoff: any
+    # subint state still live at abandon-time may leak.
+    driver_thread = threading.Thread(
+        target=_subint_target,
+        name=f'subint-driver[{interp_id}]',
+        daemon=True,
+    )
+
+    try:
+        try:
+            driver_thread.start()
+
+            try:
+                event, chan = await ipc_server.wait_for_peer(uid)
+            except trio.Cancelled:
+                cancelled_during_spawn = True
+                raise
+
+            portal = Portal(chan)
+            actor_nursery._children[uid] = (
+                subactor,
+                interp_id,  # proxy for the normal `proc` slot
+                portal,
+            )
+
+            sspec = msgtypes.SpawnSpec(
+                _parent_main_data=subactor._parent_main_data,
+                enable_modules=subactor.enable_modules,
+                reg_addrs=subactor.reg_addrs,
+                bind_addrs=bind_addrs,
+                _runtime_vars=_runtime_vars,
+            )
+            log.runtime(
+                f'Sending spawn spec to subint child\n'
+                f'{{}}=> {chan.aid.reprol()!r}\n'
+                f'\n'
+                f'{pretty_struct.pformat(sspec)}\n'
+            )
+            await chan.send(sspec)
+
+            curr_actor: Actor = current_actor()
+            curr_actor._actoruid2nursery[uid] = actor_nursery
+
+            task_status.started(portal)
+
+            with trio.CancelScope(shield=True):
+                await actor_nursery._join_procs.wait()
+
+            async with trio.open_nursery() as lifecycle_n:
+                if portal in actor_nursery._cancel_after_result_on_exit:
+                    lifecycle_n.start_soon(
+                        cancel_on_completion,
+                        portal,
+                        subactor,
+                        errors,
+                    )
+
+                # Soft-kill analog: wait for the subint to exit
+                # naturally; on cancel, send a graceful cancel
+                # via the IPC portal and then wait for the
+                # driver thread to finish so `_interpreters.destroy()`
+                # won't race with a running interpreter.
+                try:
+                    await subint_exited.wait()
+                except trio.Cancelled:
+                    # Bounded shield: we want to ATTEMPT a
+                    # graceful cancel via the portal, but we
+                    # MUST NOT let the shield trap user
+                    # Ctrl-C / parent teardown forever if the
+                    # subint is already unreachable (e.g., the
+                    # IPC channel was broken — which is exactly
+                    # what `test_ipc_channel_break_during_stream`
+                    # exercises). After `_HARD_KILL_TIMEOUT` we
+                    # drop the shield and let `Cancelled`
+                    # propagate; the outer teardown will force
+                    # things along.
+                    with (
+                        trio.CancelScope(shield=True),
+                        trio.move_on_after(
+                            _HARD_KILL_TIMEOUT,
+                        ) as cs,
+                    ):
+                        log.cancel(
+                            f'Soft-killing subint sub-actor\n'
+                            f'c)=> {chan.aid.reprol()}\n'
+                            f'   |_interp_id={interp_id}\n'
+                        )
+                        try:
+                            await portal.cancel_actor()
+                        except (
+                            trio.BrokenResourceError,
+                            trio.ClosedResourceError,
+                        ):
+                            # channel already down — subint will
+                            # exit on its own timeline (or won't,
+                            # in which case the timeout below
+                            # is our escape).
+                            pass
+                        await subint_exited.wait()
+                    if cs.cancelled_caught:
+                        log.warning(
+                            f'Soft-kill of subint sub-actor timed '
+                            f'out after {_HARD_KILL_TIMEOUT}s — '
+                            f'subint may still be running; '
+                            f'escalating to thread-abandon.\n'
+                            f'   |_interp_id={interp_id}\n'
+                            f'   |_aid={chan.aid.reprol()}\n'
+                        )
+                    raise
+                finally:
+                    lifecycle_n.cancel_scope.cancel()
+
+        finally:
+            # Ensure the driver thread is *fully* joined before
+            # destroying the subint. `subint_exited.set()` fires
+            # from inside the thread but returns to trio before
+            # the thread's bootstrap cleanup finishes; calling
+            # `destroy()` too eagerly can race with tstate
+            # teardown. Off-load the blocking `.join()` to a
+            # cache thread (which carries no subint tstate of
+            # its own, so no cache conflict).
+            #
+            # Bounded shield: if the driver thread never exits
+            # (soft-kill failed above, subint stuck in
+            # non-checkpointing Python, etc.) we MUST abandon
+            # it rather than wedge the parent forever. The
+            # thread is `daemon=True` so proc-exit won't block
+            # on it either. Subsequent `_interpreters.destroy()`
+            # on a still-running subint raises `InterpreterError`
+            # which we log and swallow — the abandoned subint
+            # will be torn down by process exit.
+            with (
+                trio.CancelScope(shield=True),
+                trio.move_on_after(_HARD_KILL_TIMEOUT) as cs,
+            ):
+                if driver_thread.is_alive():
+                    # XXX `abandon_on_cancel=True` is load-bearing:
+                    # the default (False) makes `to_thread.run_sync`
+                    # ignore the enclosing `move_on_after` and
+                    # block until `driver_thread.join()` returns —
+                    # which is exactly what we can't wait for here.
+                    await trio.to_thread.run_sync(
+                        driver_thread.join,
+                        abandon_on_cancel=True,
+                    )
+            if cs.cancelled_caught:
+                log.warning(
+                    f'Subint driver thread did not exit within '
+                    f'{_HARD_KILL_TIMEOUT}s — abandoning.\n'
+                    f'   |_interp_id={interp_id}\n'
+                    f'   |_thread={driver_thread.name}\n'
+                    f'(This usually means portal-cancel could '
+                    f'not be delivered — e.g., IPC channel was '
+                    f'already broken. The subint will continue '
+                    f'running until process exit terminates the '
+                    f'daemon thread.)'
