gstengine: Add a DumpGraph method to GstEnginePipeline

This will write a .dot graph and can be integrated with the debug console.

src/engines/gstengine.cpp

@@ -143,6 +143,14 @@ GstEngine::~GstEngine() {
 }
 
 bool GstEngine::Init() {
+  // This environment variable is required for GST_DEBUG_BIN_TO_DOT_FILE macros.
+  // Gstreamer only reads it on init, so make sure it's set now.
+  QByteArray path = qgetenv("GST_DEBUG_DUMP_DOT_DIR");
+  if (path.isEmpty()) {
+    path = QDir::currentPath().toUtf8();
+    qputenv("GST_DEBUG_DUMP_DOT_DIR", path);
+  }
+
   initialising_ = QtConcurrent::run(this, &GstEngine::InitialiseGstreamer);
   return true;
 }

src/engines/gstenginepipeline.cpp

@@ -1332,3 +1332,12 @@ void GstEnginePipeline::SetNextReq(const MediaPlaybackRequest& req,
   next_beginning_offset_nanosec_ = beginning_nanosec;
   next_end_offset_nanosec_ = end_nanosec;
 }
+
+void GstEnginePipeline::DumpGraph() {
+#ifdef GST_DISABLE_GST_DEBUG
+  qLog(Debug) << "Cannot dump graph. gstreamer debug is not enabled.";
+#else
+  qLog(Debug) << "Dumping pipeline graph";
+  GST_DEBUG_BIN_TO_DOT_FILE_WITH_TS(GST_BIN(pipeline_), GST_DEBUG_GRAPH_SHOW_ALL, "pipeline");
+#endif
+}

src/engines/gstenginepipeline.h

@@ -106,6 +106,7 @@ class GstEnginePipeline : public QObject {
 
   QString source_device() const { return source_device_; }
 
+  void DumpGraph();
  public slots:
   void SetVolumeModifier(qreal mod);