WSREP: server: node3 unallowed state transition: disconnecting -> synced

[ayurchen]

To avoid concurrent calls to provider().disconnect() wsrep-lib switches to disconnecting state before the call:

int wsrep::server_state::disconnect()
{
    {
        wsrep::unique_lock<wsrep::mutex> lock(mutex_);
        // In case of failure situations which are caused by provider
        // being shut down some failing operation may also try to shut
        // down the replication. Check the state here and
        // return success if the provider disconnect is already in progress
        // or has completed.
        if (state(lock) == s_disconnecting || state(lock) == s_disconnected)
        {
            return 0;
        }
        state(lock, s_disconnecting);
        interrupt_state_waiters(lock);
    }
    return provider().disconnect();
}

However this creates a race where wsrep-lib is already disconnecting while provider is still connected and is delivering events including state changing events:

2025-04-07 10:42:44 0 [Note] WSREP: Shutdown replication
2025-04-07 10:42:44 0 [Note] WSREP: Disconnect provider
2025-04-07 10:42:44 0 [Note] WSREP: server node3 state change: joined -> disconnecting
2025-04-07 10:42:44 0 [Note] WSREP: Server status change joined -> disconnecting
2025-04-07 10:42:44 0 [Note] WSREP: Member 1.0 (node3) synced with group.
2025-04-07 10:42:44 0 [Note] WSREP: Setting wsrep_ready to 0
2025-04-07 10:42:44 0 [Note] WSREP: wsrep_notify_status server not yet ready : wsrep_ready=0 status 8
2025-04-07 10:42:44 0 [Note] WSREP: Processing event queue:... 100.0% (1/1 events) complete.
2025-04-07 10:42:44 0 [Note] WSREP: Shifting JOINED -> SYNCED (TO: 1761)
2025-04-07 10:42:44 0 [Note] WSREP: Closing send monitor...
2025-04-07 10:42:44 0 [Note] WSREP: Closed send monitor.
2025-04-07 10:42:44 0 [Note] WSREP: gcomm: terminating thread
2025-04-07 10:42:44 0 [Note] WSREP: gcomm: joining thread
2025-04-07 10:42:44 0 [Note] WSREP: gcomm: closing backend
2025-04-07 10:42:44 1 [Note] WSREP: Server node3 synced with group
2025-04-07 10:42:44 1 [Warning] WSREP: server: node3 unallowed state transition: disconnecting -> synced

Such events should be simply ignored in disconnecting state.

Moved here from codership/galera-bugs#1063