+                )
+
+            with trio.CancelScope(shield=True):
+                try:
+                    _interpreters.destroy(interp_id)
+                    log.runtime(
+                        f'Destroyed sub-interpreter\n'
+                        f')>\n'
+                        f' |_interp_id={interp_id}\n'
+                    )
+                except _interpreters.InterpreterError as e:
+                    log.warning(
+                        f'Could not destroy sub-interpreter '
+                        f'{interp_id}: {e}\n'
+                        f'(expected if the driver thread was '
+                        f'abandoned above; the subint is still '
+                        f'running and will be reaped at process '
+                        f'exit.)'
+                    )
+
+    finally:
+        if not cancelled_during_spawn:
+            actor_nursery._children.pop(uid, None)

tractor/spawn/_subint_fork.py

@@ -0,0 +1,153 @@
+# tractor: structured concurrent "actors".
+# Copyright 2018-eternity Tyler Goodlet.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+'''
+`subint_fork` spawn backend — BLOCKED at CPython level.
+
+The idea was to use a sub-interpreter purely as a launchpad
+from which to call `os.fork()`, sidestepping the well-known
+trio+fork issues (python-trio/trio#1614 etc.) by guaranteeing
+the forking interp had never imported `trio`.
+
+**IT DOES NOT WORK ON CURRENT CPYTHON.** The fork syscall
+itself succeeds (in the parent), but the forked CHILD
+process aborts immediately during CPython's post-fork
+cleanup — `PyOS_AfterFork_Child()` calls
+`_PyInterpreterState_DeleteExceptMain()` which refuses to
+operate when the current tstate belongs to a non-main
+sub-interpreter.
+
+Full annotated walkthrough from the user-visible error
+(`Fatal Python error: _PyInterpreterState_DeleteExceptMain:
+not main interpreter`) down to the specific CPython source
+lines that enforce this is in
+`ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md`.
+
+We keep this submodule as a dedicated documentation of the
+attempt. If CPython ever lifts the restriction (e.g., via a
+force-destroy primitive or a hook that swaps tstate to main
+pre-fork), the structural sketch preserved in this file's
+git history is a concrete starting point for a working impl.
+
+See also: issue #379's "Our own thoughts, ideas for
+`fork()`-workaround/hacks..." section.
+
+'''
+from __future__ import annotations
+import sys
+from typing import (
+    Any,
+    TYPE_CHECKING,
+)
+
+import trio
+from trio import TaskStatus
+
+from tractor.runtime._portal import Portal
+from ._subint import _has_subints
+
+
+if TYPE_CHECKING:
+    from tractor.discovery._addr import UnwrappedAddress
+    from tractor.runtime._runtime import Actor
+    from tractor.runtime._supervise import ActorNursery
+
+
+async def subint_fork_proc(
+    name: str,
+    actor_nursery: ActorNursery,
+    subactor: Actor,
+    errors: dict[tuple[str, str], Exception],
+
+    bind_addrs: list[UnwrappedAddress],
+    parent_addr: UnwrappedAddress,
+    _runtime_vars: dict[str, Any],
+    *,
+    infect_asyncio: bool = False,
+    task_status: TaskStatus[Portal] = trio.TASK_STATUS_IGNORED,
+    proc_kwargs: dict[str, any] = {},
+
+) -> None:
+    '''
+    EXPERIMENTAL — currently blocked by a CPython invariant.
+
+    Attempted design
+    ----------------
+    1. Parent creates a fresh legacy-config subint.
+    2. A worker OS-thread drives the subint through a
+       bootstrap that calls `os.fork()`.
+    3. In the forked CHILD, `os.execv()` back into
+       `python -m tractor._child` (fresh process).
+    4. In the fork-PARENT, the launchpad subint is destroyed;
+       parent-side trio task proceeds identically to
+       `trio_proc()` (wait for child connect-back, send
+       `SpawnSpec`, yield `Portal`, etc.).
+
+    Why it doesn't work
+    -------------------
+    CPython's `PyOS_AfterFork_Child()` (in
+    `Modules/posixmodule.c`) calls
+    `_PyInterpreterState_DeleteExceptMain()` (in
+    `Python/pystate.c`) as part of post-fork cleanup. That
+    function requires the current `PyThreadState` belong to
+    the **main** interpreter. When `os.fork()` is called
+    from within a sub-interpreter, the child wakes up with
+    its tstate still pointing at the (now-stale) subint, and
+    this check fails with `PyStatus_ERR("not main
+    interpreter")`, triggering a `fatal_error` goto and
+    aborting the child process.
+
+    CPython devs acknowledge the fragility with a
+    `// Ideally we could guarantee tstate is running main.`
+    comment right above the call site.
+
+    See
+    `ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md`
+    for the full annotated walkthrough + upstream-report
+    draft.
+
+    Why we keep this stub
+    ---------------------
+    - Documents the attempt in-tree so the next person who
+      has this idea finds the reason it doesn't work rather
+      than rediscovering the same CPython-level dead end.
+    - If CPython ever lifts the restriction (e.g., via a
+      force-destroy primitive or a hook that swaps tstate
+      to main pre-fork), this submodule's git history holds
+      the structural sketch of what a working impl would
+      look like.
+
+    '''
+    if not _has_subints:
+        raise RuntimeError(
+            f'The {"subint_fork"!r} spawn backend requires '
+            f'Python 3.14+.\n'
+            f'Current runtime: {sys.version}'
+        )
+
+    raise NotImplementedError(
+        'The `subint_fork` spawn backend is blocked at the '
+        'CPython level — `os.fork()` from a non-main '
+        'sub-interpreter is refused by '
+        '`PyOS_AfterFork_Child()` → '
+        '`_PyInterpreterState_DeleteExceptMain()`, which '
+        'aborts the child with '
+        '`Fatal Python error: not main interpreter`.\n'
+        '\n'
+        'See '
+        '`ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md` '
+        'for the full analysis + upstream-report draft.'
