Safely close database connections and delete backends

Also fix NewClosure leak caused by disconnected object signals

src/internet/internetservice.h

@@ -40,6 +40,7 @@ class InternetService : public QObject {
   InternetService(Song::Source source, const QString &name, const QString &url_scheme, Application *app, QObject *parent = nullptr);
 
   virtual ~InternetService() {}
+  virtual void Exit() {}
 
   virtual Song::Source source() const { return source_; }
   virtual QString name() const { return name_; }
@@ -75,6 +76,7 @@ class InternetService : public QObject {
   virtual void ResetSongsRequest() {}
 
  signals:
+  void ExitFinished();
   void Login();
   void Logout();
   void Login(const QString &api_token, const QString &username, const QString &password);

src/internet/internetservices.cpp

@@ -72,3 +72,22 @@ void InternetServices::ReloadSettings() {
     service->ReloadSettings();
   }
 }
+
+void InternetServices::Exit() {
+
+  for (InternetService *service : services_.values()) {
+    wait_for_exit_ << service;
+    connect(service, SIGNAL(ExitFinished()), this, SLOT(ExitReceived()));
+    service->Exit();
+  }
+
+}
+
+void InternetServices::ExitReceived() {
+
+  InternetService *service = qobject_cast<InternetService*>(sender());
+
+  wait_for_exit_.removeAll(service);
+  if (wait_for_exit_.isEmpty()) emit ExitFinished();
+
+}

src/internet/internetservices.h

@@ -47,9 +47,17 @@ class InternetServices : public QObject {
   void AddService(InternetService *service);
   void RemoveService(InternetService *service);
   void ReloadSettings();
+  void Exit();
+
+ signals:
+  void ExitFinished();
+
+ private slots:
+  void ExitReceived();
 
  private:
   QMap<Song::Source, InternetService*> services_;
+  QList<InternetService*> wait_for_exit_;
 
 };