Wait for set state to finish before deleting pipeline

Setting state to GST_STATE_NULL sometimes blocks, to fix this use the threadpool to set the state to NULL and wait with deleting the pipeline until the state is changed.
This fixes blocking the main thread when switching Spotify songs.

src/engine/gstenginepipeline.h

@@ -46,6 +46,7 @@
 #include "core/shared_ptr.h"
 #include "enginemetadata.h"
 
+class QTimer;
 class QTimerEvent;
 class GstBufferConsumer;
 struct GstPlayBin;
@@ -80,6 +81,8 @@ class GstEnginePipeline : public QObject {
   void set_spotify_login(const QString &spotify_username, const QString &spotify_password);
 #endif
 
+  bool Finish();
+
   // Creates the pipeline, returns false on error
   bool InitFromUrl(const QUrl &media_url, const QUrl &stream_url, const QByteArray &gst_url, const qint64 end_nanosec, const double ebur128_loudness_normalizing_gain_db, QString &error);
 
@@ -89,10 +92,10 @@ class GstEnginePipeline : public QObject {
   void RemoveAllBufferConsumers();
 
   // Control the music playback
-  Q_INVOKABLE QFuture<GstStateChangeReturn> SetState(const GstState state);
-  void SetStateDelayed(const GstState state);
+  Q_INVOKABLE QFuture<GstStateChangeReturn> SetStateAsync(const GstState state);
+  Q_INVOKABLE QFuture<GstStateChangeReturn> Play(const bool pause, const quint64 offset_nanosec);
   Q_INVOKABLE bool Seek(const qint64 nanosec);
-  void SeekQueued(const qint64 nanosec);
+  void SeekAsync(const qint64 nanosec);
   void SeekDelayed(const qint64 nanosec);
   void SetEBUR128LoudnessNormalizingGain_dB(const double ebur128_loudness_normalizing_gain_db);
   void SetVolume(const uint volume_percent);
@@ -133,6 +136,8 @@ class GstEnginePipeline : public QObject {
 
   QString source_device() const { return source_device_; }
 
+  bool exclusive_mode() const { return exclusive_mode_; }
+
  public slots:
   void SetFaderVolume(const qreal volume);
 
@@ -143,7 +148,7 @@ class GstEnginePipeline : public QObject {
   void MetadataFound(const int pipeline_id, const EngineMetadata &bundle);
 
   void VolumeChanged(const uint volume);
-  void FaderFinished();
+  void FaderFinished(const int pipeline_id);
 
   void BufferingStarted();
   void BufferingProgress(const int percent);
@@ -151,6 +156,10 @@ class GstEnginePipeline : public QObject {
 
   void AboutToFinish();
 
+  void Finished();
+
+  void SetStateFinished(const GstStateChangeReturn state);
+
  protected:
   void timerEvent(QTimerEvent*) override;
 
@@ -189,7 +198,12 @@ class GstEnginePipeline : public QObject {
   void UpdateStereoBalance();
   void UpdateEqualizer();
 
+  void Disconnect();
+
+  void ResumeFaderAsync();
+
  private slots:
+  void SetStateAsyncFinished(const GstState state, const GstStateChangeReturn state_change);
   void FaderTimelineFinished();
 
  private:
@@ -290,6 +304,8 @@ class GstEnginePipeline : public QObject {
   // Also, we have to wait for the playbin to be connected.
   bool pipeline_connected_;
   bool pipeline_active_;
+
+  GstState pending_state_;
   qint64 pending_seek_nanosec_;
 
   // We can only use gst_element_query_position() when the pipeline is in
@@ -342,6 +358,10 @@ class GstEnginePipeline : public QObject {
 
   bool about_to_finish_;
 
+  bool finish_requested_;
+  bool finished_;
 };
 
+using GstEnginePipelinePtr = SharedPtr<GstEnginePipeline>;
+
 #endif  // GSTENGINEPIPELINE_H