> It seems recv() returning b"" is an alias for "connection lost". E.g. in Twisted:

To my understanding, technically the connection is not fully closed, it is just shut-down for reading but we can still perform write operations on it (that is, the client may be still waiting for the response). I can reproduce it with an http-1.0 client, I'll try to put up a test to demonstrate it more clearly.

From recv() man page:
When a stream socket peer has performed an orderly shutdown, the return value will be 0 (the traditional "end-of-file" return).