`multiprocessing.Process.is_alive()` can incorrectly return True after `join()`

Bug report

Bug description:

This came up in https://github.com/python/cpython/pull/130849#discussion_r1980137089

The problem is that popen_fork.Popen (and popen_spawn.Popen and popen_forkserver.Popen) are not thread-safe:

https://github.com/python/cpython/blob/02de9cb9a8fa5b0ae3947231b8c0677834aaee45/Lib/multiprocessing/popen_fork.py#L25-L35

The first successful call to os.waitpid() may reap the pid so that subsequent calls raise an OSError. I've only seen this on macOS (not Linux). We may not yet however have set self.returncode -- that happens a few statements later, so poll() can return None if:

1. The process has finished
2. Another thread called poll(), but hasn't yet set self.returncode

And then is_alive() can return True:

https://github.com/python/cpython/blob/02de9cb9a8fa5b0ae3947231b8c0677834aaee45/Lib/multiprocessing/process.py#L153-L170

Note that some classes like concurrent.futures.ProcessPoolExecutor use threads internally, so the user may not even know that threads are involved.

Repro:

import os
import multiprocessing as mp
import threading
import time
import sys

original_excepthook = threading.excepthook

def on_except(args):
    original_excepthook(args)
    os._exit(1)

threading.excepthook = on_except

def p1():
    pass

def thread1(p):
    while p.is_alive():
        time.sleep(0.00001)
        pass

def test():
    for i in range(1000):
        print(i)
        p = mp.Process(target=p1)
        p.start()

        t = threading.Thread(target=thread1, args=(p,))
        t.start()

        p.join()
        assert not p.is_alive()

        t.join()

def main():
    threads = [threading.Thread(target=test) for _ in range(10)]
    for t in threads:
        t.start()
    for t in threads:
        t.join()

if __name__ == "__main__":
    main()

NOTE:

• This is unrelated to free threading
• popen_fork.Popen (and subclasses) are distinct from subprocess.Popen

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-131440