Follow-up tracker companion to the module-docstring TODO
added in `372a0f32`. Catalogs why `_subint_forkserver`'s
two "non-trio thread" constraints
(`fork_from_worker_thread()` +
`run_subint_in_worker_thread()` both allocating dedicated
`threading.Thread`s; test helper named
`run_fork_in_non_trio_thread`) exist today, and which of
them would dissolve once msgspec PEP 684 support ships
(`msgspec#563`) and tractor flips to isolated-mode subints.
Deats,
- three reasons enumerated for the current constraints:
- class-A GIL-starvation — **fixed** by isolated mode:
subints don't share main's GIL so abandoned-thread
contention disappears
- destroy race / tstate-recycling from `subint_proc` —
**unclear**: `_PyXI_Enter` + `_PyXI_Exit` are
cross-mode, so isolated doesn't obviously fix it;
needs empirical retest on py3.14 + isolated API
- fork-from-main-interp-tstate (the CPython-level
`_PyInterpreterState_DeleteExceptMain` gate) — the
narrow reason for using a dedicated thread; **probably
fixed** IF the destroy-race also resolves (bc trio's
cache threads never drove subints → clean main-interp
tstate)
- TL;DR table of which constraints unwind under each
resolution branch
- four-step audit plan for when `msgspec#563` lands:
- flip `_subint` to isolated mode
- empirical destroy-race retest
- audit `_subint_forkserver.py` — drop `non_trio`
qualifier / maybe inline primitives
- doc fallout — close the three `subint_*_issue.md`
siblings w/ post-mortem notes
Also, cross-refs the three sibling `conc-anal/` docs, PEPs
684 + 734, `msgspec#563`, and `tractor#379` (the overall
subint spawn-backend tracking issue).
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
(cherry picked from commit cf2e71d87f)
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
(cherry picked from commit 25e400d526)
The smoketest (prior commit) empirically validated the
"fork-from-main-interp-worker-thread" arch on py3.14. Promote
the validated primitives out of the `ai/conc-anal/` smoketest
into `tractor.spawn._subint_forkserver` so they can eventually
be wired into a real "subint forkserver" spawn backend.
Deats,
- new module `tractor/spawn/_subint_forkserver.py` (337 LOC):
- `fork_from_worker_thread(child_target, thread_name)` —
spawn a main-interp `threading.Thread`, call `os.fork()`
from it, shuttle the child pid back to main via a pipe
- `run_trio_in_subint(bootstrap, ...)` — post-fork helper:
create a fresh subint + drive `_interpreters.exec()` on
a dedicated worker thread running the `bootstrap` str
(typically imports `trio`, defines an async entry, calls
`trio.run()`)
- `wait_child(pid, expect_exit_ok)` — `os.waitpid()` +
pass/fail classification reusable from harness AND the
eventual real spawn path
- feature-gated py3.14+ via the public
`concurrent.interpreters` presence check; matches the gate
in `tractor.spawn._subint`
- module docstring doc's the CPython-block context
(cross-refs `_subint_fork` stub + the two `conc-anal/`
docs) and status: EXPERIMENTAL, not yet registered in
`_spawn._methods`
Also, refactor the smoketest
`ai/conc-anal/subint_fork_from_main_thread_smoketest.py` to
import the primitives from the new module rather than inline
its own copies. Keeps the smoketest and the tractor-side
impl in sync as the forkserver design evolves; the smoketest
remains a zero-`tractor`-runtime CPython-level check
(imports ONLY the three primitives, no runtime bring-up).
Status: next step is to drive these from a parent-side
`trio.run()` and hook the returned child pid into the normal
actor-nursery/IPC flow — then register `subint_forkserver`
as a `SpawnMethodKey` in `_spawn.py`.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
(cherry picked from commit 82332fbceb)
Standalone script to validate the "main-interp worker-thread
forkserver + subint-hosted trio" arch proposed as a workaround
to the CPython-level refusal doc'd in
`ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md`.
Deliberately NOT a `tractor` test — zero `tractor` imports.
Uses `_interpreters` (private stdlib) + `os.fork()` directly so
pass/fail is a property of CPython alone, independent of our
runtime. Requires py3.14+.
Deats,
- four scenarios via `--scenario`:
- `control_subint_thread_fork` — the KNOWN-BROKEN case as a
harness sanity; if the child DOESN'T abort, our analysis
is wrong
- `main_thread_fork` — baseline sanity, must always succeed
- `worker_thread_fork` — architectural assertion: regular
`threading.Thread` attached to main interp calls
`os.fork()`; child should survive post-fork cleanup
- `full_architecture` — end-to-end: fork from a main-interp
worker thread, then in child create a subint driving a
worker thread running `trio.run()`
- exit code 0 on EXPECTED outcome (for `control_*` that means
"child aborted", not "child succeeded")
- each scenario prints a self-contained pass/fail banner; use
`os.waitpid()` of the parent + per-scenario status prints to
observe the child's fate
Also, log NLNet provenance for this session's three-sub-phase
work (py3.13 gate tightening, `pytest-timeout` + marker
refactor, `subint_fork` prototype → CPython-block finding).
