Move all buffer discard to MediaPeriod.discardBuffer

This is a step toward retaining a back-buffer in a way that
works for all MediaSource implementations. It's not possible
to adjust the discardBuffer calls in ExoPlayerImplInternal
to discard up to (position - backBufferDurationUs). Next steps
are to:

1. Find an appropriate place to specify the back buffer value,
   to be passed to the discardBuffer calls. I guess the
   LoadControl is the appropriate place to define such values.
2. Enhance discardBuffer to support a toKeyframe argument to
   pass through to discardTo.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=175565363

library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java

@@ -732,6 +732,7 @@ import java.io.IOException;
       setPlayingPeriodHolder(newPlayingPeriodHolder);
       if (playingPeriodHolder.hasEnabledTracks) {
         periodPositionUs = playingPeriodHolder.mediaPeriod.seekToUs(periodPositionUs);
+        playingPeriodHolder.mediaPeriod.discardBuffer(periodPositionUs);
       }
       resetRendererPosition(periodPositionUs);
       maybeContinueLoading();

library/core/src/main/java/com/google/android/exoplayer2/source/ExtractorMediaPeriod.java

@@ -569,7 +569,6 @@ import java.util.Arrays;
       if (!seekInsideQueue && (trackIsAudioVideoFlags[i] || !haveAudioVideoTracks)) {
         return false;
       }
-      sampleQueue.discardToRead();
     }
     return true;
   }

library/core/src/main/java/com/google/android/exoplayer2/source/chunk/ChunkSampleStream.java

@@ -110,21 +110,17 @@ public class ChunkSampleStream<T extends ChunkSource> implements SampleStream, S
     lastSeekPositionUs = positionUs;
   }
 
-  // TODO: Generalize this method to also discard from the primary sample queue and stop discarding
-  // from this queue in readData and skipData. This will cause samples to be kept in the queue until
-  // they've been rendered, rather than being discarded as soon as they're read by the renderer.
-  // This will make in-buffer seeks more likely when seeking slightly forward from the current
-  // position. This change will need handling with care, in particular when considering removal of
-  // chunks from the front of the mediaChunks list.
   /**
-   * Discards buffered media for embedded tracks, up to the specified position.
+   * Discards buffered media up to the specified position.
    *
    * @param positionUs The position to discard up to, in microseconds.
    */
-  public void discardEmbeddedTracksTo(long positionUs) {
+  public void discardBuffer(long positionUs) {
+    primarySampleQueue.discardTo(positionUs, false, true);
     for (int i = 0; i < embeddedSampleQueues.length; i++) {
       embeddedSampleQueues[i].discardTo(positionUs, true, embeddedTracksSelected[i]);
     }
+    discardDownstreamMediaChunks(primarySampleQueue.getFirstIndex());
   }
 
   /**
@@ -189,16 +185,15 @@ public class ChunkSampleStream<T extends ChunkSource> implements SampleStream, S
    */
   public void seekToUs(long positionUs) {
     lastSeekPositionUs = positionUs;
+    primarySampleQueue.rewind();
     // If we're not pending a reset, see if we can seek within the primary sample queue.
     boolean seekInsideBuffer = !isPendingReset() && (primarySampleQueue.advanceTo(positionUs, true,
         positionUs < getNextLoadPositionUs()) != SampleQueue.ADVANCE_FAILED);
     if (seekInsideBuffer) {
       // We succeeded. Discard samples and corresponding chunks prior to the seek position.
-      discardDownstreamMediaChunks(primarySampleQueue.getReadIndex());
-      primarySampleQueue.discardToRead();
       for (SampleQueue embeddedSampleQueue : embeddedSampleQueues) {
         embeddedSampleQueue.rewind();
-        embeddedSampleQueue.discardTo(positionUs, true, false);
+        embeddedSampleQueue.advanceTo(positionUs, true, false);
       }
     } else {
       // We failed, and need to restart.
@@ -261,13 +256,8 @@ public class ChunkSampleStream<T extends ChunkSource> implements SampleStream, S
     if (isPendingReset()) {
       return C.RESULT_NOTHING_READ;
     }
-    discardDownstreamMediaChunks(primarySampleQueue.getReadIndex());
-    int result = primarySampleQueue.read(formatHolder, buffer, formatRequired, loadingFinished,
+    return primarySampleQueue.read(formatHolder, buffer, formatRequired, loadingFinished,
         lastSeekPositionUs);
-    if (result == C.RESULT_BUFFER_READ) {
-      primarySampleQueue.discardToRead();
-    }
-    return result;
   }
 
   @Override
@@ -282,7 +272,6 @@ public class ChunkSampleStream<T extends ChunkSource> implements SampleStream, S
         skipCount = 0;
       }
     }
-    primarySampleQueue.discardToRead();
     return skipCount;
   }
 

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaPeriod.java

@@ -261,7 +261,7 @@ import java.util.Map;
   @Override
   public void discardBuffer(long positionUs) {
     for (ChunkSampleStream<DashChunkSource> sampleStream : sampleStreams) {
-      sampleStream.discardEmbeddedTracksTo(positionUs);
+      sampleStream.discardBuffer(positionUs);
     }
   }
 

library/hls/src/main/java/com/google/android/exoplayer2/source/hls/HlsSampleStreamWrapper.java

@@ -792,7 +792,6 @@ import java.util.LinkedList;
       if (!seekInsideQueue && (sampleQueueIsAudioVideoFlags[i] || !haveAudioVideoSampleQueues)) {
         return false;
       }
-      sampleQueue.discardToRead();
     }
     return true;
   }