Lands the capture-pipe workaround from the prior cluster of diagnosis
commits: switch pytest's `--capture` mode from the default `fd`
(redirects fd 1,2 to temp files, which fork children inherit and can
deadlock writing into) to `sys` (only `sys.stdout` / `sys.stderr` — fd
1,2 left alone).
Trade-off documented inline in `pyproject.toml`:
- LOST: per-test attribution of raw-fd output (C-ext writes,
`os.write(2, ...)`, subproc stdout). Still goes to terminal / CI
capture, just not per-test-scoped in the failure report.
- KEPT: `print()` + `logging` capture per-test (tractor's logger uses
`sys.stderr`).
- KEPT: `pytest -s` debugging behavior.
This allows us to re-enable `test_nested_multierrors` without
skip-marking + clears the class of pytest-capture-induced hangs for any
future fork-based backend tests.
Deats,
- `pyproject.toml`: `'--capture=sys'` added to `addopts` w/ ~20 lines of
rationale comment cross-ref'ing the post-mortem doc
- `test_cancellation`: drop `skipon_spawn_backend('subint_forkserver')`
from `test_nested_ multierrors` — no longer needed.
* file-level `pytestmark` covers any residual.
- `tests/spawn/test_subint_forkserver.py`: orphan-SIGINT test's xfail
mark loosened from `strict=True` to `strict=False` + reason rewritten.
* it passes in isolation but is session-env-pollution sensitive
(leftover subactor PIDs competing for ports / inheriting harness
FDs).
* tolerate both outcomes until suite isolation improves.
- `test_shm`: extend the existing
`skipon_spawn_backend('subint', ...)` to also skip
`'subint_forkserver'`.
* Different root cause from the cancel-cascade class:
`multiprocessing.SharedMemory`'s `resource_tracker` + internals
assume fresh- process state, don't survive fork-without-exec cleanly
- `tests/discovery/test_registrar.py`: bump timeout 3→7s on one test
(unrelated to forkserver; just a flaky-under-load bump).
- `tractor.spawn._subint_forkserver`: inline comment-only future-work
marker right before `_actor_child_main()` describing the planned
conditional stdout/stderr-to-`/dev/null` redirect for cases where
`--capture=sys` isn't enough (no code change — the redirect logic
itself is deferred).
EXTRA NOTEs
-----------
The `--capture=sys` approach is the minimum- invasive fix: just a pytest
ini change, no runtime code change, works for all fork-based backends,
trade-offs well-understood (terminal-level capture still happens, just
not pytest's per-test attribution of raw-fd output).
(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Sixth and final diagnostic pass — after all 4
cascade fixes landed (FD hygiene, pidfd wait,
`_parent_chan_cs` wiring, bounded peer-clear), the
actual last gate on
`test_nested_multierrors[subint_forkserver]`
turned out to be **pytest's default
`--capture=fd` stdout/stderr capture**, not
anything in the runtime cascade.
Empirical result: `pytest -s` → test PASSES in
6.20s. Default `--capture=fd` → hangs forever.
Mechanism: pytest replaces the parent's fds 1,2
with pipe write-ends it reads from. Fork children
inherit those pipes (since `_close_inherited_fds`
correctly preserves stdio). The error-propagation
cascade in a multi-level cancel test generates
7+ actors each logging multiple `RemoteActorError`
/ `ExceptionGroup` tracebacks — enough output to
fill Linux's 64KB pipe buffer. Writes block,
subactors can't progress, processes don't exit,
`_ForkedProc.wait` hangs.
Self-critical aside: I earlier tested w/ and w/o
`-s` and both hung, concluding "capture-pipe
ruled out". That was wrong — at that time fixes
1-4 weren't all in place, so the test was
failing at deeper levels long before reaching
the "produce lots of output" phase. Once the
cascade could actually tear down cleanly, enough
output flowed to hit the pipe limit. Order-of-
operations mistake: ruling something out based
on a test that was failing for a different
reason.
