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test_embed.test_bpo20891 is racy under free-threading #137093

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test_embed.test_bpo20891 is racy under free-threading#137093
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testsTests in the Lib/test dirtopic-free-threadingtype-bugAn unexpected behavior, bug, or error
@ZeroIntensity

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@ZeroIntensity
ZeroIntensity
opened on Jul 25, 2025

Bug report

Bug description:

I haven't seen this in CI, but when running regrtest locally, I noticed a crash on test_embed.test_bpo20891 regarding unlocking a mutex that wasn't locked.

The two offending pieces of code are here:

cpython/Programs/_testembed.c

Lines 412 to 417 in d5e75c0

unsigned long thrd = PyThread_start_new_thread(bpo20891_thread, &lock);
if (thrd == PYTHREAD_INVALID_THREAD_ID) {
error("PyThread_start_new_thread failed!");
return 1;
}
PyThread_acquire_lock(lock, WAIT_LOCK);

cpython/Programs/_testembed.c

Lines 382 to 395 in d5e75c0

static void bpo20891_thread(void *lockp)
{
PyThread_type_lock lock = *((PyThread_type_lock*)lockp);
PyGILState_STATE state = PyGILState_Ensure();
if (!PyGILState_Check()) {
error("PyGILState_Check failed!");
abort();
}
PyGILState_Release(state);
PyThread_release_lock(lock);
}

The problem is that in rare cases, the created thread can hit the call to PyThread_release_lock before it's held by the main thread, which causes a fatal error. This isn't an issue on the GILful build, because PyGILState_Ensure will block until the main thread releases the GIL.

I think the best fix would be to use a PyEvent to signal to the main thread rather than a lock.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

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    testsTests in the Lib/test dirtopic-free-threadingtype-bugAn unexpected behavior, bug, or error

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