Python 3.10 multiprocessing hangs (only sometimes) after poisoning

I've got the following toy script:

#!/usr/bin/env python3
import multiprocessing as mp


def main():
    queue = mp.Queue()
    stop = mp.Event()
    workers = []
    n = mp.cpu_count()
    print(f"starting {n} processes")
    for i in range(n):
        p = mp.Process(target=work, args=(i, queue, stop))
        workers.append(p)
        p.start()
    print("getting 1000 items from queue")
    for _ in range(1000):
        queue.get()
    print("poisoning processes")
    stop.set()
    print("joining processes")
    for worker in workers:
        # hangs occassionally if terminate not called
        # worker.terminate()
        worker.join()
    print("closing queue")
    queue.close()
    print("returning")


def work(i, queue, stop):
    while not stop.is_set():
        queue.put("something")
    print(f"exiting process {i}")


if __name__ == "__main__":
    main()

Here is some sample output where it hangs and then I kill it with Ctrl-C:

λ ./mp_template.py
starting 16 processes
getting 1000 items from queue
poisoning processes
joining processes
exiting process 1
exiting process 2
exiting process 0
exiting process 10
exiting process 9
exiting process 7
exiting process 11
exiting process 5
exiting process 12
exiting process 6
exiting process 4
exiting process 13
exiting process 3
exiting process 8
exiting process 14
exiting process 15
^CTraceback (most recent call last):
  File "/data/repos/mse-408/./mp_template.py", line 37, in <module>
    main()
  File "/data/repos/mse-408/./mp_template.py", line 24, in main
    worker.join()
  File "/usr/lib/python3.10/multiprocessing/process.py", line 149, in join
    res = self._popen.wait(timeout)
  File "/usr/lib/python3.10/multiprocessing/popen_fork.py", line 43, in wait
    return self.poll(os.WNOHANG if timeout == 0.0 else 0)
  File "/usr/lib/python3.10/multiprocessing/popen_fork.py", line 27, in poll
    pid, sts = os.waitpid(self.pid, flag)
KeyboardInterrupt
Process Process-1:
Traceback (most recent call last):
  File "/usr/lib/python3.10/multiprocessing/process.py", line 317, in _bootstrap
    util._exit_function()
  File "/usr/lib/python3.10/multiprocessing/util.py", line 360, in _exit_function
    _run_finalizers()
  File "/usr/lib/python3.10/multiprocessing/util.py", line 300, in _run_finalizers
    finalizer()
  File "/usr/lib/python3.10/multiprocessing/util.py", line 224, in __call__
    res = self._callback(*self._args, **self._kwargs)
  File "/usr/lib/python3.10/multiprocessing/queues.py", line 199, in _finalize_join
    thread.join()
  File "/usr/lib/python3.10/threading.py", line 1096, in join
    self._wait_for_tstate_lock()
  File "/usr/lib/python3.10/threading.py", line 1116, in _wait_for_tstate_lock
    if lock.acquire(block, timeout):
KeyboardInterrupt

Why does this occasionally hang and other times not? If I add worker.terminate() for each worker, it always exits. However, I thought that if the worker returned (as it does after stop.set() is called) -- why does it hang?

source https://stackoverflow.com/questions/74105679/python-3-10-multiprocessing-hangs-only-sometimes-after-poisoning