Deats,
- `subint_forkserver_test_cancellation_leak_issue
.md`: new section "Update — VERY late: pytest
capture pipe IS the final gate" w/ DIAG timeline
showing `trio.run` fully returns, diagnosis of
pipe-fill mechanism, retrospective on the
earlier wrong ruling-out, and fix direction
(redirect subactor stdout/stderr to `/dev/null`
in fork-child prelude, conditional on
pytest-detection or opt-in flag)
- `tests/test_cancellation.py`: skip-mark reason
rewritten to describe the capture-pipe gate
specifically; cross-refs the new doc section
- `tests/spawn/test_subint_forkserver.py`: the
orphan-SIGINT test regresses back to xfail.
Previously passed after the FD-hygiene fix,
but the new `wait_for_no_more_peers(
move_on_after=3.0)` bound in `async_main`'s
teardown added up to 3s latency, pushing
orphan-subactor exit past the test's 10s poll
window. Real fix: faster orphan-side teardown
OR extend poll window to 15s
No runtime code changes in this commit — just
test-mark adjustments + doc wrap-up.
(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Two coordinated improvements to the `subint_forkserver` backend:
1. Replace `trio.to_thread.run_sync(os.waitpid, ...,
abandon_on_cancel=False)` in `_ForkedProc.wait()`
with `trio.lowlevel.wait_readable(pidfd)`. The
prior version blocked a trio cache thread on a
sync syscall — outer cancel scopes couldn't
unwedge it when something downstream got stuck.
Same pattern `trio.Process.wait()` and
`proc_waiter` (the mp backend) already use.
2. Drop the `@pytest.mark.xfail(strict=True)` from
`test_orphaned_subactor_sigint_cleanup_DRAFT` —
the test now PASSES after 0cd0b633 (fork-child
FD scrub). Same root cause as the nested-cancel
hang: inherited IPC/trio FDs were poisoning the
child's event loop. Closing them lets SIGINT
propagation work as designed.
Deats,
- `_ForkedProc.__init__` opens a pidfd via
`os.pidfd_open(pid)` (Linux 5.3+, Python 3.9+)
- `wait()` parks on `trio.lowlevel.wait_readable()`,
then non-blocking `waitpid(WNOHANG)` to collect
the exit status (correct since the pidfd signal
IS the child-exit notification)
- `ChildProcessError` swallow handles the rare race
where someone else reaps first
- pidfd closed after `wait()` completes (one-shot
semantics) + `__del__` belt-and-braces for
unexpected-teardown paths
- test docstring's `@xfail` block replaced with a
`# NOTE` comment explaining the historical
context + cross-ref to the conc-anal doc; test
remains in place as a regression guard
The two changes are interdependent — the
cancellable `wait()` matters for the same nested-
cancel scenarios the FD scrub fixes, since the
original deadlock had trio cache workers wedged in
`os.waitpid` swallowing the outer cancel.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Empirical follow-up to the xfail'd orphan-SIGINT test:
the hang is **not** "trio can't install a handler on a
non-main thread" (the original hypothesis from the
`child_sigint` scaffold commit). On py3.14:
- `threading.current_thread() is threading.main_thread()`
IS True post-fork — CPython re-designates the
fork-inheriting thread as "main" correctly
- trio's `KIManager` SIGINT handler IS installed in the
subactor (`signal.getsignal(SIGINT)` confirms)
- the kernel DOES deliver SIGINT to the thread
But `faulthandler` dumps show the subactor wedged in
`trio/_core/_io_epoll.py::get_events` — trio's
wakeup-fd mechanism (which turns SIGINT into an epoll-wake)
isn't firing. So the `except KeyboardInterrupt` at
`tractor/spawn/_entry.py::_trio_main:164` — the runtime's
intentional "KBI-as-OS-cancel" path — never fires.
