python - ThreadPoolExecutor 中的 worker 并不是真正的守护进程

Question

我想不通的是，尽管 ThreadPoolExecutor 使用守护进程，但即使主线程退出，它们仍会运行。

我可以在 python3.6.4 中提供一个最小的例子:

import concurrent.futures
import time


def fn():
    while True:
        time.sleep(5)
        print("Hello")


thread_pool = concurrent.futures.ThreadPoolExecutor()
thread_pool.submit(fn)
while True:
    time.sleep(1)
    print("Wow")

主线程和工作线程都是死循环。因此，如果我使用 KeyboardInterrupt 终止主线程，我预计整个程序也会终止。但实际上工作线程仍然在运行，即使它是一个守护线程。

ThreadPoolExecutor 的源代码确认工作线程是守护线程:

t = threading.Thread(target=_worker,
                     args=(weakref.ref(self, weakref_cb),
                           self._work_queue))
t.daemon = True
t.start()
self._threads.add(t)

此外，如果我手动创建一个守护线程，它就像一个魅力:

from threading import Thread
import time


def fn():
    while True:
        time.sleep(5)
        print("Hello")


thread = Thread(target=fn)
thread.daemon = True
thread.start()
while True:
    time.sleep(1)
    print("Wow")

所以我真的无法理解这种奇怪的行为。

Answer 1

突然……我找到了原因。根据 ThreadPoolExecutor 的更多源代码:

# Workers are created as daemon threads. This is done to allow the interpreter
# to exit when there are still idle threads in a ThreadPoolExecutor's thread
# pool (i.e. shutdown() was not called). However, allowing workers to die with
# the interpreter has two undesirable properties:
#   - The workers would still be running during interpreter shutdown,
#     meaning that they would fail in unpredictable ways.
#   - The workers could be killed while evaluating a work item, which could
#     be bad if the callable being evaluated has external side-effects e.g.
#     writing to a file.
#
# To work around this problem, an exit handler is installed which tells the
# workers to exit when their work queues are empty and then waits until the
# threads finish.

_threads_queues = weakref.WeakKeyDictionary()
_shutdown = False

def _python_exit():
    global _shutdown
    _shutdown = True
    items = list(_threads_queues.items())
    for t, q in items:
        q.put(None)
    for t, q in items:
        t.join()

atexit.register(_python_exit)

有一个退出处理程序，它将加入所有未完成的 worker...