Deadlock when shutting down ThreadPoolExecutor from inside OS Signal handler

Bug report

Bug description:

When running the code below and sending a SIGTERM to the process results in what looks like a deadlock on the ThreadPoolExecutor._shutdown_lock. I guess the issue is that the SIGTERM is being handled in main thread while in the submit function where the _shutdown_lock is already locked. When I try calling shutdown inside the signal handler it waits forever.

TBH I'm not sure if this is a bug or expected behavior but I wanted to file this anyways. Thanks!

from concurrent.futures import ThreadPoolExecutor
import signal


def main():
    with ThreadPoolExecutor(max_workers=2) as executor:

        def __exit(signal, frame):
            executor.shutdown(wait=True, cancel_futures=True)

        signal.signal(signal.SIGTERM, __exit)

        while True:
            executor.submit(lambda: 1 + 1)


if __name__ == "__main__":
    main()

AFTER sending SIGTERM to the process, this is what pystack shows:

(v) rbost@fedora:~/code/CheckDuplicate$ pystack remote 3431749
Traceback for thread 3431751 (python) [] (most recent call last):
    (Python) File "/usr/lib64/python3.12/threading.py", line 1030, in _bootstrap
        self._bootstrap_inner()
    (Python) File "/usr/lib64/python3.12/threading.py", line 1073, in _bootstrap_inner
        self.run()
    (Python) File "/usr/lib64/python3.12/threading.py", line 1010, in run
        self._target(*self._args, **self._kwargs)
    (Python) File "/usr/lib64/python3.12/concurrent/futures/thread.py", line 89, in _worker
        work_item = work_queue.get(block=True)

Traceback for thread 3431750 (python) [] (most recent call last):
    (Python) File "/usr/lib64/python3.12/threading.py", line 1030, in _bootstrap
        self._bootstrap_inner()
    (Python) File "/usr/lib64/python3.12/threading.py", line 1073, in _bootstrap_inner
        self.run()
    (Python) File "/usr/lib64/python3.12/threading.py", line 1010, in run
        self._target(*self._args, **self._kwargs)
    (Python) File "/usr/lib64/python3.12/concurrent/futures/thread.py", line 89, in _worker
        work_item = work_queue.get(block=True)

Traceback for thread 3431749 (python) [] (most recent call last):
    (Python) File "/home/rbost/code/CheckDuplicate/test.py", line 18, in <module>
        main()
    (Python) File "/home/rbost/code/CheckDuplicate/test.py", line 14, in main
        executor.submit(lambda: 1 + 1)
    (Python) File "/usr/lib64/python3.12/concurrent/futures/thread.py", line 175, in submit
        f = _base.Future()
    (Python) File "/usr/lib64/python3.12/concurrent/futures/_base.py", line 328, in __init__
        def __init__(self):
    (Python) File "/home/rbost/code/CheckDuplicate/test.py", line 9, in __exit
        executor.shutdown(wait=True, cancel_futures=True)
    (Python) File "/usr/lib64/python3.12/concurrent/futures/thread.py", line 220, in shutdown
        with self._shutdown_lock:

CPython versions tested on:

3.12

Operating systems tested on:

Linux

提供された SIGTERM の例を再現し、その後 concurrent/futures/thread.py の ThreadPoolExecutor.submit()、shutdown()、_shutdown_lock 周辺を調査してください。Issue ではシグナルハンドラーに期待される動作が定義されておらず、テストも指定されていません。進展させるには、その動作について合意し、報告されたデッドロックなしに完了することを示す回帰テストを追加する必要があります。