gstengine: Move pipeline id handling to base class

Move the id and the global counter from GstEnginePipeline to GstPipelineBase.
Make counter atomic.

src/engines/gstenginepipeline.cpp

@@ -48,13 +48,11 @@ const int GstEnginePipeline::kEqBandCount = 10;
 const int GstEnginePipeline::kEqBandFrequencies[] = {
     60, 170, 310, 600, 1000, 3000, 6000, 12000, 14000, 16000};
 
-int GstEnginePipeline::sId = 1;
 GstElementDeleter* GstEnginePipeline::sElementDeleter = nullptr;
 
 GstEnginePipeline::GstEnginePipeline(GstEngine* engine)
     : GstPipelineBase(),
       engine_(engine),
-      id_(sId++),
       valid_(false),
       sink_(GstEngine::kAutoSink),
       segment_start_(0),

src/engines/gstenginepipeline.h

@@ -44,9 +44,6 @@ class GstEnginePipeline : public GstPipelineBase {
   GstEnginePipeline(GstEngine* engine);
   ~GstEnginePipeline();
 
-  // Globally unique across all pipelines.
-  int id() const { return id_; }
-
   // Call these setters before Init
   void set_output_device(const QString& sink, const QVariant& device);
   void set_replaygain(bool enabled, int mode, float preamp, bool compression);
@@ -181,14 +178,6 @@ class GstEnginePipeline : public GstPipelineBase {
 
   GstEngine* engine_;
 
-  // Using == to compare two pipelines is a bad idea, because new ones often
-  // get created in the same address as old ones.  This ID will be unique for
-  // each pipeline.
-  // Threading warning: access to the static ID field isn't protected by a
-  // mutex because all pipeline creation is currently done in the main thread.
-  static int sId;
-  int id_;
-
   // General settings for the pipeline
   bool valid_;
   QString sink_;

src/engines/gstpipelinebase.cpp

@@ -19,7 +19,10 @@
 
 #include "core/logging.h"
 
-GstPipelineBase::GstPipelineBase() : QObject(nullptr), pipeline_(nullptr) {}
+std::atomic<int> GstPipelineBase::sId(1);
+
+GstPipelineBase::GstPipelineBase()
+    : QObject(nullptr), pipeline_(nullptr), id_(sId++) {}
 
 GstPipelineBase::~GstPipelineBase() {
   if (pipeline_) {

src/engines/gstpipelinebase.h

@@ -29,10 +29,20 @@ class GstPipelineBase : public QObject {
 
   virtual bool Init(const QString& name);
 
+  // Globally unique across all pipelines.
+  int id() const { return id_; }
+
   void DumpGraph();
 
  protected:
   GstElement* pipeline_;
+
+ private:
+  // Using == to compare two pipelines is a bad idea, because new ones often
+  // get created in the same address as old ones.  This ID will be unique for
+  // each pipeline.
+  static std::atomic<int> sId;
+  const int id_;
 };
 
 #endif  // GSTPIPELINEBASE_H