threading_forking_locking.py

""" Mixing threads and processes (fork) is a bad idea.

When a process forks, only the thread that was running at the time of the
fork is copied to the child process. That means that if a lock was acquired
by a background thread in the main process, that lock will appear as locked
in the child process, with no way to unlock it.

"""

import os
import multiprocessing
import threading

lock = threading.Lock()
event = threading.Event()

def lock_then_unlock():
    print("acquiring lock in process", os.getpid())
    lock.acquire()
    event.wait()
    print("releasing lock in process", os.getpid())
    lock.release()


def main():
    acquire = threading.Thread(target=lock_then_unlock)
    acquire.start()

    pid = os.fork()
    if pid == 0:  # child
        print("child")

        print(lock.locked())

        print("acquiring lock again in child")
        lock.acquire()
    else:
        # parent
        print("parent")
        event.set()

        acquire.join()
        os.wait()

if __name__ == '__main__':
    main()