MVCE for early finalisation using Python multiprocessing and spawn/fork method

mp_gc.py

import os
import sys
import time
import multiprocessing


def print_pid(*args, **kwargs):
    print('[%s]' % os.getpid(), *args, **kwargs)


class Finalisable:
    def __init__(self, name):
        self.name = name

    def __repr__(self):
        return '<Finalisable object %s at 0x%x>' % (getattr(self, 'name', 'unknown'), id(self))

    def __del__(self):
        print_pid('finalising', self)


def drop_late():
    payload = Finalisable('late')
    child = multiprocessing.Process(target=print_pid, args=(payload,))
    child.start()
    child.join()
    del payload


def drop_early():
    payload = Finalisable('early')
    child = multiprocessing.Process(target=print_pid, args=('child sees', payload,))
    print('drop')
    del payload
    print('start')
    child.start()
    child.join()


def drop_early_shared():
    payload = Finalisable(multiprocessing.Value('i', 65))
    child = multiprocessing.Process(target=print_pid, args=('child sees', payload,))
    print('drop')
    del payload
    print('start')
    child.start()
    child.join()


if __name__ == '__main__':
    method = sys.argv[1] if len(sys.argv) > 1 else None
    multiprocessing.set_start_method(method)
    for test in (drop_late, drop_early, drop_early_shared):
        print('### test', test.__name__, 'in', os.getpid(), 'method:', method)
        test()
        print('### done', '\n'*2)

Using either fork, forkserver or spawn shows the payload passed as args to Process is finalised too early (CPython 3.7.0, MacOS). In the case of spawn, a multiprocessing.Value will incorrectly drop its resources as well.