Deats,
- new `ai/conc-anal/subint_forkserver_orphan_sigint_hang_issue.md`
(+385 LOC): full writeup — TL;DR, symptom reproducer,
the "intentional cancel path" the bug defeats,
diagnostic evidence (`faulthandler` output +
`getsignal` probe), ruled-out hypotheses
(non-main-thread issue, wakeup-fd inheritance,
KBI-as-trio-check-exception), and fix directions
- `test_orphaned_subactor_sigint_cleanup_DRAFT` xfail
`reason` + test docstring rewritten to match the
refined understanding — old wording blamed the
non-main-thread path, new wording points at the
`epoll_wait` wedge + cross-refs the new conc-anal doc
- `_subint_forkserver` module docstring's
`child_sigint='trio'` bullet updated: now notes trio's
handler is already correctly installed, so the flag may
end up a no-op / doc-only mode once the real root cause
is fixed
Closing the gap aligns with existing design intent (make
the already-designed "KBI-as-OS-cancel" behavior actually
fire), not a new feature.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Tier-4 test `test_orphaned_subactor_sigint_cleanup_DRAFT`
documents an empirical SIGINT-delivery gap in the
`subint_forkserver` backend: when the parent dies via
`SIGKILL` (no IPC `Portal.cancel_actor()` possible) and
`SIGINT` is sent to the orphan child, the child DOES NOT
unwind — CPython's default `KeyboardInterrupt` is delivered
to `threading.main_thread()`, whose tstate is dead in the
post-fork child bc fork inherited the worker thread, not
main. Trio running on the fork-inherited worker thread
therefore never observes the signal. Marked
`xfail(strict=True)` so the mark flips to XPASS→fail once
the backend grows explicit SIGINT plumbing.
Deats,
- harness runs the failure-mode sequence out-of-process:
1. harness subprocess runs a fresh Python script
that calls `try_set_start_method('subint_forkserver')`
then opens a root actor + one `sleep_forever` subactor
2. parse `PARENT_READY=<pid>` + `CHILD_PID=<pid>` markers
off harness `stdout` to confirm IPC handshake
completed
3. `SIGKILL` the parent, `proc.wait()` to reap the
zombie (otherwise `os.kill(pid, 0)` keeps reporting
it alive)
4. assert the child survived the parent-reap (i.e. was
actually orphaned, not reaped too) before moving on
5. `SIGINT` the orphan child, poll `os.kill(child_pid, 0)`
every 100ms for up to 10s
- supporting helpers: `_read_marker()` with per-proc
bytes-buffer to carry partial lines across calls,
`_process_alive()` liveness probe via `kill(pid, 0)`
- Linux-only via `platform.system() != 'Linux'` skip —
orphan-reparenting semantics don't generalize to
other platforms
- port offset (`reg_addr[1] + 17`) so the harness listener
doesn't race concurrently-running backend tests
- best-effort `finally:` cleanup: `SIGKILL` any still-alive
pids + `proc.kill()` + bounded `proc.wait()` to avoid
leaking orphans across the session
Also, tier-4 header comment documents the cross-backend
generalization path: applicable to any multi-process
backend (`trio`, `mp_spawn`, `mp_forkserver`,
`subint_forkserver`), NOT to plain `subint` (in-process
subints have no orphan OS-child). Move path: lift
harness into `tests/_orphan_harness.py`, parametrize on
session `_spawn_method`, add
`skipif _spawn_method == 'subint'`.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Promote `_subint_forkserver` from primitives-only into a
registered spawn backend: `'subint_forkserver'` is now a
`SpawnMethodKey` literal, dispatched via `_methods` to
the new `subint_forkserver_proc()` target, feature-gated
under the existing `subint`-family py3.14+ case, and
selectable via `--spawn-backend=subint_forkserver`.
