If we're not crossfading, keep the same pipeline when changing tracks and just swap out the gstreamer source - this should allow for completely gapless playback.

src/engines/gstengine.cpp

@@ -350,16 +350,25 @@ void GstEngine::UpdateScope() {
 }
 
 void GstEngine::StartPreloading(const QUrl& url) {
-  preload_pipeline_ = CreatePipeline(url);
-  if (!preload_pipeline_)
-    return;
+  if (autocrossfade_enabled_) {
+    // Have to create a new pipeline so we can crossfade between the two
 
-  // We don't want to get metadata messages before the track starts playing -
-  // we reconnect this in GstEngine::Load
-  disconnect(preload_pipeline_.get(), SIGNAL(MetadataFound(Engine::SimpleMetaBundle)), this, 0);
+    preload_pipeline_ = CreatePipeline(url);
+    if (!preload_pipeline_)
+      return;
 
-  preloaded_url_ = url;
-  preload_pipeline_->SetState(GST_STATE_PAUSED);
+    // We don't want to get metadata messages before the track starts playing -
+    // we reconnect this in GstEngine::Load
+    disconnect(preload_pipeline_.get(), SIGNAL(MetadataFound(Engine::SimpleMetaBundle)), this, 0);
+
+    preloaded_url_ = url;
+    preload_pipeline_->SetState(GST_STATE_PAUSED);
+  } else {
+    // No crossfading, so we can just queue the new URL in the existing
+    // pipeline and get gapless playback (hopefully)
+    if (current_pipeline_)
+      current_pipeline_->SetNextUrl(url);
+  }
 }
 
 bool GstEngine::Load(const QUrl& url, Engine::TrackChangeType change) {
@@ -375,6 +384,12 @@ bool GstEngine::Load(const QUrl& url, Engine::TrackChangeType change) {
                          ((crossfade_enabled_ && change == Engine::Manual) ||
                           (autocrossfade_enabled_ && change == Engine::Auto));
 
+  if (!crossfade && current_pipeline_ && current_pipeline_->url() == url) {
+    // We're not crossfading, and the pipeline is already playing the URI we
+    // want, so just do nothing.
+    return true;
+  }
+
   shared_ptr<GstEnginePipeline> pipeline;
   if (preload_pipeline_ && preloaded_url_ == url) {
     pipeline = preload_pipeline_;
@@ -545,8 +560,10 @@ void GstEngine::HandlePipelineError(const QString& message) {
 }
 
 
-void GstEngine::EndOfStreamReached() {
-  current_pipeline_.reset();
+void GstEngine::EndOfStreamReached(bool has_next_track) {
+  if (!has_next_track)
+    current_pipeline_.reset();
+  ClearScopeBuffers();
   emit TrackEnded();
 }
 
@@ -603,7 +620,7 @@ shared_ptr<GstEnginePipeline> GstEngine::CreatePipeline(const QUrl& url) {
   ret->set_forwards_buffers(true);
   ret->set_output_device(sink_, device_);
 
-  connect(ret.get(), SIGNAL(EndOfStreamReached()), SLOT(EndOfStreamReached()));
+  connect(ret.get(), SIGNAL(EndOfStreamReached(bool)), SLOT(EndOfStreamReached(bool)));
   connect(ret.get(), SIGNAL(BufferFound(GstBuffer*)), SLOT(AddBufferToScope(GstBuffer*)));
   connect(ret.get(), SIGNAL(Error(QString)), SLOT(HandlePipelineError(QString)));
   connect(ret.get(), SIGNAL(MetadataFound(Engine::SimpleMetaBundle)),

src/engines/gstengine.h

@@ -97,7 +97,7 @@ class GstEngine : public Engine::Base {
   void timerEvent(QTimerEvent*);
 
  private slots:
-  void EndOfStreamReached();
+  void EndOfStreamReached(bool has_next_track);
   void HandlePipelineError(const QString& message);
   void NewMetaData(const Engine::SimpleMetaBundle& bundle);
   void AddBufferToScope(GstBuffer* buf);

src/engines/gstenginepipeline.cpp

@@ -36,8 +36,7 @@ GstEnginePipeline::GstEnginePipeline(GstEngine* engine)
     equalizer_(NULL),
     volume_(NULL),
     audioscale_(NULL),
-    audiosink_(NULL),
-    event_cb_id_(0)
+    audiosink_(NULL)
 {
 }
 