+    )

tractor/spawn/_subint_forkserver.py

@@ -0,0 +1,891 @@
+# tractor: structured concurrent "actors".
+# Copyright 2018-eternity Tyler Goodlet.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Affero General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Affero General Public License for more details.
+
+# You should have received a copy of the GNU Affero General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+'''
+Forkserver-style `os.fork()` primitives for the `subint`-hosted
+actor model.
+
+Background
+----------
+CPython refuses `os.fork()` from a non-main sub-interpreter:
+`PyOS_AfterFork_Child()` →
+`_PyInterpreterState_DeleteExceptMain()` gates on the calling
+thread's tstate belonging to the main interpreter and aborts
+the forked child otherwise. The full walkthrough (with source
+refs) lives in
+`ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md`.
+
+However `os.fork()` from a regular `threading.Thread` attached
+to the *main* interpreter — i.e. a worker thread that has
+never entered a subint — works cleanly. Empirically validated
+across four scenarios by
+`ai/conc-anal/subint_fork_from_main_thread_smoketest.py` on
+py3.14.
+
+This submodule lifts the validated primitives out of the
+smoke-test and into tractor proper, so they can eventually be
+wired into a real "subint forkserver" spawn backend — where:
+
+- A dedicated main-interp worker thread owns all `os.fork()`
+  calls (never enters a subint).
+- The tractor parent-actor's `trio.run()` lives in a
+  sub-interpreter on a different worker thread.
+- When a spawn is requested, the trio-task signals the
+  forkserver thread; the forkserver forks; child re-enters
+  the same pattern (trio in a subint + forkserver on main).
+
+This mirrors the stdlib `multiprocessing.forkserver` design
+but keeps the forkserver in-process for faster spawn latency
+and inherited parent state.
+
+Status
+------
+**EXPERIMENTAL** — wired as the `'subint_forkserver'` entry
+in `tractor.spawn._spawn._methods` and selectable via
+`try_set_start_method('subint_forkserver')` / `--spawn-backend
+=subint_forkserver`. Parent-side spawn, child-side runtime
+bring-up and normal portal-RPC teardown are validated by the
+backend-tier test in
+`tests/spawn/test_subint_forkserver.py::test_subint_forkserver_spawn_basic`.
+
+Still-open work (tracked on tractor #379):
+
+- no cancellation / hard-kill stress coverage yet (counterpart
+  to `tests/test_subint_cancellation.py` for the plain
+  `subint` backend),
+- `child_sigint='trio'` mode (flag scaffolded below; default
+  is `'ipc'`). Originally intended as a manual SIGINT →
+  trio-cancel bridge, but investigation showed trio's handler
+  IS already correctly installed in the fork-child subactor
+  — the orphan-SIGINT hang is actually a separate bug where
+  trio's event loop stays wedged in `epoll_wait` despite
+  delivery. See
+  `ai/conc-anal/subint_forkserver_orphan_sigint_hang_issue.md`
+  for the full trace + fix directions. Once that root cause
+  is fixed, this flag may end up a no-op / doc-only mode.
+- child-side "subint-hosted root runtime" mode (the second
+  half of the envisioned arch — currently the forked child
+  runs plain `_trio_main` via `spawn_method='trio'`; the
+  subint-hosted variant is still the future step gated on
+  msgspec PEP 684 support),
+- thread-hygiene audit of the two `threading.Thread`
+  primitives below, gated on the same msgspec unblock
+  (see TODO section further down).
+
+TODO — cleanup gated on msgspec PEP 684 support
+-----------------------------------------------
+Both primitives below allocate a dedicated
+`threading.Thread` rather than using
+`trio.to_thread.run_sync()`. That's a cautious design
+rooted in three distinct-but-entangled issues (GIL
+starvation from legacy-config subints, tstate-recycling
+destroy race on trio cache threads, fork-from-main-tstate
+invariant). Some of those dissolve under PEP 684
+isolated-mode subints; one requires empirical re-testing
+to know.
+
+Full analysis + audit plan for when we can revisit is in
+`ai/conc-anal/subint_forkserver_thread_constraints_on_pep684_issue.md`.
+Intent: file a follow-up GH issue linked to #379 once
+[jcrist/msgspec#563](https://github.com/jcrist/msgspec/issues/563)
+unblocks isolated-mode subints in tractor.
+
+See also
+--------
+- `tractor.spawn._subint_fork` — the stub for the
+  fork-from-subint strategy that DIDN'T work (kept as
+  in-tree documentation of the attempt + CPython-level
+  block).
+- `ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md`
+  — the CPython source walkthrough.
+- `ai/conc-anal/subint_fork_from_main_thread_smoketest.py`
+  — the standalone feasibility check (now delegates to
+  this module for the primitives it exercises).
+
+'''
+from __future__ import annotations
+import os
+import signal
+import sys
+import threading
+from functools import partial
+from typing import (
+    Any,
+    Callable,
+    Literal,
+    TYPE_CHECKING,
+)
+
+import trio
+from trio import TaskStatus
+
+from tractor.log import get_logger
+from tractor.msg import (
+    types as msgtypes,
+    pretty_struct,
+)
+from tractor.runtime._state import current_actor
+from tractor.runtime._portal import Portal
+from ._spawn import (
+    cancel_on_completion,
+    soft_kill,
+)
+
+if TYPE_CHECKING:
+    from tractor.discovery._addr import UnwrappedAddress
+    from tractor.ipc import (
+        _server,
+    )
+    from tractor.runtime._runtime import Actor
+    from tractor.runtime._supervise import ActorNursery
+
+
+log = get_logger('tractor')
+
+
+# Configurable child-side SIGINT handling for forkserver-spawned
+# subactors. Threaded through `subint_forkserver_proc`'s
+# `proc_kwargs` under the `'child_sigint'` key.
+#
+# - `'ipc'` (default, currently the only implemented mode):
+#   child has NO trio-level SIGINT handler — trio.run() is on
+#   the fork-inherited non-main thread, `signal.set_wakeup_fd()`
+#   is main-thread-only. Cancellation flows exclusively via
+#   the parent's `Portal.cancel_actor()` IPC path. Safe +
+#   deterministic for nursery-structured apps where the parent
+#   is always the cancel authority. Known gap: orphan
+#   (post-parent-SIGKILL) children don't respond to SIGINT
+#   — see `test_orphaned_subactor_sigint_cleanup_DRAFT`.
+#
+# - `'trio'` (**not yet implemented**): install a manual
+#   SIGINT → trio-cancel bridge in the child's fork prelude
+#   (pre-`trio.run()`) so external Ctrl-C reaches stuck
+#   grandchildren even with a dead parent. Adds signal-
+#   handling surface the `'ipc'` default cleanly avoids; only
+#   pay for it when externally-interruptible children actually
+#   matter (e.g. CLI tool grandchildren).
+ChildSigintMode = Literal['ipc', 'trio']
+_DEFAULT_CHILD_SIGINT: ChildSigintMode = 'ipc'
+
+
+# Feature-gate: py3.14+ via the public `concurrent.interpreters`
+# wrapper. Matches the gate in `tractor.spawn._subint` —
+# see that module's docstring for why we require the public
+# API's presence even though we reach into the private
+# `_interpreters` C module for actual calls.
+try:
+    from concurrent import interpreters as _public_interpreters  # noqa: F401  # type: ignore
+    import _interpreters  # type: ignore
+    _has_subints: bool = True
+except ImportError:
+    _interpreters = None  # type: ignore
+    _has_subints: bool = False
+
+
+def _close_inherited_fds(
+    keep: frozenset[int] = frozenset({0, 1, 2}),
+) -> int:
+    '''
+    Close every open file descriptor in the current process
+    EXCEPT those in `keep` (default: stdio only).
+
+    Intended as the first thing a post-`os.fork()` child runs
+    after closing any communication pipes it knows about. This
+    is the fork-child FD hygiene discipline that
+    `subprocess.Popen(close_fds=True)` applies by default for
+    its exec-based children, but which we have to implement
+    ourselves because our `fork_from_worker_thread()` primitive
+    deliberately does NOT exec.
+
+    Why it matters
+    --------------
+    Without this, a forkserver-spawned subactor inherits the
+    parent actor's IPC listener sockets, trio-epoll fd, trio
+    wakeup-pipe, peer-channel sockets, etc. If that subactor
+    then itself forkserver-spawns a grandchild, the grandchild
+    inherits the FDs transitively from *both* its direct
+    parent AND the root actor — IPC message routing becomes
+    ambiguous and the cancel cascade deadlocks. See
+    `ai/conc-anal/subint_forkserver_test_cancellation_leak_issue.md`
+    for the full diagnosis + the empirical repro.
+
+    Fresh children will open their own IPC sockets via
+    `_actor_child_main()`, so they don't need any of the
+    parent's FDs.
+
+    Returns the count of fds that were successfully closed —
+    useful for sanity-check logging at callsites.
+
+    '''
+    # Enumerate open fds via `/proc/self/fd` on Linux (the fast +
+    # precise path); fall back to `RLIMIT_NOFILE` range close on
+    # other platforms. Matches stdlib
+    # `subprocess._posixsubprocess.close_fds` strategy.
+    try:
+        fd_names: list[str] = os.listdir('/proc/self/fd')
+        candidates: list[int] = [
+            int(n) for n in fd_names if n.isdigit()
+        ]
+    except (FileNotFoundError, PermissionError):
+        import resource
+        soft, _ = resource.getrlimit(resource.RLIMIT_NOFILE)
+        candidates = list(range(3, soft))
+
+    closed: int = 0
+    for fd in candidates:
+        if fd in keep:
+            continue
+        try:
+            os.close(fd)
+            closed += 1
+        except OSError:
+            # fd was already closed (race with listdir) or otherwise
+            # unclosable — either is fine.
+            log.exception(
+                f'Failed to close inherited fd in child ??\n'
+                f'{fd!r}\n'
+            )
+
+    return closed
+
+
+def _format_child_exit(
+    status: int,
+) -> str:
+    '''
+    Render `os.waitpid()`-returned status as a short human
+    string (`'rc=0'` / `'signal=SIGABRT'` / etc.) for log
+    output.
+
+    '''
+    if os.WIFEXITED(status):
+        return f'rc={os.WEXITSTATUS(status)}'
+    elif os.WIFSIGNALED(status):
+        sig: int = os.WTERMSIG(status)
+        return f'signal={signal.Signals(sig).name}'
+    else:
+        return f'raw_status={status}'
+
+
+def wait_child(
+    pid: int,
+    *,
+    expect_exit_ok: bool = True,
+) -> tuple[bool, str]:
+    '''
+    `os.waitpid()` + classify the child's exit as
+    expected-or-not.
+
+    `expect_exit_ok=True` → expect clean `rc=0`. `False` →
+    expect abnormal death (any signal or nonzero rc). Used
+    by the control-case smoke-test scenario where CPython
+    is meant to abort the child.
+
+    Returns `(ok, status_str)` — `ok` reflects whether the
+    observed outcome matches `expect_exit_ok`, `status_str`
+    is a short render of the actual status.
+
+    '''
+    _, status = os.waitpid(pid, 0)
+    exited_normally: bool = (
+        os.WIFEXITED(status)
+        and
+        os.WEXITSTATUS(status) == 0
+    )
+    ok: bool = (
+        exited_normally
+        if expect_exit_ok
+        else not exited_normally
+    )
+    return ok, _format_child_exit(status)
+
+
+def fork_from_worker_thread(
+    child_target: Callable[[], int] | None = None,
+    *,
+    thread_name: str = 'subint-forkserver',
+    join_timeout: float = 10.0,
+
+) -> int:
+    '''
+    `os.fork()` from a main-interp worker thread; return the
+    forked child's pid.