Deats,
- new `subint_forkserver_proc()` spawn target in
`_subint_forkserver`:
- mirrors `trio_proc()`'s supervision model — real OS
subprocess so `Portal.cancel_actor()` + `soft_kill()`
on graceful teardown, `os.kill(SIGKILL)` on hard-reap
(no `_interpreters.destroy()` race to fuss over bc the
child lives in its own process)
- only real diff from `trio_proc` is the spawn mechanism:
fork from a main-interp worker thread via
`fork_from_worker_thread()` (off-loaded to trio's
thread pool) instead of `trio.lowlevel.open_process()`
- child-side `_child_target` closure runs
`tractor._child._actor_child_main()` with
`spawn_method='trio'` — the child is a regular trio
actor, "subint_forkserver" names how the parent
spawned, not what the child runs
- new `_ForkedProc` class — thin `trio.Process`-compatible
shim around a raw OS pid: `.poll()` via
`waitpid(WNOHANG)`, async `.wait()` off-loaded to a trio
cache thread, `.kill()` via `SIGKILL`, `.returncode`
cached for repeat calls. `.stdin`/`.stdout`/`.stderr`
are `None` (fork-w/o-exec inherits parent FDs; we don't
marshal them) which matches `soft_kill()`'s `is not None`
guards
Also, new backend-tier test
`test_subint_forkserver_spawn_basic` drives the registered
backend end-to-end via `open_root_actor` + `open_nursery` +
`run_in_actor` w/ a trivial portal-RPC round-trip. Uses a
`forkserver_spawn_method` fixture to flip
`_spawn_method`/`_ctx` for the test's duration + restore on
teardown (so other session-level tests don't observe the
global flip). Test module docstring reworked to describe
the three tiers now covered: (1) primitive-level, (2)
parent-trio-driven primitives, (3) full registered backend.
Status: still-open work (tracked on `tractor#379`) doc'd
inline in the module docstring — no cancel/hard-kill stress
coverage yet, child-side subint-hosted root runtime still
future (gated on `msgspec#563`), thread-hygiene audit
pending the same unblock.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
New pytest module `tests/spawn/test_subint_forkserver.py`
drives the forkserver primitives from inside a real
`trio.run()` in the parent — the runtime shape tractor will
actually use when we wire up a `subint_forkserver` spawn
backend proper. Complements the standalone no-trio-in-parent
`ai/conc-anal/subint_fork_from_main_thread_smoketest.py`.
Deats,
- new test pkg `tests/spawn/` (+ empty `__init__.py`)
- two tests, both `@pytest.mark.timeout(30, method='thread')`
for the GIL-hostage safety reason doc'd in
`ai/conc-anal/subint_sigint_starvation_issue.md`:
- `test_fork_from_worker_thread_via_trio` — parent-side
plumbing baseline. `trio.run()` off-loads forkserver
prims via `trio.to_thread.run_sync()` + asserts the
child reaps cleanly
- `test_fork_and_run_trio_in_child` — end-to-end: forked
child calls `run_subint_in_worker_thread()` with a
bootstrap str that does `trio.run()` in a fresh subint
- both tests wrap the inner `trio.run()` in a
`dump_on_hang()` for post-mortem if the outer
`pytest-timeout` fires
- intentionally NOT using `--spawn-backend` — the tests
drive the primitives directly rather than going through
tractor's spawn-method registry (which the forkserver
isn't plugged into yet)
Also, rename `run_trio_in_subint()` →
`run_subint_in_worker_thread()` for naming consistency with
the sibling `fork_from_worker_thread()`. The action is really
"host a subint on a worker thread", not specifically "run
trio" — trio just happens to be the typical payload.
Propagate the rename to the smoketest.
Further, add a "TODO — cleanup gated on msgspec PEP 684
support" section to the `_subint_forkserver` module
docstring: flags the dedicated-`threading.Thread` design as
potentially-revisable once isolated-mode subints are viable
in tractor. Cross-refs `msgspec#563` + `tractor#379` and
points at an audit-plan conc-anal doc we'll add next.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Gud Boi