Prompt-IO: ai/prompt-io/claude/20260422T200723Z_797f57c_prompt_io.md
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
(cherry picked from commit de4f470b6c)
Empirical finding: the WIP `subint_fork_proc` scaffold
landed in `cf0e3e6f` does *not* work on current CPython.
The `fork()` syscall succeeds in the parent, but the
CHILD aborts immediately during
`PyOS_AfterFork_Child()` →
`_PyInterpreterState_DeleteExceptMain()`, which gates
on the current tstate belonging to the main interp —
the child dies with `Fatal Python error: not main
interpreter`.
CPython devs acknowledge the fragility with an in-source
comment (`// Ideally we could guarantee tstate is running
main.`) but expose no user-facing hook to satisfy the
precondition — so the strategy is structurally dead until
upstream changes.
Rather than delete the scaffold, reshape it into a
documented dead-end so the next person with this idea
lands on the reason rather than rediscovering the same
CPython-level refusal.
Deats,
- Move `subint_fork_proc` out of `tractor.spawn._subint`
into a new `tractor.spawn._subint_fork` dedicated
module (153 LOC). Module + fn docstrings now describe
the blockage directly; the fn body is trimmed to a
`NotImplementedError` pointing at the analysis doc —
no more dead-code `bootstrap` sketch bloating
`_subint.py`.
- `_spawn.py`: keep `'subint_fork'` in `SpawnMethodKey`
+ the `_methods` dispatch so
`--spawn-backend=subint_fork` routes to a clean
`NotImplementedError` rather than "invalid backend";
comment calls out the blockage. Collapse the duplicate
py3.14 feature-gate in `try_set_start_method()` into a
combined `case 'subint' | 'subint_fork':` arm.
- New 337-line analysis:
`ai/conc-anal/subint_fork_blocked_by_cpython_post_fork_issue.md`.
Annotated walkthrough from the user-visible fatal
error down to the specific `Modules/posixmodule.c` +
`Python/pystate.c` source lines enforcing the refusal,
plus an upstream-report draft.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
(cherry picked from commit 0f48ed2eb9)
Add two more tests to the catalog in
`conc-anal/subint_sigint_starvation_issue.md` — same
signal-wakeup-fd-saturation fingerprint (abandoned legacy-subint driver
threads → shared-GIL starvation → `write() = EAGAIN` on the wakeup pipe
→ silent SIGINT drop), different load patterns.
Deats,
- `test_cancel_while_childs_child_in_sync_sleep[subint-False]`: nested
actor-tree + sync-sleeping grandchild. Under `trio`/`mp_*` the "zombie
reaper" is a subproc `SIGKILL`; no equivalent exists under subint, so
the grandchild persists in its abandoned driver thread. Often only
manifests under full-suite runs (earlier tests seed the
abandoned-thread pool).
- `test_multierror_fast_nursery[subint-25-0.5]`: 25 concurrent subactors
all go through teardown on the multierror. Bounded hard-kills run in
parallel — so the total budget is ~3s, not 3s × 25. Leaves 25
abandoned driver threads simultaneously alive, an extreme pressure
multiplier. `strace` shows several successful `write(16, "\2", 1) = 1`
(GIL round-robin IS giving main brief slices) before finally
saturating with `EAGAIN`.
Also include a `pstree -snapt <pid>` capture showing
16+ live `{subint-driver[<interp_id>}` threads at the
moment of hang — the direct GIL-contender population.
(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
(cherry picked from commit f3cea714bc)
Classify and write up the two distinct hang modes hit during Phase
B subint bringup (issue #379) so future triage doesn't re-derive them
from scratch.
Deats, two new `ai/conc-anal/` docs,
- `subint_sigint_starvation_issue.md`: abandoned legacy-subint thread
+ shared GIL → main trio loop starves → signal-wakeup-fd pipe fills
→ `SIGINT` silently dropped (`strace` shows `write() = EAGAIN` on the
wakeup-fd). Un- Ctrl-C-able. Structurally a CPython limit; blocked on
`msgspec` PEP 684 (jcrist/msgspec#563)
- `subint_cancel_delivery_hang_issue.md`: parent-side trio task parks on
an orphaned IPC channel after subint teardown — no clean EOF delivered
to the waiting receive. Ctrl-C-able (main loop iterates fine); OUR bug
to fix. Candidate fix: explicit parent-side channel abort in
`subint_proc`'s hard-kill teardown
Cross-link the docs from their test reproducers,
- `test_stale_entry_is_deleted` (→ starvation class): wrap
`trio.run(main)` in `dump_on_hang(seconds=20)` so a future regression
captures a stack dump. Kept un- skipped so the dump file is
inspectable
- `test_subint_non_checkpointing_child` (→ delivery class): extend
docstring with a "KNOWN ISSUE" block pointing at the analysis
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
(cherry picked from commit 4a3254583b)
(MTF-only portion: kept ai/conc-anal/subint_cancel_delivery_hang_issue.md ai/conc-anal/subint_sigint_starvation_issue.md tests/test_subint_cancellation.py)
trio 0.29 → 0.33 lock bump (c7741bba) slowed the
depth=3 cancel-cascade in `test_nested_multierrors`
from <6s to ~7-8s; the 6s deadline was firing and its
`Cancelled(source='deadline')` (trio 0.33's new
cancel-reason metadata) collapsed a BEG branch,
breaking the `RemoteActorError` assertion downstream.
