libjack: Detect when LD_PRELOAD does not work

source/interposer/interposer-jack-x11.cpp

@@ -228,19 +228,21 @@ int jack_carla_interposed_action(int action, void* ptr)
     {
     case 1: // set callback
         gInterposedCallback = (CarlaInterposedCallback)ptr;
-        break;
+        return 1;
 
     case 2: // show gui
         gCurrentWindowVisible = true;
         if (gCurrentlyMappedDisplay == nullptr || gCurrentlyMappedWindow == 0)
-            break;
+            return 0;
+
         gCurrentWindowMapped = true;
         return real_XMapWindow(gCurrentlyMappedDisplay, gCurrentlyMappedWindow);
 
     case 3: // hide gui
         gCurrentWindowVisible = false;
         if (gCurrentlyMappedDisplay == nullptr || gCurrentlyMappedWindow == 0)
-            break;
+            return 0;
+
         gCurrentWindowMapped = false;
         return real_XUnmapWindow(gCurrentlyMappedDisplay, gCurrentlyMappedWindow);
 

source/libjack/libjack.cpp

@@ -30,7 +30,7 @@ CARLA_EXPORT
 int jack_carla_interposed_action(int, void*)
 {
     carla_stderr2("Non-export jack_carla_interposed_action called, this should not happen!!");
-    return 0;
+    return 1337;
 }
 
 CARLA_BACKEND_START_NAMESPACE
@@ -843,7 +843,22 @@ bool CarlaJackAppClient::handleNonRtData()
             break;
 
         case kPluginBridgeNonRtClientShowUI:
-            jack_carla_interposed_action(2, nullptr);
+            if (jack_carla_interposed_action(2, nullptr) == 1337)
+            {
+                // failed, LD_PRELOAD did not work?
+                const char* const message("Cannot show UI, LD_PRELOAD not working?");
+                const std::size_t messageSize(std::strlen(message));
+
+                const CarlaMutexLocker _cml(fShmNonRtServerControl.mutex);
+
+                fShmNonRtServerControl.writeOpcode(kPluginBridgeNonRtServerUiClosed);
+                fShmNonRtServerControl.commitWrite();
+
+                fShmNonRtServerControl.writeOpcode(kPluginBridgeNonRtServerError);
+                fShmNonRtServerControl.writeUInt(messageSize);
+                fShmNonRtServerControl.writeCustomData(message, messageSize);
+                fShmNonRtServerControl.commitWrite();
+            }
             break;
 
         case kPluginBridgeNonRtClientHideUI: