我用python写了一个简单的pub/sub草图。下面是完整的源代码，但我遇到的问题是，当代理通过send()广播消息时，客户端不会recv()任何东西。其他的一切似乎都很正常：listen()和accept()正在工作，当客户端调用send()时，代理的recv()会得到消息。
下面是两个客户端和代理之间交互的序列图：

Broker                      Client 62863        Client 62867
------------------------------------------------------------
start()
                            start()
(62863) joined.
                                                start()
(62867) joined.
                            Hello from 62863
(62863): Hello from 62863
(62863) => (62867)

在最后一步中，Broker调用send('Hello from 62863')，但客户端62867的recv()函数没有接收到它。
有什么建议吗？
以下是完整的Broker代码：

import socket
import threading

class Broker(object):

    def __init__(self, host='', port=5000):
        self._host = host
        self._port = port
        self._subscribers = []

    def start(self):
        self._socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self._socket.bind((self._host, self._port))

        while True:
            """
            Wait for a client to request a connection and spawn a thread to
            receive and forward messages from that client.
            """
            self._socket.listen()
            subscriber, addr = self._socket.accept()
            print(f'{addr} joined.', flush=True)
            self._subscribers.append(subscriber)
            threading.Thread(target=self.listen_thread, args=(subscriber,)).start()

    def listen_thread(self, publisher):
        """
        Wait for a message to arrive from a publisher, broadcast to all other
        subscribers.
        """
        while True:
            msg = publisher.recv(1024).decode()
            if msg is not None:
                print(f'{publisher.getpeername()} published: {msg}', end='', flush=True)
                self.broadcast(publisher, msg)
            else:
                print(f'{publisher.getpeername()} has disconnected')
                return

    def broadcast(self, publisher, msg):
        for subscriber in self._subscribers:
            if publisher != subscriber:        # don't send to yourself
                print(f'{publisher.getpeername()} => {subscriber.getpeername()}', flush=True)
                try:
                    subscriber.send(msg) # NOTE: See solution below!!!
                except:
                    # broken socket, remove from subscriber list
                    self._subscribers.remove(subscriber)

if __name__ == "__main__":
    Broker().start()

下面是相应的客户端代码：

import socket
import threading
import sys

class StdioClient(object):
    """
    A simple pub/sub client:
    Anything received on stdin is published to the broker.
    Concurrently, anything broadcast by the broker is printed on stdout.
    """

    def __init__(self, host='localhost', port=5000):
        self._host = host
        self._port = port

    def start(self):
        self._sock = socket.socket()
        self._sock.connect((self._host, self._port))
        threading.Thread(target=self.stdin_to_sock).start()
        threading.Thread(target=self.sock_to_stdout).start()

    def stdin_to_sock(self):
        """
        Send anything received on stdin to the broker.
        """
        for msg in sys.stdin:
            self._sock.send(bytes(msg, 'utf-8'))

    def sock_to_stdout(self):
        """
        Print anything received from the broker on stdout.
        """
        while True:
            msg = self._sock.recv(1024) # <<<= This never gets a message
            print(msg.decode('utf-8'), eol='', flush=True)

if __name__ == '__main__':
    StdioClient().start()