In MongoDB 4.0, when the killCursors command is issued as part of a transaction, it checks out the session and uses the same locker as the transaction. This is important because killCursors must take locks, so if it used a different locker, then it could block behind a DDL operation that is blocked behind the transaction, preventing the killCursors (and the transaction that it is part of) from making progress.

When all cursors are globally managed, killCursors will no longer need to take locks. This means it no longer needs to use the same locker as the transaction. It also means that killCursors will trigger an invariant if it is the first operation in the transaction, since we expect the locker to be non-empty when we release its WT ticket and stash transaction resources.

We have a few options for how to address this. We could ban killCursors from being the first statement in a transaction. We could ban killCursors in transactions entirely (though this would require drivers changes). Or we could allow transactions to stash an empty locker.