https://github.com/mongodb/specifications/blob/master/source/transactions/transactions.rst#endsession-changes says:

> endSession changes
>
> This method ends a ClientSession. Drivers MUST call abortTransaction if this session is in the "transaction in progress" state in order to release resources on the server. Drivers MUST ignore any errors raised by abortTransaction while ending a session.

In a driver implementing server session pool, endSession returns the session to the pool rather than ends it. The session becomes available for future use.

If a session had a transaction on it, and aborting the transaction failed, should the session still be returned to the pool? I think in this case the session should be discarded.