+
+    The calling context **must** be the main interpreter
+    (not a subinterpreter) — that's the whole point of this
+    primitive. A regular `threading.Thread(target=...)`
+    spawned from main-interp code satisfies this
+    automatically because Python attaches the thread's
+    tstate to the *calling* interpreter, and our main
+    thread's calling interp is always main.
+
+    If `child_target` is provided, it runs IN the forked
+    child process before `os._exit` is called. The callable
+    should return an int used as the child's exit rc. If
+    `child_target` is None, the child `_exit(0)`s immediately
+    (useful for the baseline sanity case).
+
+    On the PARENT side, this function drives the worker
+    thread to completion (`fork()` returns near-instantly;
+    the thread is expected to exit promptly) and then
+    returns the forked child's pid. Raises `RuntimeError`
+    if the worker thread fails to return within
+    `join_timeout` seconds — that'd be an unexpected CPython
+    pathology.
+
+    '''
+    if not _has_subints:
+        raise RuntimeError(
+            'subint-forkserver primitives require Python '
+            '3.14+ (public `concurrent.interpreters` module '
+            'not present on this runtime).'
+        )
+
+    # Use a pipe to shuttle the forked child's pid from the
+    # worker thread back to the caller.
+    rfd, wfd = os.pipe()
+
+    def _worker() -> None:
+        '''
+        Runs on the forkserver worker thread. Forks; child
+        runs `child_target` (if any) and exits; parent side
+        writes the child pid to the pipe so the main-thread
+        caller can retrieve it.
+
+        '''
+        pid: int = os.fork()
+        if pid == 0:
+            # CHILD: close the pid-pipe ends (we don't use
+            # them here), then scrub ALL other inherited FDs
+            # so the child starts with a clean slate
+            # (stdio-only). Critical for multi-level spawn
+            # trees — see `_close_inherited_fds()` docstring.
+            os.close(rfd)
+            os.close(wfd)
+            _close_inherited_fds()
+            rc: int = 0
+            if child_target is not None:
+                try:
+                    rc = child_target() or 0
+                except BaseException as err:
+                    log.error(
+                        f'subint-forkserver child_target '
+                        f'raised:\n'
+                        f'|_{type(err).__name__}: {err}'
+                    )
+                    rc = 2
+            os._exit(rc)
+        else:
+            # PARENT (still inside the worker thread):
+            # hand the child pid back to main via pipe.
+            os.write(wfd, pid.to_bytes(8, 'little'))
+
+    worker: threading.Thread = threading.Thread(
+        target=_worker,
+        name=thread_name,
+        daemon=False,
+    )
+    worker.start()
+    worker.join(timeout=join_timeout)
+    if worker.is_alive():
+        # Pipe cleanup best-effort before bail.
+        try:
+            os.close(rfd)
+        except OSError:
+            log.exception(
+                f'Failed to close PID-pipe read-fd in parent ??\n'
+                f'{rfd!r}\n'
+            )
+        try:
+            os.close(wfd)
+        except OSError:
+            log.exception(
+                f'Failed to close PID-pipe write-fd in parent ??\n'
+                f'{wfd!r}\n'
+            )
+        raise RuntimeError(
+            f'subint-forkserver worker thread '
+            f'{thread_name!r} did not return within '
+            f'{join_timeout}s — this is unexpected since '
+            f'`os.fork()` should return near-instantly on '
+            f'the parent side.'
+        )
+
+    pid_bytes: bytes = os.read(rfd, 8)
+    os.close(rfd)
+    os.close(wfd)
+    pid: int = int.from_bytes(pid_bytes, 'little')
+    log.runtime(
+        f'subint-forkserver forked child\n'
+        f'(>\n'
+        f' |_pid={pid}\n'
+    )
+    return pid
+
+
+def run_subint_in_worker_thread(
+    bootstrap: str,
+    *,
+    thread_name: str = 'subint-trio',
+    join_timeout: float = 10.0,
+
+) -> None:
+    '''
+    Create a fresh legacy-config sub-interpreter and drive
+    the given `bootstrap` code string through
+    `_interpreters.exec()` on a dedicated worker thread.
+
+    Naming mirrors `fork_from_worker_thread()`:
+    "<action>_in_worker_thread" — the action here is "run a
+    subint", not "run trio" per se. Typical `bootstrap`
+    content does import `trio` + call `trio.run()`, but
+    nothing about this primitive requires trio; it's a
+    generic "host a subint on a worker thread" helper.
+    Intended mainly for use inside a fork-child (see
+    `tractor.spawn._subint_forkserver` module docstring) but
+    works anywhere.
+
+    See `tractor.spawn._subint.subint_proc` for the matching
+    pattern tractor uses at the sub-actor level.
+
+    Destroys the subint after the thread joins.
+
+    '''
+    if not _has_subints:
+        raise RuntimeError(
+            'subint-forkserver primitives require Python '
+            '3.14+.'
+        )
+
+    interp_id: int = _interpreters.create('legacy')
+    log.runtime(
+        f'Created child-side subint for trio.run()\n'
+        f'(>\n'
+        f' |_interp_id={interp_id}\n'
+    )
+
+    err: BaseException | None = None
+
+    def _drive() -> None:
+        nonlocal err
+        try:
+            _interpreters.exec(interp_id, bootstrap)
+        except BaseException as e:
+            err = e
+            log.exception(
+                f'Failed to .exec() in subint ??\n'
+                f'_interpreters.exec(\n'
+                f'    interp_id={interp_id!r},\n'
+                f'    bootstrap={bootstrap!r},\n'
+                f') => {err!r}\n'
+            )
+
+    worker: threading.Thread = threading.Thread(
+        target=_drive,
+        name=thread_name,
+        daemon=False,
+    )
+    worker.start()
+    worker.join(timeout=join_timeout)
+
+    try:
+        _interpreters.destroy(interp_id)
+    except _interpreters.InterpreterError as e:
+        log.warning(
+            f'Could not destroy child-side subint '
+            f'{interp_id}: {e}'
+        )
+
+    if worker.is_alive():
+        raise RuntimeError(
+            f'child-side subint trio-driver thread '
+            f'{thread_name!r} did not return within '
+            f'{join_timeout}s.'
