Multiprocessing Client connection, send call is blocking when multiple threads try to connect to IPC Server which is offline/down/not running

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Issue 类型
缺陷
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基本清楚
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技术栈
python
领域
backend, networking

调研方向

从 multiprocessing.connection.Client 入口点开始，在 IPC 服务器离线时使用三个线程在 Windows 10 上复现该问题。跟踪为什么只有一个调用会返回连接被拒绝异常；完成的标准是每个线程都会返回，而不是一直阻塞，并为此场景提供回归覆盖。

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描述

topic-multiprocessing type-bug

Bug report

When multiple threads access send call of multiprocessing Client to connect to server via IPC, the threads are blocked forever. This occurs when IPC server is down and the client tries to connect to server.

CODE:

from multiprocessing.connection import Client
def ipc_send(data={}):
    try:
        address = ('localhost', 56000)
        print("ipc address: ", address)
        conn = Client(address, authkey="SOMESECRET")
        conn.send(data)
        res = conn.recv()
        conn.close()
        return True
    except Exception as e:
        print(e)
    return None

As server is down, when ipc_send() is called by 3 different threads(including main thread), suprisingly only one thread has returned None with exception: [WinError 10061] No connection could be made because the target machine actively refused it.

It is trivial that Error would be connection refused as server is down, but only one thread has thrown the error and rest of the 2 threads are possibly blocked at

conn = Client(address, authkey="SOMESECRET")

Your environment

Python 3.8.8
OS: Windows 10.

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