review connection lifecycle

[python-trio]

review connection lifecycle

[smurfix]

Well, the connection may be usable when we get it out of the pool and the socket is not readable, but that's not a guarantee. I once had a cute race condition where the server timed out after 60 seconds … and the client sent a request every minute … so the new request and the server's FIN crossed each other.

Meaning, "send a request and immediately get EOF" is a hard error on a new connection but the request must be retried when the connection is from the pool.

[njsmith]

I don't think there's any general solution for that race condition, but we can certainly keep whatever strategy urllib3 is currently using.

[njsmith]

(in general when you get a blank eof after sending a request, you don't know whether it was processed by the server or not, and if the request isn't idempotent then automatically retrying it could be dangerous.)