René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"ProcessPoolExecutor shutdown hangs after future cancel was requested","articleBody":"**Bug report**\r\n\r\nWith a ProcessPoolExecutor, after submitting and quickly canceling a future, a call to `shutdown(wait=True)` would hang indefinitely.\r\nThis happens pretty much on all platforms and all recent Python versions.\r\n\r\nHere is a minimal reproduction:\r\n\r\n```py\r\nimport concurrent.futures\r\n\r\nppe = concurrent.futures.ProcessPoolExecutor(1)\r\nppe.submit(int).result()\r\nppe.submit(int).cancel()\r\nppe.shutdown(wait=True)\r\n```\r\n\r\nThe first submission gets the executor going and creates its internal `queue_management_thread`.\r\nThe second submission appears to get that thread to loop, enter a wait state, and never receive a wakeup event.\r\n\r\nIntroducing a tiny sleep between the second submit and its cancel request makes the issue disappear. From my initial observation it looks like something in the way the `queue_management_worker` internal loop is structured doesn't handle this edge case well.\r\n\r\nShutting down with `wait=False` would return immediately as expected, but the `queue_management_thread` would then die with an unhandled `OSError: handle is closed` exception.\r\n\r\n**Environment**\r\n\r\n* Discovered on macOS-12.2.1 with cpython 3.8.5.\r\n* Reproduced in Ubuntu and Windows (x64) as well, and in cpython versions 3.7 to 3.11.0-beta.3.\r\n* Reproduced in pypy3.8 as well, but not consistently. Seen for example in Ubuntu with Python 3.8.13 (PyPy 7.3.9).\r\n\r\n**Additional info**\r\n\r\nWhen tested with `pytest-timeout` under Ubuntu and cpython 3.8.13, these are the tracebacks at the moment of timing out:\r\n\r\n\u003cdetails\u003e\r\n\r\n```pytb\r\n_____________________________________ test _____________________________________\r\n    @pytest.mark.timeout(10)\r\n    def test():\r\n        ppe = concurrent.futures.ProcessPoolExecutor(1)\r\n        ppe.submit(int).result()\r\n        ppe.submit(int).cancel()\r\n\u003e       ppe.shutdown(wait=True)\r\ntest_reproduce_python_bug.py:14:\r\n_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _\r\n/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/concurrent/futures/process.py:686: in shutdown\r\n    self._queue_management_thread.join()\r\n/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/threading.py:1011: in join\r\n    self._wait_for_tstate_lock()\r\n_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _\r\nself = \u003cThread(QueueManagerThread, started daemon 140003176535808)\u003e\r\nblock = True, timeout = -1\r\n    def _wait_for_tstate_lock(self, block=True, timeout=-1):\r\n        # Issue #18808: wait for the thread state to be gone.\r\n        # At the end of the thread's life, after all knowledge of the thread\r\n        # is removed from C data structures, C code releases our _tstate_lock.\r\n        # This method passes its arguments to _tstate_lock.acquire().\r\n        # If the lock is acquired, the C code is done, and self._stop() is\r\n        # called.  That sets ._is_stopped to True, and ._tstate_lock to None.\r\n        lock = self._tstate_lock\r\n        if lock is None:  # already determined that the C code is done\r\n            assert self._is_stopped\r\n\u003e       elif lock.acquire(block, timeout):\r\nE       Failed: Timeout \u003e10.0s\r\n/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/threading.py:1027: Failed\r\n----------------------------- Captured stderr call -----------------------------\r\n+++++++++++++++++++++++++++++++++++ Timeout ++++++++++++++++++++++++++++++++++++\r\n~~~~~~~~~~~~~~~~~ Stack of QueueFeederThread (140003159754496) ~~~~~~~~~~~~~~~~~\r\n  File \"/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/threading.py\", line 890, in _bootstrap\r\n    self._bootstrap_inner()\r\n  File \"/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/threading.py\", line 932, in _bootstrap_inner\r\n    self.run()\r\n  File \"/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/threading.py\", line 870, in run\r\n    self._target(*self._args, **self._kwargs)\r\n  File \"/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/multiprocessing/queues.py\", line 227, in _feed\r\n    nwait()\r\n  File \"/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/threading.py\", line 302, in wait\r\n    waiter.acquire()\r\n~~~~~~~~~~~~~~~~ Stack of QueueManagerThread (140003176535808) ~~~~~~~~~~~~~~~~~\r\n  File \"/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/threading.py\", line 890, in _bootstrap\r\n    self._bootstrap_inner()\r\n  File \"/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/threading.py\", line 932, in _bootstrap_inner\r\n    self.run()\r\n  File \"/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/threading.py\", line 870, in run\r\n    self._target(*self._args, **self._kwargs)\r\n  File \"/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/concurrent/futures/process.py\", line 362, in _queue_management_worker\r\n    ready = mp.connection.wait(readers + worker_sentinels)\r\n  File \"/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/multiprocessing/connection.py\", line 931, in wait\r\n    ready = selector.select(timeout)\r\n  File \"/opt/hostedtoolcache/Python/3.8.13/x64/lib/python3.8/selectors.py\", line 415, in select\r\n    fd_event_list = self._selector.poll(timeout)\r\n+++++++++++++++++++++++++++++++++++ Timeout ++++++++++++++++++++++++++++++++++++\r\n```\r\n\r\n\u003c/details\u003e\r\n\r\nTracebacks in PyPy are similar on the `concurrent.futures.process` level. Tracebacks in Windows are different in the lower-level areas, but again similar on the `concurrent.futures.process` level.\r\n\r\nLinked PRs:\r\n- #94468\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-94468\n* gh-102746\n* gh-102747\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/yonatanp","@type":"Person","name":"yonatanp"},"datePublished":"2022-06-30T13:42:21.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":13},"url":"https://github.com/94440/cpython/issues/94440"}