@@ -46,8 +45,36 @@ void GstEnginePipeline::set_output_device(const QString &sink, const QString &de
   device_ = device;
 }
 
+bool GstEnginePipeline::StopUriDecodeBin(gpointer bin) {
+  gst_element_set_state(GST_ELEMENT(bin), GST_STATE_NULL);
+  return false; // So it doesn't get called again
+}
+
+bool GstEnginePipeline::ReplaceDecodeBin(const QUrl& url) {
+  GstElement* new_bin = engine_->CreateElement("uridecodebin");
+  if (!new_bin) return false;
+
+  // Destroy the old one, if any
+  if (uridecodebin_) {
+    gst_bin_remove(GST_BIN(pipeline_), uridecodebin_);
+
+    // Set its state to NULL later in the main thread
+    g_idle_add(GSourceFunc(StopUriDecodeBin), uridecodebin_);
+  }
+
+  uridecodebin_ = new_bin;
+  gst_bin_add(GST_BIN(pipeline_), uridecodebin_);
+
+  g_object_set(G_OBJECT(uridecodebin_), "uri", url.toEncoded().constData(), NULL);
+  g_signal_connect(G_OBJECT(uridecodebin_), "pad-added", G_CALLBACK(NewPadCallback), this);
+  g_signal_connect(G_OBJECT(uridecodebin_), "drained", G_CALLBACK(SourceDrainedCallback), this);
+
+  return true;
+}
+
 bool GstEnginePipeline::Init(const QUrl &url) {
   pipeline_ = gst_pipeline_new("pipeline");
+  url_ = url;
 
   // Here we create all the parts of the gstreamer pipeline - from the source
   // to the sink.  The parts of the pipeline are split up into bins:
@@ -59,16 +86,7 @@ bool GstEnginePipeline::Init(const QUrl &url) {
   //   audiosink
 
   // Decode bin
-  if (!(uridecodebin_ = engine_->CreateElement("uridecodebin", pipeline_))) { return false; }
-  g_object_set(G_OBJECT(uridecodebin_), "uri", url.toEncoded().constData(), NULL);
-  g_signal_connect(G_OBJECT(uridecodebin_), "pad-added", G_CALLBACK(NewPadCallback), this);
-
-  // Does some stuff with ghost pads
-  /*GstPad* pad = gst_element_get_pad(uridecodebin_, "sink");
-  if (pad) {
-    event_cb_id_ = gst_pad_add_event_probe (pad, G_CALLBACK(EventCallback), this);
-    gst_object_unref(pad);
-  }*/
+  if (!ReplaceDecodeBin(url)) return false;
 
   // Audio bin
   audiobin_ = gst_bin_new("audiobin");
@@ -119,13 +137,6 @@ bool GstEnginePipeline::Init(const QUrl &url) {
 }
 
 GstEnginePipeline::~GstEnginePipeline() {
-  // We don't want an EOS signal from the decodebin
-  if (uridecodebin_) {
-    GstPad *p = gst_element_get_pad(uridecodebin_, "sink");
-    if (p)
-      gst_pad_remove_event_probe(p, event_cb_id_);
-  }
-
   if (pipeline_) {
     gst_bus_set_sync_handler(gst_pipeline_get_bus(GST_PIPELINE(pipeline_)), NULL, NULL);
     g_source_remove(bus_cb_id_);
@@ -158,7 +169,7 @@ GstBusSyncReply GstEnginePipeline::BusCallbackSync(GstBus*, GstMessage* msg, gpo
   GstEnginePipeline* instance = reinterpret_cast<GstEnginePipeline*>(self);
   switch (GST_MESSAGE_TYPE(msg)) {
     case GST_MESSAGE_EOS:
-      emit instance->EndOfStreamReached();
+      emit instance->EndOfStreamReached(false);
       break;
 
     case GST_MESSAGE_TAG:
@@ -249,7 +260,7 @@ void GstEnginePipeline::EventCallback(GstPad*, GstEvent* event, gpointer self) {
 
   switch(event->type) {
     case GST_EVENT_EOS:
-      emit instance->EndOfStreamReached();
+      emit instance->EndOfStreamReached(false);
       break;
 
     default:
@@ -257,6 +268,21 @@ void GstEnginePipeline::EventCallback(GstPad*, GstEvent* event, gpointer self) {
   }
 }
 
