If you do the following:

• Create a client pool
• Pop a client
• Use that client to create a session
• Destroy the session
• Return the client to the pool
• Destroy the pool

A warning is logged:

WARNING:       client: Couldn’t send “endSessions”: Could not find node 127.0.0.1:27017"

The warning is coming from the call to mongoc_cluster_stream_for_server in _mongoc_client_end_sessions.

This does not happen in single-threaded mode.

Haven't experienced any issues because of this, but it seems like the warning should not be logged in this case.

Repro code attached