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

Start with materials/python-sockets-tutorial/multiconn-client.py and multiconn-server.py, then reproduce the issue using the two commands shown with the server running on 127.0.0.1:65432. Trace how the client sends and receives the two messages and how the server logs them; done means the second run reports each message separately, matching the first run.