ppe_deadlock.py

"""Warning: executor.shutdown(wait=False) causes deadlocks for instances of
concurrent.futures.ProcessPoolExecutor.

Not intended for usage outside of debugging and demonstration purposes.
Tested on Python 3.7.6 and 3.8.1.
"""

import concurrent.futures as cf
import time


def wait_return(duration, result):
    time.sleep(duration)
    return result


def show_executor_shutdown(executor_type, wait):
    executor = executor_type(max_workers=3)
    futures = [executor.submit(wait_return, .1, 42) for _ in range(9)]
    executor.shutdown(wait=wait)
    for fut in futures:
        print(f"fut._result = {fut._result}, fut._state = {fut._state}")


if __name__ == "__main__":
    # Behaves as expected
    print("ThreadPoolExecutor:")
    print("wait=False")
    show_executor_shutdown(executor_type=cf.ThreadPoolExecutor,
                            wait=False)
    print()

    print("wait=True")
    show_executor_shutdown(executor_type=cf.ThreadPoolExecutor,
                            wait=True)

    print("ProcessPoolExecutor:")
    print("wait=True")
    show_executor_shutdown(executor_type=cf.ProcessPoolExecutor,
                            wait=True)
    print()
    
    print("wait=False")
    # Deadlock
    show_executor_shutdown(executor_type=cf.ProcessPoolExecutor,
                            wait=False)
    print()