gh-155477: Fix multiprocessing.Pool deadlock on close() with a buffersize imap - #155478
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serhiy-storchaka merged 2 commits intoAug 16, 2026
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…buffersize imap close() did not release the buffersize semaphores that throttle the task generator, so a partially-consumed imap left the task handler blocked and join() deadlocked. Release them in close() and stop the generator once the pool leaves the RUN state.
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Thank you for your PR. In general LGTM, I have only few nitpicks.
This is a fix for an unrealized feature, so NEWS is not required. It will only confuse late readers.
| 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): |
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Why tuple() instead of list() or .copy()?
The fix is for an unrealized feature, so no NEWS is needed.
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Done in 306d869: switched to |
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The buffersize variant of
Pool.imap()/imap_unordered()throttles the taskgenerator on a semaphore.
close()did not release those semaphores (onlyterminate()did), so a partially-consumed iterator left the task handlerblocked in
sema.acquire()andjoin()deadlocked. Release the buffersizesemaphores in
close(), and stop the task generator once the pool leaves theRUN state. Introduced in gh-64192.