The way _runningSessionCount is updated here permits a concurrent call to waitForRunningSessions to observe _runningSessionCount == 0 before the last thread has called notify().

Then, the SessionHandler may be destroyed, making the notify() call unsafe.

To prevent this, _notificationMutex should be acquired for the duration of the decrement and notify call.

A concurrent waitForRunningSessions call will then either:

• Block on the lock, then correctly observe sessions == 0 once the dying thread releases the lock and it is safe to destroy the session handler.
• Acquire the lock, observe sessions == 1, wait on CV (releases the lock), wake when dying thread calls notify, re-acquire lock, observe sessions == 0.