QmlDebug: Allow retrieval of blocked engines from EngineControl

Also, don't accidentally add engines to the list on engineAdded() and
engineRemoved().

Change-Id: Ida4362eb003b85282dad17afbf0116c82f024288
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/libs/qmldebug/qmlenginecontrolclient.cpp

@@ -65,6 +65,11 @@ void QmlEngineControlClient::releaseEngine(int engineId)
     }
 }
 
+QList<int> QmlEngineControlClient::blockedEngines() const
+{
+    return m_blockedEngines.keys();
+}
+
 void QmlEngineControlClient::messageReceived(const QByteArray &data)
 {
     QPacket stream(dataStreamVersion(), data);
@@ -77,30 +82,36 @@ void QmlEngineControlClient::messageReceived(const QByteArray &data)
     if (!stream.atEnd())
         stream >> name;
 
-    EngineState &state = m_blockedEngines[id];
+    auto handleWaiting = [&](CommandType command, std::function<void()> emitter) {
-    QTC_ASSERT(state.blockers == 0 && state.releaseCommand == InvalidCommand, /**/);
+        EngineState &state = m_blockedEngines[id];
+        QTC_CHECK(state.blockers == 0);
+        QTC_CHECK(state.releaseCommand == InvalidCommand);
+        state.releaseCommand = command;
+        emitter();
+        if (state.blockers == 0) {
+            sendCommand(state.releaseCommand, id);
+            m_blockedEngines.remove(id);
+        }
+    };
 
     switch (message) {
     case EngineAboutToBeAdded:
-        state.releaseCommand = StartWaitingEngine;
+        handleWaiting(StartWaitingEngine, [&](){
-        emit engineAboutToBeAdded(id, name);
+            emit engineAboutToBeAdded(id, name);
+        });
         break;
     case EngineAdded:
         emit engineAdded(id, name);
         break;
     case EngineAboutToBeRemoved:
-        state.releaseCommand = StopWaitingEngine;
+        handleWaiting(StopWaitingEngine, [&](){
-        emit engineAboutToBeRemoved(id, name);
+            emit engineAboutToBeRemoved(id, name);
+        });
         break;
     case EngineRemoved:
         emit engineRemoved(id, name);
         break;
     }
-
-    if (state.blockers == 0 && state.releaseCommand != InvalidCommand) {
-        sendCommand(state.releaseCommand, id);
-        m_blockedEngines.remove(id);
-    }
 }
 
 void QmlEngineControlClient::sendCommand(QmlEngineControlClient::CommandType command, int engineId)

src/libs/qmldebug/qmlenginecontrolclient.h

@@ -52,6 +52,8 @@ public:
     void blockEngine(int engineId);
     void releaseEngine(int engineId);
 
+    QList<int> blockedEngines() const;
+
     void messageReceived(const QByteArray &) override;
 
 signals: