Allow LoadControl to configure the back-buffer.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=175833398

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

@@ -25,7 +25,7 @@ import com.google.android.exoplayer2.util.Util;
 /**
  * The default {@link LoadControl} implementation.
  */
-public final class DefaultLoadControl implements LoadControl {
+public class DefaultLoadControl implements LoadControl {
 
   /**
    * The default minimum duration of media that the player will attempt to ensure is buffered at all
@@ -164,6 +164,16 @@ public final class DefaultLoadControl implements LoadControl {
   }
 
   @Override
+  public long getBackBufferDurationUs() {
+    return 0;
+  }
+
+  @Override
+  public boolean retainBackBufferFromKeyframe() {
+    return false;
+  }
+
+  @Override
   public boolean shouldStartPlayback(long bufferedDurationUs, boolean rebuffering) {
     long minBufferDurationUs = rebuffering ? bufferForPlaybackAfterRebufferUs : bufferForPlaybackUs;
     return minBufferDurationUs <= 0 || bufferedDurationUs >= minBufferDurationUs;

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

@@ -107,6 +107,8 @@ import java.io.IOException;
   private final Timeline.Window window;
   private final Timeline.Period period;
   private final MediaPeriodInfoSequence mediaPeriodInfoSequence;
+  private final long backBufferDurationUs;
+  private final boolean retainBackBufferFromKeyframe;
 
   private PlaybackInfo playbackInfo;
   private PlaybackParameters playbackParameters;
@@ -147,6 +149,9 @@ import java.io.IOException;
     this.state = Player.STATE_IDLE;
     this.player = player;
 
+    backBufferDurationUs = loadControl.getBackBufferDurationUs();
+    retainBackBufferFromKeyframe = loadControl.retainBackBufferFromKeyframe();
+
     playbackInfo = new PlaybackInfo(null, null, 0, C.TIME_UNSET);
     rendererCapabilities = new RendererCapabilities[renderers.length];
     for (int i = 0; i < renderers.length; i++) {
@@ -541,7 +546,8 @@ import java.io.IOException;
     TraceUtil.beginSection("doSomeWork");
 
     updatePlaybackPositions();
-    playingPeriodHolder.mediaPeriod.discardBuffer(playbackInfo.positionUs, false);
+    playingPeriodHolder.mediaPeriod.discardBuffer(playbackInfo.positionUs - backBufferDurationUs,
+        retainBackBufferFromKeyframe);
 
     boolean allRenderersEnded = true;
     boolean allRenderersReadyOrEnded = true;
@@ -732,7 +738,8 @@ import java.io.IOException;
       setPlayingPeriodHolder(newPlayingPeriodHolder);
       if (playingPeriodHolder.hasEnabledTracks) {
         periodPositionUs = playingPeriodHolder.mediaPeriod.seekToUs(periodPositionUs);
-        playingPeriodHolder.mediaPeriod.discardBuffer(periodPositionUs, false);
+        playingPeriodHolder.mediaPeriod.discardBuffer(periodPositionUs - backBufferDurationUs,
+            retainBackBufferFromKeyframe);
       }
       resetRendererPosition(periodPositionUs);
       maybeContinueLoading();

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

@@ -56,6 +56,38 @@ public interface LoadControl {
   Allocator getAllocator();
 
   /**
+   * Returns the duration of media to retain in the buffer prior to the current playback position,
+   * for fast backward seeking.
+   * <p>
+   * Note: If {@link #retainBackBufferFromKeyframe()} is false then seeking in the back-buffer will
+   * only be fast if the back-buffer contains a keyframe prior to the seek position.
+   * <p>
+   * Note: Implementations should return a single value. Dynamic changes to the back-buffer are not
+   * currently supported.
+   *
+   * @return The duration of media to retain in the buffer prior to the current playback position,
+   *     in microseconds.
+   */
+  long getBackBufferDurationUs();
+
+  /**
+   * Returns whether media should be retained from the keyframe before the current playback position
+   * minus {@link #getBackBufferDurationUs()}, rather than any sample before or at that position.
+   * <p>
+   * Warning: Returning true will cause the back-buffer size to depend on the spacing of keyframes
+   * in the media being played. Returning true is not recommended unless you control the media and
+   * are comfortable with the back-buffer size exceeding {@link #getBackBufferDurationUs()} by as
+   * much as the maximum duration between adjacent keyframes in the media.
+   * <p>
+   * Note: Implementations should return a single value. Dynamic changes to the back-buffer are not
+   * currently supported.
+   *
+   * @return Whether media should be retained from the keyframe before the current playback position
+   * minus {@link #getBackBufferDurationUs()}, rather than any sample before or at that position.
+   */
+  boolean retainBackBufferFromKeyframe();
+
+  /**
    * Called by the player to determine whether sufficient media is buffered for playback to be
    * started or resumed.
    *