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

Question

我想不通的是，虽然 ThreadPoolExecutor 使用守护进程，但即使主线程退出，它们仍然运行。

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是运行ning 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工作人员...

Answer 2

这是避免此问题的方法。糟糕的设计可以被另一个糟糕的设计打败。人们只有在真正知道工作人员不会损坏任何对象或文件时才会写 daemon=True。

在我的例子中，我用一个工人创建了 TreadPoolExecutor，在一个 submit 之后，我刚刚从队列中删除了新创建的线程，这样解释器就不会等到这个线程停止在其自己的。请注意，工作线程是在 submit 之后创建的，而不是在 TreadPoolExecutor.

的初始化之后创建的

import concurrent.futures.thread
from concurrent.futures import ThreadPoolExecutor

...

executor = ThreadPoolExecutor(max_workers=1)
future = executor.submit(lambda: self._exec_file(args))
del concurrent.futures.thread._threads_queues[list(executor._threads)[0]]

它适用于 Python 3.8，但可能不适用于 3.9+，因为此代码正在访问私有变量。

查看工作代码on github

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

The workers in ThreadPoolExecutor is not really daemon

python

multithreading

daemon

concurrent.futures