Try to fix rare GUI crash, likely caused by pushing a screen in the

network thread while the gui is being drawn. Made the messages
causing this synchronous, i.e. executed by the main thread.

src/network/protocols/client_lobby.cpp

@@ -237,6 +237,7 @@ bool ClientLobby::notifyEvent(Event* event)
               message_type);
     switch(message_type)
     {
+        case LE_START_SELECTION:       startSelection(event);      break;
         case LE_KART_SELECTION_UPDATE: kartSelectionUpdate(event); break;
         case LE_LOAD_WORLD:            loadWorld();                break;
         case LE_RACE_FINISHED:         raceFinished(event);        break;
@@ -266,7 +267,6 @@ bool ClientLobby::notifyEventAsynchronous(Event* event)
             case LE_NEW_PLAYER_CONNECTED: newPlayer(event);              break;
             case LE_PLAYER_DISCONNECTED : disconnectedPlayer(event);     break;
             case LE_START_RACE: startGame(event);                        break;
-            case LE_START_SELECTION: startSelection(event);              break;
             case LE_CONNECTION_REFUSED: connectionRefused(event);        break;
             case LE_CONNECTION_ACCEPTED: connectionAccepted(event);      break;
             case LE_KART_SELECTION_REFUSED: kartSelectionRefused(event); break;

src/network/protocols/server_lobby.cpp

@@ -122,6 +122,29 @@ void ServerLobby::setup()
 
 }   // setup
 
+//-----------------------------------------------------------------------------
+bool ServerLobby::notifyEvent(Event* event)
+{
+    assert(m_game_setup); // assert that the setup exists
+    if (event->getType() != EVENT_TYPE_MESSAGE)
+        return false;
+
+    NetworkString &data = event->data();
+    assert(data.size()); // message not empty
+    uint8_t message_type;
+    message_type = data.getUInt8();
+    Log::info("ServerLobby", "Synchronous message received with type %d.",
+              message_type);
+    switch (message_type)
+    {
+    case LE_REQUEST_BEGIN: startSelection(event); break;
+    default: Log::error("ServerLobby", "Unknown message type %d - ignored.",
+                        message_type);
+             break;
+    }   // switch message_type
+    return true;
+}   // notifyEvent
+
 //-----------------------------------------------------------------------------
 
 bool ServerLobby::notifyEventAsynchronous(Event* event)
@@ -138,7 +161,6 @@ bool ServerLobby::notifyEventAsynchronous(Event* event)
         switch(message_type)
         {
         case LE_CONNECTION_REQUESTED: connectionRequested(event); break;
-        case LE_REQUEST_BEGIN: startSelection(event);             break;
         case LE_KART_SELECTION: kartSelectionRequested(event);    break;
         case LE_CLIENT_LOADED_WORLD: finishedLoadingWorldClient(event); break;
         case LE_STARTED_RACE:  startedRaceOnClient(event);        break;
@@ -357,7 +379,9 @@ void ServerLobby::startSelection(const Event *event)
     }
     const std::vector<STKPeer*> &peers = STKHost::get()->getPeers();
     NetworkString *ns = getNetworkString(1);
-    // start selection
+    // Start selection - must be synchronous since the receiver pushes
+    // a new screen, which must be donefrom the main thread.
+    ns->setSynchronous(true);
     ns->addUInt8(LE_START_SELECTION);
     sendMessageToPeersChangingToken(ns, /*reliable*/true);
     delete ns;

src/network/protocols/server_lobby.hpp

@@ -76,6 +76,7 @@ public:
     virtual ~ServerLobby();
 
     virtual bool notifyEventAsynchronous(Event* event) OVERRIDE;
+    virtual bool notifyEvent(Event* event) OVERRIDE;
     virtual void setup() OVERRIDE;
     virtual void update(float dt) OVERRIDE;
     virtual void asynchronousUpdate() OVERRIDE {};

src/states_screens/networking_lobby.cpp

@@ -170,6 +170,7 @@ void NetworkingLobby::eventCallback(Widget* widget, const std::string& name,
         {
             // Send a message to the server to start
             NetworkString start(PROTOCOL_LOBBY_ROOM);
+            start.setSynchronous(true);
             start.addUInt8(LobbyProtocol::LE_REQUEST_BEGIN);
             STKHost::get()->sendToServer(&start, true);
         }