- Split the `('trio', _)` case-match into per-depth
arms: `('trio', 1)` keeps 6s (still finishes in
~3s); `('trio', 3)` → 12s.
- Updated inline NOTE explains the version pivot +
links the tracking issue
`ai/conc-anal/trio_033_cancel_cascade_slowdown_depth3_issue.md`.
- Existing MTF/`subint_forkserver` budgets unchanged.
(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
(cherry picked from commit ea67f1b67b)
(cherry picked from commit 57b3ea59ea)
New `ai/conc-anal/spawn_time_boot_death_dup_name_issue.md`
documenting the spawn-time rc=2 race under rapid
same-name spawning against a forkserver + registrar
— the `wait_for_peer_or_proc_death` helper now surfaces
the death instead of parking forever on the handshake
wait.
Also,
- extract inline `xfail` into module-level
`_DOGGY_BOOT_RACE_XFAIL` marker.
- apply it to `n_dups=8` too (previously bare) bc
larger N widens the race window enough to fire
occasionally.
- link to tracking issue #456.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
(cherry picked from commit 92443dc4ef)
With a seminal patch fixing `trio`'s `WakeupSocketpair.drain()` which
can busy-loop due to lack of handling `EOF`.
New `tractor.trionics.patches` subpkg housing defensive monkey-patches
for upstream `trio` bugs we've encountered while running `tractor`
— particularly as of recent, fork-survival edge cases that haven't been
filed/fixed upstream yet. Each patch is idempotent, version-gated via
`is_needed()`, and carries a `# REMOVE WHEN:` marker pointing at the
upstream release whose adoption allows deletion.
Subpkg layout + per-patch contract documented in
`tractor/trionics/patches/README.md` — `apply()` / `is_needed()`
/ `repro()` API, registry pattern via `_PATCHES` in `__init__.py`,
single-call entry point `apply_all()`.
First patch, `_wakeup_socketpair`:
- `trio`'s `WakeupSocketpair.drain()` loops on `recv(64KB)` and exits
ONLY on `BlockingIOError`, NEVER on `recv() == b''` (peer-closed FIN).
- under `fork()`-spawning backends the COW-inherited socketpair fds
& `_close_inherited_fds()` teardown can leave a `WakeupSocketpair`
instance whose write-end is closed, and `drain()` then **spins forever
in C with no Python checkpoints**,
- this obviously burns 100% CPU and no signal delivery.
Standalone repro:
from trio._core._wakeup_socketpair import WakeupSocketpair
ws = WakeupSocketpair()
ws.write_sock.close()
ws.drain() # spins forever
Patch is one-line — break the drain loop on b'' EOF.
Manifested as two distinct test failures:
- `tests/test_multi_program.py::test_register_duplicate_name` hung at
100% CPU on the busy-loop directly (fork child's worker thread)
- `tests/test_infected_asyncio.py::test_aio_simple_error` Mode-A
deadlock — busy-loop wedged trio's scheduler inside `start_guest_run`,
both threads parked in `epoll_wait`, no TCP connect-back to parent
ever happened.
Same patch fixes both. Restored 99.7% pass rate on full
suite under `--spawn-backend=main_thread_forkserver`
(was hanging indefinitely before).
Wired into `tractor._child._actor_child_main` via `apply_all()` BEFORE
any trio runtime init. Harmless on non-fork backends.
Conc-anal write-ups, including strace + py-spy evidence:
- `ai/conc-anal/trio_wakeup_socketpair_busy_loop_under_fork_issue.md`
- `ai/conc-anal/infected_asyncio_under_main_thread_forkserver_hang_issue.md`
Regression tests in `tests/trionics/test_patches.py`: each test asserts
(a) the bug exists pre-patch (or is fixed upstream — skip cleanly), (b)
the patch fixes it with a SIGALRM wall-clock cap so a regression hangs
loud instead of silently.
TODO:
- [ ] file the upstream `python-trio/trio` issue + PR.
- [ ] use the `repro()` callable in `_wakeup_socketpair.py` IS the issue
body's evidence section.
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
(cherry picked from commit 0ef549fadb)
(factored: dropped spawn-backend-only paths: ai/conc-anal/infected_asyncio_under_main_thread_forkserver_hang_issue.md)
Document the intermittent connect-refused failure in the registrar
daemon test — root cause is the `daemon` fixture's blind `time.sleep()`
readiness gate racing against the subproc's `bind()`/ `listen()`
completion. Distinct from the cancel- cascade `TooSlowError` flake
class.
(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
(cherry picked from commit 29f9928524)
(cherry picked from commit 15c3b670e1)
Gud Boi