_runningSessionCount is atomically updated to track the number of sessions currently active.

However, it is incremented by a per-session thread created in createSession. This means after createSession returns, there may be a window where _runningSessionCount reports 0 while a session really exists.

Increment the counter in createSession instead.

That is, src/mongo/replay/session_handler.cpp:55

    std::thread([session = std::move(session), this] {
        ++_runningSessionCount;

becomes:

    ++_runningSessionCount;
    std::thread([session = std::move(session), this] {