+        )
+    if err is not None:
+        raise err
+
+
+class _ForkedProc:
+    '''
+    Thin `trio.Process`-compatible shim around a raw OS pid
+    returned by `fork_from_worker_thread()`, exposing just
+    enough surface for the `soft_kill()` / hard-reap pattern
+    borrowed from `trio_proc()`.
+
+    Unlike `trio.Process`, we have no direct handles on the
+    child's std-streams (fork-without-exec inherits the
+    parent's FDs, but we don't marshal them into this
+    wrapper) — `.stdin`/`.stdout`/`.stderr` are all `None`,
+    which matches what `soft_kill()` handles via its
+    `is not None` guards.
+
+    '''
+    def __init__(self, pid: int):
+        self.pid: int = pid
+        self._returncode: int | None = None
+        # `soft_kill`/`hard_kill` check these for pipe
+        # teardown — all None since we didn't wire up pipes
+        # on the fork-without-exec path.
+        self.stdin = None
+        self.stdout = None
+        self.stderr = None
+        # pidfd (Linux 5.3+, Python 3.9+) — a file descriptor
+        # referencing this child process which becomes readable
+        # once the child exits. Enables a fully trio-cancellable
+        # wait via `trio.lowlevel.wait_readable()` — same
+        # pattern `trio.Process.wait()` uses under the hood, and
+        # the same pattern `multiprocessing.Process.sentinel`
+        # uses for `tractor.spawn._spawn.proc_waiter()`. Without
+        # this, waiting via `trio.to_thread.run_sync(os.waitpid,
+        # ...)` blocks a cache thread on a sync syscall that is
+        # NOT trio-cancellable, which prevents outer cancel
+        # scopes from unwedging a stuck-child cancel cascade.
+        self._pidfd: int = os.pidfd_open(pid)
+
+    def poll(self) -> int | None:
+        '''
+        Non-blocking liveness probe. Returns `None` if the
+        child is still running, else its exit code (negative
+        for signal-death, matching `subprocess.Popen`
+        convention).
+
+        '''
+        if self._returncode is not None:
+            return self._returncode
+        try:
+            waited_pid, status = os.waitpid(self.pid, os.WNOHANG)
+        except ChildProcessError:
+            # already reaped (or never existed) — treat as
+            # clean exit for polling purposes.
+            self._returncode = 0
+            return 0
+        if waited_pid == 0:
+            return None
+        self._returncode = self._parse_status(status)
+        return self._returncode
+
+    @property
+    def returncode(self) -> int | None:
+        return self._returncode
+
+    async def wait(self) -> int:
+        '''
+        Async, fully-trio-cancellable wait for the child's
+        exit. Uses `trio.lowlevel.wait_readable()` on the
+        `pidfd` sentinel — same pattern as `trio.Process.wait`
+        and `tractor.spawn._spawn.proc_waiter` (mp backend).
+
+        Safe to call multiple times; subsequent calls return
+        the cached rc without re-issuing the syscall.
+
+        '''
+        if self._returncode is not None:
+            return self._returncode
+        # Park until the pidfd becomes readable — the OS
+        # signals this exactly once on child exit. Cancellable
+        # via any outer trio cancel scope (this was the key
+        # fix vs. the prior `to_thread.run_sync(os.waitpid,
+        # abandon_on_cancel=False)` which blocked a thread on
+        # a sync syscall and swallowed cancels).
+        await trio.lowlevel.wait_readable(self._pidfd)
+        # pidfd signaled → reap non-blocking to collect the
+        # exit status. `WNOHANG` here is correct: by the time
+        # the pidfd is readable, `waitpid()` won't block.
+        try:
+            _, status = os.waitpid(self.pid, os.WNOHANG)
+        except ChildProcessError:
+            # already reaped by something else
+            status = 0
+        self._returncode = self._parse_status(status)
+        # pidfd is one-shot; close it so we don't leak fds
+        # across many spawns.
+        try:
+            os.close(self._pidfd)
+        except OSError:
+            pass
+        self._pidfd = -1
+        return self._returncode
+
+    def kill(self) -> None:
+        '''
+        OS-level `SIGKILL` to the child. Swallows
+        `ProcessLookupError` (already dead).
+
+        '''
+        try:
+            os.kill(self.pid, signal.SIGKILL)
+        except ProcessLookupError:
+            pass
+
+    def __del__(self) -> None:
+        # belt-and-braces: close the pidfd if `wait()` wasn't
+        # called (e.g. unexpected teardown path).
+        fd: int = getattr(self, '_pidfd', -1)
+        if fd >= 0:
+            try:
+                os.close(fd)
+            except OSError:
+                pass
+
+    def _parse_status(self, status: int) -> int:
+        if os.WIFEXITED(status):
+            return os.WEXITSTATUS(status)
+        elif os.WIFSIGNALED(status):
+            # negative rc by `subprocess.Popen` convention
+            return -os.WTERMSIG(status)
+        return 0
+
+    def __repr__(self) -> str:
+        return (
+            f'<_ForkedProc pid={self.pid} '
+            f'returncode={self._returncode}>'
+        )
+
+
+async def subint_forkserver_proc(
+    name: str,
+    actor_nursery: ActorNursery,
+    subactor: Actor,
+    errors: dict[tuple[str, str], Exception],
+
+    # passed through to actor main
+    bind_addrs: list[UnwrappedAddress],
+    parent_addr: UnwrappedAddress,
+    _runtime_vars: dict[str, Any],
+    *,
+    infect_asyncio: bool = False,
+    task_status: TaskStatus[Portal] = trio.TASK_STATUS_IGNORED,
+    proc_kwargs: dict[str, any] = {},
+
+) -> None:
+    '''
+    Spawn a subactor via `os.fork()` from a non-trio worker
+    thread (see `fork_from_worker_thread()`), with the forked
+    child running `tractor._child._actor_child_main()` and
+    connecting back via tractor's normal IPC handshake.
