8000 [3.14] gh-135099: Only wait on `_PyOS_SigintEvent()` in main thread (GH-135100) by miss-islington · Pull Request #135116 · python/cpython · GitHub
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[3.14] gh-135099: Only wait on _PyOS_SigintEvent() in main thread (GH-135100) #135116

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Merged
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Jun 4, 2025
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[3.14] gh-135099: Only wait on _PyOS_SigintEvent() in main thread (GH-135100) #135116

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gh-135099: Only wait on _PyOS_SigintEvent() in main thread (GH-135100) 
On Windows, the `_PyOS_SigintEvent()` event handle is used to interrupt
the main thread when Ctrl-C is pressed. Previously, we also waited on
the event from other threads, but ignored the result. However, this can
race with interpreter shutdown because the main thread closes the handle
in `_PySignal_Fini` and threads may still be running and using mutexes
during interpreter shtudown.

Only use `_PyOS_SigintEvent()` in the main thread in parking_lot.c, like
we do in other places in the CPython codebase.
(cherry picked from commit cc581f3)

Co-authored-by: Sam Gross <colesbury@gmail.com>
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@colesbury @miss-islington
colesbury authored and miss-islington committed Jun 4, 2025
commit c46a14c2078cc7e99592b89c6747c5f44e208f01
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Windows/2025-06-03-18-26-54.gh-issue-135099.Q9usKm.rst
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@@ -0,0 +1,2 @@
Fix a crash that could occur on Windows when a background thread waits on a
:c:type:`PyMutex` while the main thread is shutting down the interpreter.
22 changes: 16 additions & 6 deletions Python/parking_lot.c
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8000
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Expand Up @@ -112,17 +112,27 @@ _PySemaphore_PlatformWait(_PySemaphore *sema, PyTime_t timeout)
}
}

// NOTE: we wait on the sigint event even in non-main threads to match the
// behavior of the other platforms. Non-main threads will ignore the
// Py_PARK_INTR result.
HANDLE sigint_event = _PyOS_SigintEvent();
HANDLE handles[2] = { sema->platform_sem, sigint_event };
DWORD count = sigint_event != NULL ? 2 : 1;
HANDLE handles[2] = { sema->platform_sem, NULL };
HANDLE sigint_event = NULL;
DWORD count = 1;
if (_Py_IsMainThread()) {
// gh-135099: Wait on the SIGINT event only in the main thread. Other
// threads would ignore the result anyways, and accessing
// `_PyOS_SigintEvent()` from non-main threads may race with
// interpreter shutdown, which closes the event handle. Note that
// non-main interpreters will ignore the result.
sigint_event = _PyOS_SigintEvent();
if (sigint_event != NULL) {
handles[1] = sigint_event;
count = 2;
}
}
wait = WaitForMultipleObjects(count, handles, FALSE, millis);
if (wait == WAIT_OBJECT_0) {
res = Py_PARK_OK;
}
else if (wait == WAIT_OBJECT_0 + 1) {
assert(sigint_event != NULL);
ResetEvent(sigint_event);
res = Py_PARK_INTR;
}
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