[Chromecast] Fade in after a mixer input stream has skipped

This reduces audio glitches after a stream has skipped (due to
the stream not having enough data).

BUG= internal b/30747742

Review-Url: https://codereview.chromium.org/2344993003
Cr-Commit-Position: refs/heads/master@{#419084}

chromecast/media/cma/backend/alsa/stream_mixer_alsa.cc

@@ -785,6 +785,7 @@ bool StreamMixerAlsa::TryWriteFrames() {
           frames_in_buffer < min_frames_in_buffer) {
         // If there has been (or soon will be) an underrun, continue without the
         // empty primary input stream.
+        input->OnSkipped();
         continue;
       }
 
@@ -793,16 +794,13 @@ bool StreamMixerAlsa::TryWriteFrames() {
           FROM_HERE, base::TimeDelta::FromMilliseconds(kMinBufferedDataMs / 2),
           base::Bind(&StreamMixerAlsa::WriteFrames, base::Unretained(this)));
       return false;
+    } else {
+      input->OnSkipped();
     }
   }
 
   if (active_inputs.empty()) {
-    // No inputs have any data to provide.
-    if (!inputs_.empty()) {
-      return false;  // If there are some inputs, don't fill with silence.
-    }
-
-    // If we have no inputs, fill with silence to avoid underrun.
+    // No inputs have any data to provide. Fill with silence to avoid underrun.
     chunk_size = kPreventUnderrunChunkSize;
     if (!mixed_ || mixed_->frames() < chunk_size) {
       mixed_ = ::media::AudioBus::Create(kNumOutputChannels, chunk_size);

chromecast/media/cma/backend/alsa/stream_mixer_alsa.h

@@ -93,6 +93,11 @@ class StreamMixerAlsa {
     // MaxReadSize(), and |dest->frames()| shall be >= |frames|.
     virtual void GetResampledData(::media::AudioBus* dest, int frames) = 0;
 
+    // Called when this input has been skipped for output due to not having any
+    // data available. This indicates that there will be a gap in the playback
+    // from this stream.
+    virtual void OnSkipped() = 0;
+
     // This is called for every InputQueue when the mixer writes data to ALSA
     // for any of its input streams.
     virtual void AfterWriteFrames(

chromecast/media/cma/backend/alsa/stream_mixer_alsa_input_impl.cc

@@ -235,6 +235,14 @@ void StreamMixerAlsaInputImpl::DidQueueData(bool end_of_stream) {
   mixer_->OnFramesQueued();
 }
 
+void StreamMixerAlsaInputImpl::OnSkipped() {
+  DCHECK(mixer_task_runner_->BelongsToCurrentThread());
+  if (state_ == kStateNormalPlayback) {
+    // Fade in once this input starts providing data again.
+    fade_frames_remaining_ = NormalFadeFrames();
+  }
+}
+
 void StreamMixerAlsaInputImpl::AfterWriteFrames(
     const MediaPipelineBackendAlsa::RenderingDelay& mixer_rendering_delay) {
   DCHECK(mixer_task_runner_->BelongsToCurrentThread());

chromecast/media/cma/backend/alsa/stream_mixer_alsa_input_impl.h

@@ -120,6 +120,7 @@ class StreamMixerAlsaInputImpl : public StreamMixerAlsa::InputQueue {
                       mixer_rendering_delay) override;
   int MaxReadSize() override;
   void GetResampledData(::media::AudioBus* dest, int frames) override;
+  void OnSkipped() override;
   void AfterWriteFrames(const MediaPipelineBackendAlsa::RenderingDelay&
                             mixer_rendering_delay) override;
   void SignalError(StreamMixerAlsaInput::MixerError error) override;

chromecast/media/cma/backend/alsa/stream_mixer_alsa_unittest.cc

@@ -151,6 +151,7 @@ class MockInputQueue : public StreamMixerAlsa::InputQueue {
                         mixer_rendering_delay));
   int MaxReadSize() override { return max_read_size_; }
   MOCK_METHOD2(GetResampledData, void(::media::AudioBus* dest, int frames));
+  MOCK_METHOD0(OnSkipped, void());
   MOCK_METHOD1(AfterWriteFrames,
                void(const MediaPipelineBackendAlsa::RenderingDelay&
                         mixer_rendering_delay));
@@ -376,6 +377,7 @@ TEST_F(StreamMixerAlsaTest, WriteFrames) {
   // to ALSA.
   inputs[1]->SetPrimary(false);
   inputs[1]->SetMaxReadSize(0);
+  EXPECT_CALL(*inputs[1], OnSkipped());
   inputs[2]->SetPrimary(false);
   for (auto* input : inputs) {
     if (input != inputs[1])
@@ -654,6 +656,7 @@ TEST_F(StreamMixerAlsaTest, StuckStreamWithUnderrun) {
   EXPECT_CALL(*inputs[0], GetResampledData(_, kNumFrames));
   EXPECT_CALL(*inputs[0], AfterWriteFrames(_));
   EXPECT_CALL(*inputs[1], GetResampledData(_, _)).Times(0);
+  EXPECT_CALL(*inputs[1], OnSkipped());
   EXPECT_CALL(*inputs[1], AfterWriteFrames(_));
 
   EXPECT_CALL(*mock_alsa(), PcmWritei(_, _, kNumFrames)).Times(1);
@@ -688,6 +691,7 @@ TEST_F(StreamMixerAlsaTest, StuckStreamWithLowBuffer) {
   EXPECT_CALL(*inputs[0], GetResampledData(_, kNumFrames));
   EXPECT_CALL(*inputs[0], AfterWriteFrames(_));
   EXPECT_CALL(*inputs[1], GetResampledData(_, _)).Times(0);
+  EXPECT_CALL(*inputs[1], OnSkipped());
   EXPECT_CALL(*inputs[1], AfterWriteFrames(_));
 
   EXPECT_CALL(*mock_alsa(), PcmWritei(_, _, kNumFrames)).Times(1);