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gh-155477: Fix multiprocessing.Pool deadlock on close() with a buffersize imap #155478

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gh-155477: Fix multiprocessing.Pool deadlock on close() with a buffersize imap #155478

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9 changes: 9 additions & 0 deletions Lib/multiprocessing/pool.py
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Original file line number Diff line number Diff line change
Expand Up @@ -403,6 +403,11 @@ def _guarded_task_generation(self, result_job, func, iterable, sema=None):
enumerated_iter = iter(enumerate(iterable))
while True:
sema.acquire()
if self._state != RUN:
# The pool is closing or terminating; stop submitting
# the still-throttled tasks so the task handler can
# finish instead of blocking here forever.
break
try:
i, x = next(enumerated_iter)
except StopIteration:
Expand Down Expand Up @@ -661,6 +666,10 @@ def close(self):
self._state = CLOSE
self._worker_handler._state = CLOSE
self._change_notifier.put(None)
# Wake any task generator throttled on a buffersize semaphore so
# it observes the CLOSE state and stops submitting.
for sema in tuple(self._taskqueue_buffersize_semaphores):
sema.release()

def terminate(self):
util.debug('terminating pool')
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21 changes: 21 additions & 0 deletions Lib/test/_test_multiprocessing.py
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Expand Up @@ -3227,6 +3227,27 @@ def produce_args():
p.terminate()
p.join()

@warnings_helper.ignore_fork_in_thread_deprecation_warnings()
@support.subTests('method_name', ("imap", "imap_unordered"))
def test_imap_with_buffersize_close_after_partial_consumption(
self, method_name
):
# close()/join() must not deadlock when a buffersize iterator is
# only partially consumed (the throttled task generator must stop).
p = self.Pool(2)
method = getattr(p, method_name)
it = method(sqr, range(1000), buffersize=2)
next(it)
finished = threading.Event()
def finalize():
p.close()
p.join()
finished.set()
t = threading.Thread(target=finalize)
t.start()
t.join(support.SHORT_TIMEOUT)
self.assertTrue(finished.is_set(), "close()/join() deadlocked")

@support.subTests('method_name', ("imap", "imap_unordered"))
def test_imap_and_imap_unordered_with_buffersize_on_empty_iterable(
self, method_name
Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-08-10-17-54-16.gh-issue-155477.Mp6Db2.rst
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@@ -0,0 +1,3 @@
Fix a deadlock when ``multiprocessing.pool.Pool.close()`` is followed by
``join()`` after only partially consuming an ``imap()`` or ``imap_unordered()``
iterator that was created with a *buffersize*. Patch by tonghuaroot.
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