multiconn-client.py sometimes concatenates messages

@dbader, thanks very much for your example code.

Description of the bug

Running the multi-connection example from the python-sockets-tutorial, ie,

• materials/python-sockets-tutorial/multiconn-client.py

sometimes, but not always, concatenates the two messages.

To Reproduce

Start the server and view the output from when the client is run (see below)

python -u multiconn-server.py 127.0.0.1 65432

  listening on ('127.0.0.1', 65432)

  accepted connection from ('127.0.0.1', 50484)
  echoing b'Message 1 from client.' to ('127.0.0.1', 50484)
  echoing b'Message 2 from client.' to ('127.0.0.1', 50484)
  closing connection to ('127.0.0.1', 50484)

  accepted connection from ('127.0.0.1', 50486)
  echoing b'Message 1 from client.Message 2 from client.' to ('127.0.0.1', 50486)
  closing connection to ('127.0.0.1', 50486)

Run the client twice with 1 connection (see below). The first run gives expected results, the second run concatenates the two messages (as can be seen below). The concatenation is also evident in the output from multiconn-server.py above.

python -u multiconn-client.py 127.0.0.1 65432 1

  starting connection 1 to ('127.0.0.1', 65432)
  sending b'Message 1 from client.' to connection 1
  sending b'Message 2 from client.' to connection 1
  received b'Message 1 from client.' from connection 1
  received b'Message 2 from client.' from connection 1
  closing connection 1

python -u multiconn-client.py 127.0.0.1 65432 1

  starting connection 1 to ('127.0.0.1', 65432)
  sending b'Message 1 from client.' to connection 1
  sending b'Message 2 from client.' to connection 1
  received b'Message 1 from client.Message 2 from client.' from connection 1
  closing connection 1

Expected behavior

The output form the second run of the multiconn-client.py should be identical to the output from the first run.

Desktop (please complete the following information):

• OS: CentOS 7
• Browser: Firefox
• Version 91.2.0esr (64-bit)

Additional context
Python 3.8.0

从 materials/python-sockets-tutorial/multiconn-client.py 和 multiconn-server.py 开始，然后在 server 运行于 127.0.0.1:65432 时使用所示的两个命令重现该问题。跟踪 client 如何发送和接收两条消息，以及 server 如何记录它们；完成的标准是第二次运行分别报告每条消息，与第一次运行一致。