Скрипт зависает при выходе при использовании atexit для завершения потоков

Я играю с потоками на python 3.7.4 и хочу использовать atexit для регистрации функции очистки, которая (чисто) завершит потоки.

Например:

# example.py
import threading
import queue
import atexit
import sys

Terminate = object()

class Worker(threading.Thread):
    def __init__(self):
        super().__init__()
        self.queue = queue.Queue()

    def send_message(self, m):
        self.queue.put_nowait(m)

    def run(self):
        while True:
            m = self.queue.get()
            if m is Terminate:
                break
            else:
                print("Received message: ", m)


def shutdown_threads(threads):
    for t in threads:
        print(f"Terminating thread {t}")
        t.send_message(Terminate)
    for t in threads:
        print(f"Joining on thread {t}")
        t.join()
    else:
        print("All threads terminated")

if __name__ == "__main__":
    threads = [
        Worker()
        for _ in range(5)
    ]
    atexit.register(shutdown_threads, threads)

    for t in threads:
        t.start()

    for t in threads:
        t.send_message("Hello")
        #t.send_message(Terminate)

    sys.exit(0)

Однако кажется, что взаимодействие с потоками и очередями в обратном вызове atexit создает взаимоблокировку с некоторой внутренней процедурой завершения работы:

$ python example.py
Received message:  Hello
Received message:  Hello
Received message:  Hello
Received message:  Hello
Received message:  Hello
^CException ignored in: <module 'threading' from '/usr/lib64/python3.7/threading.py'>
Traceback (most recent call last):
  File "/usr/lib64/python3.7/threading.py", line 1308, in _shutdown
    lock.acquire()
KeyboardInterrupt
Terminating thread <Worker(Thread-1, started 140612492904192)>
Terminating thread <Worker(Thread-2, started 140612484511488)>
Terminating thread <Worker(Thread-3, started 140612476118784)>
Terminating thread <Worker(Thread-4, started 140612263212800)>
Terminating thread <Worker(Thread-5, started 140612254820096)>
Joining on thread <Worker(Thread-1, stopped 140612492904192)>
Joining on thread <Worker(Thread-2, stopped 140612484511488)>
Joining on thread <Worker(Thread-3, stopped 140612476118784)>
Joining on thread <Worker(Thread-4, stopped 140612263212800)>
Joining on thread <Worker(Thread-5, stopped 140612254820096)>
All threads terminated

(KeyboardInterrupt - это я использую ctrl-c, так как процесс, кажется, зависает на неопределенный срок).

Однако, если я отправлю сообщение Terminate перед выходом (раскомментируйте строку после t.send_message("Hello")), программа не зависнет и корректно завершится:

$ python example.py
Received message:  Hello
Received message:  Hello
Received message:  Hello
Received message:  Hello
Received message:  Hello
Terminating thread <Worker(Thread-1, stopped 140516051592960)>
Terminating thread <Worker(Thread-2, stopped 140516043200256)>
Terminating thread <Worker(Thread-3, stopped 140515961992960)>
Terminating thread <Worker(Thread-4, stopped 140515953600256)>
Terminating thread <Worker(Thread-5, stopped 140515945207552)>
Joining on thread <Worker(Thread-1, stopped 140516051592960)>
Joining on thread <Worker(Thread-2, stopped 140516043200256)>
Joining on thread <Worker(Thread-3, stopped 140515961992960)>
Joining on thread <Worker(Thread-4, stopped 140515953600256)>
Joining on thread <Worker(Thread-5, stopped 140515945207552)>
All threads terminated

Возникает вопрос: когда выполняется эта процедура threading._shutdown относительно обработчиков atexit? Имеет ли смысл взаимодействовать с потоками в обработчиках atexit?