fix: fixes for buffer level handling, player lifecycle handling

app/src/main/cpp/PlaybackEngine.h

@@ -18,7 +18,7 @@
 /** Provides music through a regular callback to oboe. Called from separate oboe thread. */
 class MusicProvider : public AudioCallbackProvider {
 public:
-    explicit MusicProvider(RubberBand::RubberBandStretcher *stretcher, size_t buf_size_samples, int num_ch_out);
+    explicit MusicProvider(RubberBand::RubberBandStretcher *stretcher, size_t buf_size_samples, int num_ch_out, std::atomic<int> *back_pressure);
     ~MusicProvider() override;
 
     /** Called from separate oboe thread. */
@@ -30,6 +30,8 @@ private:
     int idebug;
     size_t buf_size_samples;
     int num_ch_out;
+    /** contains the current available() frames from 'stretcher' in the audio callback thread 2 (oboe) */
+    std::atomic<int> *back_pressure;
 };
 
 class PlaybackEngine : public StepListener {
@@ -47,12 +49,16 @@ private:
     std::atomic<bool> haveMusicFile;
     std::unique_ptr<std::thread> musicFeed;
     std::atomic<bool> exitMusicFeedThread;
+    /** where musicFeedThread() keeps track of the fact that we have music set -- will start the audio cb */
+    std::atomic<bool> isSetMusic;
     int android_fd;
     std::atomic<bool> haveTimeRatio;
     std::atomic<double> timeRatio;
     std::atomic<int> playbackRate;
     std::atomic<int> numOutChannels;
     std::atomic<int> numInChannels;
+    /** contains the current available() frames from 'stretcher' in the audio callback thread 2 (oboe) */
+    std::atomic<int> back_pressure;
     /** this is actually in frames, not samples */
     static size_t constexpr buf_size_samples = 1024;
     void initRubberBand();