Threading server should close connections on exit

[aaugustin]

Essentially, it should behave like the asyncio server. It could also gain the connections attribute.

[aaugustin]

This would also be more elegant than f9cea9c.

[aaugustin]

Implementing this may hit #1596.

[darkhaniop]

Hi, I just ran some tests to try to understand how the (sync) connections are handled in WebSockets (I described one in #1596).

So, what about the server side? I noticed that a server process with active connections also has some issues when exiting. I think graceful server shutdowns/restarts can be handled within the library.

I have an idea on how to fix it. Would it be okay to work on it?

Proposed approach

We can store a set of initiated threads/connections in the Server instance. Then, gracefully close all active connections in Server.shutdown(). This set, e.g. called connections, must also be accessible from conn_handler(), so we can initiate it within def serve() and either set the connections property of the newly created Server instance from this function, or (maybe more reasonable way) pass it as a constructor argument.

With this fix, it would be easier for users to implement graceful shutdowns/service restarts. Of course, threads can still be blocked in the handler functions, but that'd be a different issue.

What do you think?