Prevent GreenletExit exceptions (and improve shutdown time) when shutting down with active keepalive connections

[chriskuehl]

Internal ticket: SVF-438

This fixes an issue where a Baseplate WSGI server in shutdown doesn't proactively close existing "keepalive" connections and continues to process requests from them. This causes two issues:

1. Requests can be killed in-flight with GreenletExit if the server's stop_time is reached.
2. Shutdowns are unnecessarily slow because the gevent server waits for connections to be closed (but does nothing to close them itself).

This fix causes gevent to close connections once a server begins graceful shutdown (i.e. once server.stop() is called). It does this by adding the Connection: close header on responses and by closing connections in the pool.

Commentary

There doesn't appear to be an easy way to fix this without subclassing gevent's WSGIHandler and/or WSGIServer classes. These classes are designed to be subclassed according to the documentation, though it would still be better to fix this in gevent.

I believe most gevent users don't hit this as it is fairly rare (and not recommended) to use gevent's built-in WSGI server instead of something like gunicorn.

I've emailed the gevent mailing list to ask about this issue. If the reception is positive I'll submit a patch upstream.

Validation

• First commit adds a regression test (expected to fail on develop branch).
• Second commit adds the fix, which also fixes the regression test.

[mjbonifacio]

I'm informed and this appears sensible -- excited to try it. I wouldn't consider my approval authoritative as I don't think I'd be able to foresee other issues this might cause.

[salomon-smekecohen]

I believe the way the pywsgi and event modules are accessed will cause issues.

[salomon-smekecohen]

ty! sorry for not re-reviewing quickly here