---
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")
```