+
+    Supervision model mirrors `trio_proc()` — we manage a
+    real OS subprocess, so `Portal.cancel_actor()` +
+    `soft_kill()` on graceful teardown and `os.kill(SIGKILL)`
+    on hard-reap both apply directly (no
+    `_interpreters.destroy()` voodoo needed since the child
+    is in its own process).
+
+    The only real difference from `trio_proc` is the spawn
+    mechanism: fork from a known-clean main-interp worker
+    thread instead of `trio.lowlevel.open_process()`.
+
+    '''
+    if not _has_subints:
+        raise RuntimeError(
+            f'The {"subint_forkserver"!r} spawn backend '
+            f'requires Python 3.14+.\n'
+            f'Current runtime: {sys.version}'
+        )
+
+    # Backend-scoped config pulled from `proc_kwargs`. Using
+    # `proc_kwargs` (vs a first-class kwarg on this function)
+    # matches how other backends expose per-spawn tuning
+    # (`trio_proc` threads it to `trio.lowlevel.open_process`,
+    # etc.) and keeps `ActorNursery.start_actor(proc_kwargs=...)`
+    # as the single ergonomic entry point.
+    child_sigint: ChildSigintMode = proc_kwargs.get(
+        'child_sigint',
+        _DEFAULT_CHILD_SIGINT,
+    )
+    if child_sigint not in ('ipc', 'trio'):
+        raise ValueError(
+            f'Invalid `child_sigint={child_sigint!r}` for '
+            f'`subint_forkserver` backend.\n'
+            f'Expected one of: {ChildSigintMode}.'
+        )
+    if child_sigint == 'trio':
+        raise NotImplementedError(
+            "`child_sigint='trio'` mode — trio-native SIGINT "
+            "plumbing in the fork-child — is scaffolded but "
+            "not yet implemented. See the xfail'd "
+            "`test_orphaned_subactor_sigint_cleanup_DRAFT` "
+            "and the TODO in this module's docstring."
+        )
+
+    uid: tuple[str, str] = subactor.aid.uid
+    loglevel: str | None = subactor.loglevel
+
+    # Closure captured into the fork-child's memory image.
+    # In the child this is the first post-fork Python code to
+    # run, on what was the fork-worker thread in the parent.
+    # `child_sigint` is captured here so the impl lands inside
+    # this function once the `'trio'` mode is wired up —
+    # nothing above this comment needs to change.
+    def _child_target() -> int:
+        # Dispatch on the captured SIGINT-mode closure var.
+        # Today only `'ipc'` is reachable (the `'trio'` branch
+        # is fenced off at the backend-entry guard above); the
+        # match is in place so the future `'trio'` impl slots
+        # in as a plain case arm without restructuring.
+        match child_sigint:
+            case 'ipc':
+                pass  # <- current behavior: no child-side
+                      #    SIGINT plumbing; rely on parent
+                      #    `Portal.cancel_actor()` IPC path.
+            case 'trio':
+                # Unreachable today (see entry-guard above);
+                # this stub exists so that lifting the guard
+                # is the only change required to enable
+                # `'trio'` mode once the SIGINT wakeup-fd
+                # bridge is implemented.
+                raise NotImplementedError(
+                    "`child_sigint='trio'` fork-prelude "
+                    "plumbing not yet wired."
+                )
+        # Lazy import so the parent doesn't pay for it on
+        # every spawn — it's module-level in `_child` but
+        # cheap enough to re-resolve here.
+        from tractor._child import _actor_child_main
+        # XXX, `os.fork()` inherits the parent's entire memory
+        # image, including `tractor.runtime._state._runtime_vars`
+        # (which in the parent encodes "this process IS the root
+        # actor"). A fresh `exec`-based child starts cold; we
+        # replicate that here by explicitly resetting runtime
+        # vars to their fresh-process defaults — otherwise
+        # `Actor.__init__` takes the `is_root_process() == True`
+        # branch, pre-populates `self.enable_modules`, and trips
+        # the `assert not self.enable_modules` gate at the top
+        # of `Actor._from_parent()` on the subsequent parent→
+        # child `SpawnSpec` handshake. (`_state._current_actor`
+        # is unconditionally overwritten by `_trio_main` → no
+        # reset needed for it.)
+        from tractor.runtime._state import (
+            get_runtime_vars,
+            set_runtime_vars,
+        )
+        set_runtime_vars(get_runtime_vars(clear_values=True))
+        _actor_child_main(
+            uid=uid,
+            loglevel=loglevel,
+            parent_addr=parent_addr,
+            infect_asyncio=infect_asyncio,
+            # The child's runtime is trio-native (uses
+            # `_trio_main` + receives `SpawnSpec` over IPC),
+            # but label it with the actual parent-side spawn
+            # mechanism so `Actor.pformat()` / log lines
+            # reflect reality. Downstream runtime gates that
+            # key on `_spawn_method` group `subint_forkserver`
+            # alongside `trio`/`subint` where the SpawnSpec
+            # IPC handshake is concerned — see
+            # `runtime._runtime.Actor._from_parent()`.
