assistant: fix garbage data at end of audio stream.

Bug: b/112431554
Test: Manual
Change-Id: I10e52dad59b2989fe9ba223b3c8cf130143ea9b8
Reviewed-on: https://chromium-review.googlesource.com/1170031
Commit-Queue: Muyuan Li <muyuanli@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581962}

chromeos/services/assistant/platform/audio_output_provider_impl.cc

@@ -279,7 +279,23 @@ int AudioDeviceOwner::Render(base::TimeDelta delay,
     // very beginning.
     return 0;
   }
-  audio_fifo_->Consume()->CopyTo(dest);
+
+  int available_frames = audio_fifo_->GetAvailableFrames();
+  if (available_frames < dest->frames()) {
+    // audio_fifo_->Consume() returns a whole audio bus. In our case,
+    // bus->frames() == dest->frames(). If there aren't enough frames to fill
+    // the bus, the rest of it would be garbage data. In such case,
+    // audio_fifo_->GetAvailableFrames() < dest->frames(). We use
+    // bus->CopyPartialFrames() to only copy the valid frames and zero out
+    // the rest to avoid playing the garbage.
+    const media::AudioBus* bus = audio_fifo_->Consume();
+
+    DCHECK(available_frames <= bus->frames());
+
+    bus->CopyPartialFramesTo(0, available_frames, 0, dest);
+  } else {
+    audio_fifo_->Consume()->CopyTo(dest);
+  }
   ScheduleFillLocked(base::TimeTicks::Now());
   return dest->frames();
 }