Bug report
Bug description:
Starting in Python 3.11 when the max_tasks_per_child parameter was introduced, ProcessPoolExecutor hangs when max_tasks_per_child>1 and enough tasks have been submitted to trigger a worker restart.
The following reproducer hangs on a fresh installation of Python 3.11 on Linux.
importosfromconcurrent.futuresimportProcessPoolExecutorwithProcessPoolExecutor(1, max_tasks_per_child=2) asexe: futs= [exe.submit(os.getpid) for_inrange(10)] forfutinfuts: print(fut.result())Issuing a keyboard interrupt results in the following stack trace:
Traceback (most recent call last): File "<string>", line 7, in <module> File "/usr/lib/python3.11/concurrent/futures/_base.py", line 451, in result self._condition.wait(timeout) File "/usr/lib/python3.11/threading.py", line 320, in wait waiter.acquire() KeyboardInterrupt During handling of the above exception, another exception occurred: Traceback (most recent call last): File "<string>", line 4, in <module> File "/usr/lib/python3.11/concurrent/futures/_base.py", line 647, in __exit__ self.shutdown(wait=True) File "/usr/lib/python3.11/concurrent/futures/process.py", line 825, in shutdown self._executor_manager_thread.join() File "/usr/lib/python3.11/threading.py", line 1112, in join self._wait_for_tstate_lock() File "/usr/lib/python3.11/threading.py", line 1132, in _wait_for_tstate_lock if lock.acquire(block, timeout): ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ KeyboardInterrupt Notes:
Interleaving submissions and result checking does not help.
Adding a timeout to the result() calls does not help.
Increasing the pool size does not help, unless it's made large enough to not require any worker restarts.
Interestingly, setting max_tasks_per_child=1 works great. It never hangs, and a new process correctly used for each task.
It does not hang if max_tasks_per_child is set high enough so that no worker restarts happen.
I have reproduced this problem in the following test environments: On GitHub's default Linux CI environment using Python 3.11.
On GitHub's default Linux CI environment using Python 3.12.
On Ubuntu 22.04.3 LTS running under WSL2 using a fresh installation of Python 3.11.
CPython versions tested on:
3.11, 3.12
Operating systems tested on:
Linux
Linked PRs
gh-115634: Force the process pool to adjust when a process worker exits #115642
gh-115634: document ProcessPoolExecutor max_tasks_per_child bug #140897
gh-115634: Fix ProcessPoolExecutor deadlock with max_tasks_per_child #140900
[3.14] gh-115634: document ProcessPoolExecutor max_tasks_per_child bug (GH-140897) #143302
[3.13] gh-115634: document ProcessPoolExecutor max_tasks_per_child bug (GH-140897) #143303
[3.15] gh-115634: Fix ProcessPoolExecutor deadlock with max_tasks_per_child (GH-140900) #152926
[3.14] gh-115634: Fix ProcessPoolExecutor deadlock with max_tasks_per_child (GH-140900) #152927
[3.13] gh-115634: Fix ProcessPoolExecutor deadlock with max_tasks_per_child (GH-140900) #152928