+            spawn_method='subint_forkserver',
+        )
+        return 0
+
+    cancelled_during_spawn: bool = False
+    proc: _ForkedProc | None = None
+    ipc_server: _server.Server = actor_nursery._actor.ipc_server
+
+    try:
+        try:
+            pid: int = await trio.to_thread.run_sync(
+                partial(
+                    fork_from_worker_thread,
+                    _child_target,
+                    thread_name=(
+                        f'subint-forkserver[{name}]'
+                    ),
+                ),
+                abandon_on_cancel=False,
+            )
+            proc = _ForkedProc(pid)
+            log.runtime(
+                f'Forked subactor via forkserver\n'
+                f'(>\n'
+                f' |_{proc}\n'
+            )
+
+            event, chan = await ipc_server.wait_for_peer(uid)
+
+        except trio.Cancelled:
+            cancelled_during_spawn = True
+            raise
+
+        assert proc is not None
+
+        portal = Portal(chan)
+        actor_nursery._children[uid] = (
+            subactor,
+            proc,
+            portal,
+        )
+
+        sspec = msgtypes.SpawnSpec(
+            _parent_main_data=subactor._parent_main_data,
+            enable_modules=subactor.enable_modules,
+            reg_addrs=subactor.reg_addrs,
+            bind_addrs=bind_addrs,
+            _runtime_vars=_runtime_vars,
+        )
+        log.runtime(
+            f'Sending spawn spec to forkserver child\n'
+            f'{{}}=> {chan.aid.reprol()!r}\n'
+            f'\n'
+            f'{pretty_struct.pformat(sspec)}\n'
+        )
+        await chan.send(sspec)
+
+        curr_actor: Actor = current_actor()
+        curr_actor._actoruid2nursery[uid] = actor_nursery
+
+        task_status.started(portal)
+
+        with trio.CancelScope(shield=True):
+            await actor_nursery._join_procs.wait()
+
+        async with trio.open_nursery() as nursery:
+            if portal in actor_nursery._cancel_after_result_on_exit:
+                nursery.start_soon(
+                    cancel_on_completion,
+                    portal,
+                    subactor,
+                    errors,
+                )
+
+            # reuse `trio_proc`'s soft-kill dance — `proc`
+            # is our `_ForkedProc` shim which implements the
+            # same `.poll()` / `.wait()` / `.kill()` surface
+            # `soft_kill` expects.
+            await soft_kill(
+                proc,
+                _ForkedProc.wait,
+                portal,
+            )
+            nursery.cancel_scope.cancel()
+
+    finally:
+        # Hard reap: SIGKILL + waitpid. Cheap since we have
+        # the real OS pid, unlike `subint_proc` which has to
+        # fuss with `_interpreters.destroy()` races.
+        if proc is not None and proc.poll() is None:
+            log.cancel(
+                f'Hard killing forkserver subactor\n'
+                f'>x)\n'
+                f' |_{proc}\n'
+            )
+            with trio.CancelScope(shield=True):
+                proc.kill()
+                await proc.wait()
+
+        if not cancelled_during_spawn:
+            actor_nursery._children.pop(uid, None)

tractor/spawn/_trio.py

@@ -246,7 +246,11 @@ async def trio_proc(
                             await proc.wait()
 
                 await debug.maybe_wait_for_debugger(
-                    child_in_debug=get_runtime_vars().get(
+                    # NOTE: use the child's `_runtime_vars`
+                    # (the fn-arg dict shipped via `SpawnSpec`)
+                    # — NOT `get_runtime_vars()` which returns
+                    # the *parent's* live runtime state.
+                    child_in_debug=_runtime_vars.get(
                         '_debug_mode', False
                     ),
                     header_msg=(

uv.lock

@@ -1,6 +1,10 @@
 version = 1
 revision = 3
-requires-python = ">=3.12, <3.14"
+requires-python = ">=3.13, <3.15"
+resolution-markers = [
+    "python_full_version >= '3.14'",
+    "python_full_version < '3.14'",
+]
 
 [[package]]
 name = "async-generator"
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     { name = "prompt-toolkit" },
     { name = "psutil" },
     { name = "pyperclip" },
     { name = "pytest" },
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+    { name = "greenback", marker = "python_full_version < '3.14'" },
+]
 testing = [
     { name = "pexpect" },
     { name = "pytest" },
+    { name = "pytest-timeout" },
 ]
 
 [package.metadata]
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     { name = "colorlog", specifier = ">=6.8.2,<7" },
-    { name = "msgspec", specifier = ">=0.21.0" },
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     { name = "multiaddr", specifier = ">=0.2.0" },
     { name = "pdbp", specifier = ">=1.8.2,<2" },
     { name = "platformdirs", specifier = ">=4.4.0" },
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     { name = "pyperclip", specifier = ">=1.9.0" },
     { name = "pytest", specifier = ">=8.3.5" },
+    { name = "pytest-timeout", specifier = ">=2.3" },
     { name = "stackscope", specifier = ">=0.2.2,<0.3" },
     { name = "typing-extensions", specifier = ">=4.14.1" },
-    { name = "xonsh", specifier = ">=0.22.2" },
+    { name = "xonsh", editable = "../xonsh" },
 ]
 devx = [
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     { name = "stackscope", specifier = ">=0.2.2,<0.3" },
     { name = "typing-extensions", specifier = ">=4.14.1" },
 ]
+eventfd = [{ name = "cffi", marker = "python_full_version == '3.13.*'", specifier = ">=1.17.1" }]
 lint = [{ name = "ruff", specifier = ">=0.9.6" }]
 repl = [
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     { name = "psutil", specifier = ">=7.0.0" },
     { name = "pyperclip", specifier = ">=1.9.0" },
-    { name = "xonsh", specifier = ">=0.22.2" },
+    { name = "xonsh", editable = "../xonsh" },
 ]
+subints = [{ name = "msgspec", marker = "python_full_version >= '3.14'", specifier = ">=0.21.0" }]
+sync-pause = [{ name = "greenback", marker = "python_full_version == '3.13.*'", specifier = ">=1.2.1,<2" }]
 testing = [
     { name = "pexpect", specifier = ">=4.9.0,<5" },
     { name = "pytest", specifier = ">=8.3.5" },
+    { name = "pytest-timeout", specifier = ">=2.3" },
 ]
 
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 [[package]]
 name = "zipp"