PYTHON-2360 Ensure ConnectionCreatedEvents are emitted before ConnectionReadyEvents

[ShaneHarvey]

This uncovered a minor oversight in our minPoolSize connection creation logic. We did create the connection but we did not ensure that the connection was authenticated. Connections created in the background (for minPoolSize) are now authenticated. This is a minor performance improvement (lower latency on first connection use).

[prashantmital]

Can you explain how this results in improved performance? It seems to me that with this change check_auth will be called twice - once in the background on socket creation and then again when the socket is checked out (i.e. in Pool._get_socket()). I'm guessing that the second call would result in a no-op but it is not clear to me how?

[ShaneHarvey]

Can you explain how this results in improved performance?

Before this change the background thread which creates minPoolSize connections would only create the connection and run the handshake. This means that the first operation which using that connection needs to authenticate which incurs added latency.

After this change, the background thread creates the connection and authenticates it too.

I'm guessing that the second call would result in a no-op but it is not clear to me how?

Yes the second call is a no-op. Each SocketInfo caches the set of credentials which have already been authenticated. See the implementation of check_auth:

mongo-python-driver/pymongo/pool.py

Lines 784 to 793 in 83578dc

	cached = set(itervalues(all_credentials))
	authset = self.authset.copy()
	# Logout any credentials that no longer exist in the cache.
	for credentials in authset - cached:
	auth.logout(credentials.source, self)
	self.authset.discard(credentials)
	for credentials in cached - authset:
	self.authenticate(credentials)