为什么我在这里得到一个ConnectionResetError？

Question

我是Python 3的新手,正在玩asyncio.因此,我遇到了以下服务器端代码的奇怪行为:

import asyncio


@asyncio.coroutine
def handle_client(reader, writer):
    print('Client connected.')
    client_connected = True
    while client_connected:
        print('Waiting for client event.')
        line = yield from reader.readline()
        if line:
            print('Got: {}'.format(line))
            if line.decode() == 'echo\n':
                print('Sending back echo.')
                writer.write(line)
            else:
                print('Not sending back anything.')
        else:
            print('Client disconnected.')
            client_connected = False


if __name__ == '__main__':
    asyncio.async(asyncio.start_server(handle_client, 'localhost', 8888))
    asyncio.get_event_loop().run_forever()

当我运行这个客户端代码时(编辑:客户端代码手动输入到IPython会话中,服务器肯定有时间在关闭套接字之前写入)...

import socket
client = socket.create_connection(('localhost', 8888))
client.sendall('echo\n'.encode())
client.close()

...我从服务器收到错误回溯:

C:\Users\Gnar\Anaconda3\python.exe C:/Users/Gnar/Code/echo.py
Client connected.
Waiting for client event.
Got: b'echo\n'
Sending back echo.
Waiting for client event.
Task exception was never retrieved
future: <Task finished coro=<handle_client() done, defined at C:/Users/Gnar/Code/echo.py:4> exception=ConnectionResetError(10054, 'An existing connection was forcibly closed by the remote host', None, 10054, None)>
Traceback (most recent call last):
  File "C:\Users\Gnar\Anaconda3\lib\asyncio\tasks.py", line 234, in _step
    result = coro.throw(exc)
  File "C:/Users/Gnar/Code/echo.py", line 10, in handle_client
    line = yield from reader.readline()
  File "C:\Users\Gnar\Anaconda3\lib\asyncio\streams.py", line 425, in readline
    yield from self._wait_for_data('readline')
  File "C:\Users\Gnar\Anaconda3\lib\asyncio\streams.py", line 393, in _wait_for_data
    yield from self._waiter
  File "C:\Users\Gnar\Anaconda3\lib\asyncio\futures.py", line 386, in __iter__
    yield self  # This tells Task to wait for completion.
  File "C:\Users\Gnar\Anaconda3\lib\asyncio\tasks.py", line 287, in _wakeup
    value = future.result()
  File "C:\Users\Gnar\Anaconda3\lib\asyncio\futures.py", line 275, in result
    raise self._exception
  File "C:\Users\Gnar\Anaconda3\lib\asyncio\selector_events.py", line 662, in _read_ready
    data = self._sock.recv(self.max_size)
ConnectionResetError: [WinError 10054] An existing connection was forcibly closed by the remote host

问题必须以某种方式与之相关writer.write,因为当我调用以下客户端代码(使服务器跳过写入)时,没有错误:

import socket
client = socket.create_connection(('localhost', 8888))
client.sendall('foo\n'.encode())
client.close()

相应的服务器日志:

C:\Users\Gnar\Anaconda3\python.exe C:/Users/Gnar/Code/echo.py
Client connected.
Waiting for client event.
Got: b'foo\n'
Not sending back anything.
Waiting for client event.
Client disconnected.

我错过了什么？我不正确地使用asyncio吗？

谢谢!

Answer 1

之所以会出现异常，是因为您试图将一些数据写回到服务器端的客户端，但是客户端socket在发送完后立即关闭'echo'，而实际上并没有收到服务器的响应。如果套接字连接已关闭，而线路上未接收到数据，则发送方会收到错误消息，以便您知道远程端可能没有收到上次发送的任何内容。

如果在调用socket.recv(1024)之前在客户端上添加了对的调用socket.close()，问题就消失了，以便客户端在关闭套接字之前等待服务器的响应。如果您只想优雅地处理异常，即使客户端做错了事情，也可以在服务器端的写调用周围使用try/ except。