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Fix: exit a forked worker child once its parent is gone #1495

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Merged
ChaoWao merged 1 commit into from
Jul 26, 2026
+135 −418
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Fix: exit a forked worker child once its parent is gone #1495

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27 changes: 27 additions & 0 deletions python/simpler/worker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1014,6 +1014,13 @@ def _read_args_from_mailbox(buf) -> TaskArgs:
return read_args_from_blob(mailbox_addr + _OFF_TASK_ARGS_BLOB)


# Idle mailbox polls between `getppid()` samples in a forked child. One poll
# costs ~0.1 us, so this samples roughly every 100 us — fast enough that an
# orphan is reaped before it is noticeable, cheap enough to be lost in the
# noise of the poll itself.
_PARENT_LIVENESS_POLL_INTERVAL = 1000


def _run_mailbox_loop(
buf,
state_addr: int,
Expand All @@ -1038,7 +1045,14 @@ def _run_mailbox_loop(
`on_shutdown()` runs on SHUTDOWN before the loop exits, for children that
own a nested Worker; per-child resource teardown that must survive an
exception belongs in the caller's own ``finally``.

A parent that dies without sending SHUTDOWN (SIGKILL from a timeout, an OOM
kill, a cancelled CI job) would otherwise leave this loop polling a mailbox
nobody writes to, for the lifetime of the machine. The loop therefore
samples its own parent and leaves by the SHUTDOWN path once it changes.
"""
parent_pid = os.getppid()
liveness_countdown = _PARENT_LIVENESS_POLL_INTERVAL
Comment on lines +1054 to +1055

@coderabbitai coderabbitai Bot Jul 26, 2026

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🩺 Stability & Availability | 🟡 Minor | ⚡ Quick win

Parent PID sampled after fork leaves a small orphan window.

If the parent dies between fork() and this first os.getppid(), the sampled parent_pid is already the reparenting init/subreaper, so it never changes and the child spins forever — precisely the case #1493 targets. Capturing the parent's PID before the fork and passing it in closes the window.

🛡️ Sketch 
 def _run_mailbox_loop(
     buf,
     state_addr: int,
     *,
     handle_task,
     handle_control,
     on_shutdown=None,
+    parent_pid: int | None = None,
 ) -> None:
@@
-    parent_pid = os.getppid()
+    # Prefer the pid captured by the forking parent; falling back to
+    # getppid() here can already observe the reparented ancestor.
+    parent_pid = parent_pid if parent_pid is not None else os.getppid()
🤖 Prompt for AI Agents 
Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@python/simpler/worker.py` around lines 1054 - 1055, Capture the original
parent PID before the fork and pass it into the child worker initialization
instead of sampling it with os.getppid() after fork. Update the worker setup
around parent_pid and liveness_countdown to use this inherited PID, while
preserving the existing liveness polling behavior.

while True:
state = _mailbox_load_i32(state_addr)
if state == _TASK_READY:
Expand All @@ -1054,6 +1068,19 @@ def _run_mailbox_loop(
if on_shutdown is not None:
on_shutdown()
break
else:
liveness_countdown -= 1
if liveness_countdown <= 0:
liveness_countdown = _PARENT_LIVENESS_POLL_INTERVAL
# Comparing against the pid captured at entry rather than
# testing for pid 1: a subreaper (container init, systemd
# user session) adopts orphans instead of init, so the pid
# changes but never becomes 1. A live parent's pid cannot
# change, so this cannot fire spuriously.
if os.getppid() != parent_pid:
if on_shutdown is not None:
on_shutdown()
break


def _sub_worker_loop(
Expand Down
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