Do not try to quit DatabaseWorkerThreads where we have never started the event loop

src/libtomahawk/database/Database.cpp

@@ -143,11 +143,17 @@ Database::~Database()
     delete m_idWorker;
 
     if ( m_workerRW )
+    {
+        // Ensure event loop was started so quit() has any effect.
+        m_workerRW->waitForEventLoopStart();
         m_workerRW.data()->quit();
+    }
     foreach ( QPointer< DatabaseWorkerThread > workerThread, m_workerThreads )
     {
-        if ( workerThread && workerThread.data()->worker() )
-            workerThread.data()->quit();
+        // Ensure event loop was started so quit() has any effect.
+        workerThread->waitForEventLoopStart();
+        // If event loop already was killed, the following is just a no-op.
+        workerThread->quit();
     }
 
     if ( m_workerRW )

src/libtomahawk/database/DatabaseWorker.cpp

@@ -46,6 +46,7 @@ DatabaseWorkerThread::DatabaseWorkerThread( Database* db, bool mutates )
     , m_db( db )
     , m_mutates( mutates )
 {
+    m_startupMutex.lock();
 }
 
 
@@ -54,6 +55,7 @@ DatabaseWorkerThread::run()
 {
     tDebug() << Q_FUNC_INFO << "DatabaseWorkerThread starting...";
     m_worker = QPointer< DatabaseWorker >( new DatabaseWorker( m_db, m_mutates ) );
+    m_startupMutex.unlock();
     exec();
     tDebug() << Q_FUNC_INFO << "DatabaseWorkerThread finishing...";
     if ( m_worker )
@@ -73,6 +75,15 @@ DatabaseWorkerThread::worker() const
 }
 
 
+void
+DatabaseWorkerThread::waitForEventLoopStart()
+{
+    m_startupMutex.lock();
+    // no-op just to block on locking.
+    m_startupMutex.unlock();
+}
+
+
 DatabaseWorker::DatabaseWorker( Database* db, bool mutates )
     : QObject()
     , m_db( db )

src/libtomahawk/database/DatabaseWorker.h

@@ -70,6 +70,13 @@ public:
 
     QPointer< DatabaseWorker > worker() const;
 
+    /**
+     * Waits until the event loop was started.
+     *
+     * Blocking, i.e. do not call from within the DatabaseWorkerThread thread.
+     */
+    void waitForEventLoopStart();
+
 protected:
     void run();
 
@@ -77,6 +84,11 @@ private:
     QPointer< DatabaseWorker > m_worker;
     Database* m_db;
     bool m_mutates;
+
+    /**
+     * Locks until we've started the event loop.
+     */
+    QMutex m_startupMutex;
 };
 
 }