+void GstEnginePipeline::SourceDrainedCallback(GstURIDecodeBin* bin, gpointer self) {
+  GstEnginePipeline* instance = reinterpret_cast<GstEnginePipeline*>(self);
+
+  if (instance->next_url_.isValid()) {
+    instance->ReplaceDecodeBin(instance->next_url_);
+    gst_element_set_state(instance->uridecodebin_, GST_STATE_PLAYING);
+
+    instance->url_ = instance->next_url_;
+    instance->next_url_ = QUrl();
+
+    // This just tells the UI that we've moved on to the next song
+    emit instance->EndOfStreamReached(true);
+  }
+}
+
 qint64 GstEnginePipeline::position() const {
   GstFormat fmt = GST_FORMAT_TIME;
   gint64 value = 0;

src/engines/gstenginepipeline.h

@@ -27,6 +27,8 @@
 
 class GstEngine;
 
+struct GstURIDecodeBin;
+
 class GstEnginePipeline : public QObject {
   Q_OBJECT
 
@@ -51,7 +53,12 @@ class GstEnginePipeline : public QObject {
                   QTimeLine::Direction direction = QTimeLine::Forward,
                   QTimeLine::CurveShape shape = QTimeLine::LinearCurve);
 
+  // If this is set then it will be loaded automatically when playback finishes
+  // for gapless playback
+  void SetNextUrl(const QUrl& url) { next_url_ = url; }
+
   // Get information about the music playback
+  QUrl url() const { return url_; }
   bool is_valid() const { return valid_; }
   qint64 position() const;
   qint64 length() const;
@@ -61,7 +68,7 @@ class GstEnginePipeline : public QObject {
   void SetVolumeModifier(qreal mod);
 
  signals:
-  void EndOfStreamReached();
+  void EndOfStreamReached(bool has_next_track);
   void MetadataFound(const Engine::SimpleMetaBundle& bundle);
   void BufferFound(GstBuffer* buf);
   void Error(const QString& message);
@@ -75,11 +82,14 @@ class GstEnginePipeline : public QObject {
   static void NewPadCallback(GstElement*, GstPad*, gpointer);
   static bool HandoffCallback(GstPad*, GstBuffer*, gpointer);
   static void EventCallback(GstPad*, GstEvent*, gpointer);
+  static void SourceDrainedCallback(GstURIDecodeBin*, gpointer);
+  static bool StopUriDecodeBin(gpointer bin);
   void TagMessageReceived(GstMessage*);
   QString ParseTag(GstTagList* list, const char* tag) const;
   void ErrorMessageReceived(GstMessage*);
 
   void UpdateVolume();
+  bool ReplaceDecodeBin(const QUrl& url);
 
  private:
   static const int kGstStateTimeoutNanosecs = 10000000;
@@ -91,6 +101,9 @@ class GstEnginePipeline : public QObject {
   QString device_;
   bool forwards_buffers_;
 
+  QUrl url_;
+  QUrl next_url_;
+
   int volume_percent_;
   qreal volume_modifier_;
 
@@ -111,7 +124,6 @@ class GstEnginePipeline : public QObject {
   GstElement* audioscale_;
   GstElement* audiosink_;
 
-  uint event_cb_id_;
   uint bus_cb_id_;
 };
 

src/player.cpp

@@ -156,7 +156,7 @@ void Player::NextInternal(Engine::TrackChangeType change) {
     return;
   }
 
-  NextItem(Engine::Manual);
+  NextItem(change);
 }
 
 void Player::NextItem(Engine::TrackChangeType change) {