Fourth diagnostic pass — instrument `_worker`'s
fork-child branch (`pre child_target()` / `child_
target RETURNED rc=N` / `about to os._exit(rc)`)
and `_trio_main` boundaries (`about to trio.run` /
`trio.run RETURNED NORMALLY` / `FINALLY`). Test
config: depth=1/breadth=2 = 1 root + 14 forked =
15 actors total.
Fresh-run results,
- **9 processes complete the full flow**:
`trio.run RETURNED NORMALLY` → `child_target
RETURNED rc=0` → `os._exit(0)`. These are tree
LEAVES (errorers) plus their direct parents
(depth-0 spawners) — they actually exit
- **5 processes stuck INSIDE `trio.run(trio_
main)`**: hit "about to trio.run" but never
see "trio.run RETURNED NORMALLY". These are
root + top-level spawners + one intermediate
The deadlock is in `async_main` itself, NOT the
peer-channel loops. Specifically, the outer
`async with root_tn:` in `async_main` never exits
for the 5 stuck actors, so the cascade wedges:
trio.run never returns
→ _trio_main finally never runs
→ _worker never reaches os._exit(rc)
→ process never dies
→ parent's _ForkedProc.wait() blocks
→ parent's nursery hangs
→ parent's async_main hangs
→ (recurse up)
The precise new question: **what task in the 5
stuck actors' `async_main` never completes?**
Candidates:
1. shielded parent-chan `process_messages` task
in `root_tn` — but we cancel it via
`_parent_chan_cs.cancel()` in `Actor.cancel()`,
which 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 in
this path.
2. `actor_nursery._join_procs.wait()` or similar
inline in the backend `*_proc` flow.
3. `_ForkedProc.wait()` on a grandchild that DID
exit — but pidfd_open watch didn't fire (race
between `pidfd_open` and the child exiting?).
Most specific next probe: add DIAG around
`_ForkedProc.wait()` enter/exit to see whether
pidfd-based wait returns for every grandchild
exit. If a stuck parent's `_ForkedProc.wait()`
never returns despite its child exiting → pidfd
mechanism has a race bug under nested forkserver.
Asymmetry observed in the cascade tree: some d=0
spawners exit cleanly, others stick, even though
they started identically. Not purely depth-
determined — some race condition in nursery
teardown when multiple siblings error
simultaneously.
No code changes — diagnosis-only.
(this commit msg was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Gud Boi
4d0555435b