When logical sessions are implicitly created in the shell, they bump the ref-count of the client that created them, so that they can end the session on the same connection. The close() method on the Mongo connection works by resetting the shared_ptr<DBClientBase> that holds the underlying C++ client. Since the implicit session now holds a copy of that shared_ptr, close() basically does nothing.

There seem to be two options:

• Have the Session object hold a weak_ptr to its DBClientBase, and lock/check the client before trying to end the session. The downside here is that the session might be finalized before the client and we'd leak the session on the server.
• Have the client keep track of its default session and end it during its finalizer.