multiprocessing.connection.Client deadlocks when trying to connect to a listener without a password

Bug report

Bug description:

We set up a multiprocessing.connection.Listener with no authkey. When we try to connect to it with a Client with an authkey, the Client deadlocks. Here is a working example:

import socket
import threading
import time
from multiprocessing.connection import Client, Listener


def _test(*, listener: str | None, client: str | None) -> tuple[Exception | None, Exception | None, list]:
    _autkey_listener = listener.encode() if listener else None
    _authkey_client = client.encode() if client else None

    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
        s.bind(("localhost", 0))
        address = s.getsockname()

    thread_exc = None
    thread_got = []
    client_exc = None

    def target() -> None:
        try:
            with Listener(address, authkey=_autkey_listener) as l:
                c = l.accept()
                while True:
                    value = c.recv()
                    if not value:
                        break
                    thread_got.append(value)
        except Exception as e:
            nonlocal thread_exc
            thread_exc = e

    t = threading.Thread(target=target)
    t.start()

    try:
        with Client(address, authkey=_authkey_client) as c:
            c.send("hello")
            time.sleep(0.1)
            c.send(None)

        t.join()
    except Exception as e:
        client_exc = e

    return thread_exc, client_exc, thread_got


if __name__ == "__main__":
    print(" no passwords ".center(40, "-"))
    print(_test(listener=None, client=None))

    print(" two matching passwords ".center(40, "-"))
    print(_test(listener="password", client="password"))

    print(" client has wrong password ".center(40, "-"))
    print(_test(listener="password", client="wrong"))

    print(" client has no password ".center(40, "-"))
    print(_test(listener="password", client=None))

    # This is the case which deadlocks
    print(" listener has no password ".center(40, "-"))
    print(_test(listener=None, client="password"))

which outputs:

------------- no passwords -------------
(None, None, ['hello'])
-------- two matching passwords --------
(None, None, ['hello'])
------ client has wrong password -------
(AuthenticationError('digest received was wrong'), AuthenticationError('digest sent was rejected'), [])
-------- client has no password --------
(AuthenticationError("expected 'md5' of length 16 got 20"), None, [])
------- listener has no password -------

and then stops.

Upon some investigation, the following bit of code from Client is to blame:

  if authkey is not None:
      answer_challenge(c, authkey)
      deliver_challenge(c, authkey)

since the listener is not configured to deliver the challenge to the connections. I think this would be resolved with an optional timeout kwarg in answer_challenge. If this suggestion is accepted I would be very happy to work on it 👍

CPython versions tested on:

3.9, 3.10, 3.11, 3.12

Operating systems tested on:

macOS, Windows

Bắt đầu với multiprocessing.connection.Client và đường dẫn answer_challenge được xác định trong báo cáo, sau đó chạy reproducer được cung cấp qua các tổ hợp xác thực của nó. Được xem là hoàn tất khi trường hợp listener-without-password và client-with-password không còn bị treo, trong khi các trường hợp khác đã nêu vẫn giữ nguyên kết quả được chứng minh.