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[3.13] gh-137017: Ensure Thread.is_alive() only returns False after the underlying OS thread exits (gh-137315) #138917

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Merged
merged 2 commits into from
Sep 15, 2025
Merged

[3.13] gh-137017: Ensure Thread.is_alive() only returns False after the underlying OS thread exits (gh-137315) #138917

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aae4ad7
gh-137017: Ensure `Thread.is_alive()` only returns False after the un…
Abdoulrasheed Sep 15, 2025
c7cb7f3
Fix function pointer type
colesbury Sep 15, 2025
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2025-08-01-23-11-25.gh-issue-137017.0yGcNc.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fix :obj:`threading.Thread.is_alive` to remain ``True`` until the underlying OS
thread is fully cleaned up. This avoids false negatives in edge cases
involving thread monitoring or premature :obj:`threading.Thread.is_alive` calls.
9 changes: 6 additions & 3 deletions Modules/_threadmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -453,8 +453,9 @@ ThreadHandle_start(ThreadHandle *self, PyObject *func, PyObject *args,
}

static int
join_thread(ThreadHandle *handle)
join_thread(void *arg)
{
ThreadHandle *handle = (ThreadHandle*)arg;
assert(get_thread_handle_state(handle) == THREAD_HANDLE_RUNNING);
PyThread_handle_t os_handle;
if (ThreadHandle_get_os_handle(handle, &os_handle)) {
Expand Down Expand Up @@ -528,8 +529,7 @@ ThreadHandle_join(ThreadHandle *self, PyTime_t timeout_ns)
}
}

if (_PyOnceFlag_CallOnce(&self->once, (_Py_once_fn_t *)join_thread,
self) == -1) {
if (_PyOnceFlag_CallOnce(&self->once, join_thread, self) == -1) {
return -1;
}
assert(get_thread_handle_state(self) == THREAD_HANDLE_DONE);
Expand Down Expand Up @@ -657,6 +657,9 @@ PyThreadHandleObject_is_done(PyThreadHandleObject *self,
PyObject *Py_UNUSED(ignored))
{
if (_PyEvent_IsSet(&self->handle->thread_is_exiting)) {
if (_PyOnceFlag_CallOnce(&self->handle->once, join_thread, self->handle) == -1) {
return NULL;
}
Py_RETURN_TRUE;
}
else {
Expand Down