import queue
from multiprocessing import Process, Queue
from threading import Thread
from time import sleep


def main():
    # You don't really need two queues to show the problem
    # If you comment out in_queue.get/put, the failure still happens, just less neatly
    # Loads of items get placed on out_queue so that might cause other problems
    # and you also need to comment out `assert out_queue.qsize() == i` below
    in_queue = Queue()
    out_queue = Queue()

    # Setting target=worker_loop (i.e. not using a Thread within a Process)
    # fixes the problem, but isn't an option for my real use case
    # Besides, are sub-processes not allowed to start threads?
    process = Process(target=worker_loop_in_thread, args=(in_queue, out_queue))
    process.start()

    # Uncommenting this sleep fixes the problem
    # I'm doing something similar to fix my real program which feels very bad
    # sleep(0.01)

    # The failure always first happens in the second out_queue.get
    # (see `assert i > 0` below)
    # so range(2) is enough to reproduce, but this shows the consistency once failure starts
    for i in range(5):
        in_queue.put(1)
        try:
            assert out_queue.get(timeout=1) == 2
        except queue.Empty:
            print("Fail!", out_queue.qsize())
            # qsize seems to be perfectly accurate here - it gives the number
            # of items that I expect to be on the queue, but .get doesn't see them
            assert out_queue.qsize() == i
            assert i > 0

            # Like .get, .empty doesn't see the items that were placed there
            assert out_queue.empty()

            # Just showing that the problem isn't caused by the process dying
            assert process.is_alive()

    process.terminate()


def worker_loop_in_thread(*args):
    Thread(target=worker_loop, args=args).start()


def worker_loop(in_queue, out_queue):
    while True:
        assert in_queue.get() == 1
        out_queue.put(2)


if __name__ == '__main__':
    # Most of the time main() works fine
    # This loop is required to show it breaking
    for attempt in range(1000):
        print(attempt)
        main()