The following code hangs on Python 3.8.0.rc0 on Ubuntu 19.10 when exiting the interpreter:


from multiprocessing import Pool

class A(object):
    def __init__(self):
        self.pool = Pool(processes=2)

solver = A()


When you eventually do ctrl-C, the traceback is:


^CProcess ForkPoolWorker-2:
Error in atexit._run_exitfuncs:
Process ForkPoolWorker-1:
Traceback (most recent call last):
  File "/usr/lib/python3.8/multiprocessing/util.py", line 277, in _run_finalizers
    finalizer()
  File "/usr/lib/python3.8/multiprocessing/util.py", line 201, in __call__
    res = self._callback(*self._args, **self._kwargs)
  File "/usr/lib/python3.8/multiprocessing/pool.py", line 689, in _terminate_pool
    cls._help_stuff_finish(inqueue, task_handler, len(pool))
  File "/usr/lib/python3.8/multiprocessing/pool.py", line 674, in _help_stuff_finish
    inqueue._rlock.acquire()
KeyboardInterrupt
Traceback (most recent call last):
  File "/usr/lib/python3.8/multiprocessing/process.py", line 313, in _bootstrap
    self.run()
  File "/usr/lib/python3.8/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3.8/multiprocessing/pool.py", line 114, in worker
    task = get()
  File "/usr/lib/python3.8/multiprocessing/queues.py", line 355, in get
    with self._rlock:
  File "/usr/lib/python3.8/multiprocessing/synchronize.py", line 95, in __enter__
    return self._semlock.__enter__()
KeyboardInterrupt
Traceback (most recent call last):
  File "/usr/lib/python3.8/multiprocessing/process.py", line 313, in _bootstrap
    self.run()
  File "/usr/lib/python3.8/multiprocessing/process.py", line 108, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3.8/multiprocessing/pool.py", line 114, in worker
    task = get()
  File "/usr/lib/python3.8/multiprocessing/queues.py", line 356, in get
    res = self._reader.recv_bytes()
  File "/usr/lib/python3.8/multiprocessing/connection.py", line 216, in recv_bytes
    buf = self._recv_bytes(maxlength)
  File "/usr/lib/python3.8/multiprocessing/connection.py", line 414, in _recv_bytes
    buf = self._recv(4)
  File "/usr/lib/python3.8/multiprocessing/connection.py", line 379, in _recv
    chunk = read(handle, remaining)
KeyboardInterrupt


A similar problem seems to occur when garbage collecting when it's part of a class, and that class has not yet called `self.pool.terminate()` and `self.pool.close()`. Hopefully fixing the `atexit` behavior will similarly fix the `gc.collect()` behavior.

Cross-ref is in SciPy, https://github.com/scipy/scipy/issues/10927. It appears to also cause hangs on Travis 3.8-dev builds: https://travis-ci.org/scipy/scipy/jobs/598786785