Move ExtractorMediaPeriod to new SampleQueue methods

This change allows you to enable/disable tracks within which
all samples are key-frames without any re-buffering (e.g. audio,
text and metadata). This effectively reverts V2 back to the
behavior in V1, only this time we're doing it properly. []ly
disabling/enabling, or disabling/enabling whilst paused, no longer
cause samples to get "lost" between the source and renderers.

Note it also becomes really easy to support a few other things,
although support is not exposed in this change:

- Enable/disable video tracks without any re-buffering, by
  changing the toKeyframe argument passed to discardTo to true.
- Retain media in the buffer for some time after it's been played
  (e.g. to support a single back-5s-seek efficiently), by
  subtracting the desired back-buffer time from the value that's
  passed to discardTo.

Issue: #2956
Issue: #2926

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=160128586

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

@@ -47,7 +47,8 @@ import java.io.IOException;
  * A {@link MediaPeriod} that extracts data using an {@link Extractor}.
  */
 /* package */ final class ExtractorMediaPeriod implements MediaPeriod, ExtractorOutput,
-    Loader.Callback<ExtractorMediaPeriod.ExtractingLoadable>, UpstreamFormatChangedListener {
+    Loader.Callback<ExtractorMediaPeriod.ExtractingLoadable>, Loader.ReleaseCallback,
+    UpstreamFormatChangedListener {
 
   /**
    * When the source's duration is unknown, it is calculated by adding this value to the largest
@@ -146,21 +147,27 @@ import java.io.IOException;
   }
 
   public void release() {
-    final ExtractorHolder extractorHolder = this.extractorHolder;
-    loader.release(new Runnable() {
-      @Override
-      public void run() {
-        extractorHolder.release();
-        int trackCount = sampleQueues.size();
-        for (int i = 0; i < trackCount; i++) {
-          sampleQueues.valueAt(i).disable();
-        }
+    boolean releasedSynchronously = loader.release(this);
+    if (!releasedSynchronously) {
+      // Discard as much as we can synchronously.
+      int trackCount = sampleQueues.size();
+      for (int i = 0; i < trackCount; i++) {
+        sampleQueues.valueAt(i).discardToEnd();
       }
-    });
+    }
     handler.removeCallbacksAndMessages(null);
     released = true;
   }
 
+  @Override
+  public void onLoaderReleased() {
+    extractorHolder.release();
+    int trackCount = sampleQueues.size();
+    for (int i = 0; i < trackCount; i++) {
+      sampleQueues.valueAt(i).reset(true);
+    }
+  }
+
   @Override
   public void prepare(Callback callback, long positionUs) {
     this.callback = callback;
@@ -182,19 +189,21 @@ import java.io.IOException;
   public long selectTracks(TrackSelection[] selections, boolean[] mayRetainStreamFlags,
       SampleStream[] streams, boolean[] streamResetFlags, long positionUs) {
     Assertions.checkState(prepared);
-    // Disable old tracks.
+    int oldEnabledTrackCount = enabledTrackCount;
+    // Deselect old tracks.
     for (int i = 0; i < selections.length; i++) {
       if (streams[i] != null && (selections[i] == null || !mayRetainStreamFlags[i])) {
         int track = ((SampleStreamImpl) streams[i]).track;
         Assertions.checkState(trackEnabledStates[track]);
         enabledTrackCount--;
         trackEnabledStates[track] = false;
-        sampleQueues.valueAt(track).disable();
         streams[i] = null;
       }
     }
-    // Enable new tracks.
-    boolean selectedNewTracks = false;
+    // We'll always need to seek if this is a first selection to a non-zero position, or if we're
+    // making a selection having previously disabled all tracks.
+    boolean seekRequired = seenFirstTrackSelection ? oldEnabledTrackCount == 0 : positionUs != 0;
+    // Select new tracks.
     for (int i = 0; i < selections.length; i++) {
       if (streams[i] == null && selections[i] != null) {
         TrackSelection selection = selections[i];
@@ -206,16 +215,12 @@ import java.io.IOException;
         trackEnabledStates[track] = true;
         streams[i] = new SampleStreamImpl(track);
         streamResetFlags[i] = true;
-        selectedNewTracks = true;
-      }
-    }
-    if (!seenFirstTrackSelection) {
-      // At the time of the first track selection all queues will be enabled, so we need to disable
-      // any that are no longer required.
-      int trackCount = sampleQueues.size();
-      for (int i = 0; i < trackCount; i++) {
-        if (!trackEnabledStates[i]) {
-          sampleQueues.valueAt(i).disable();
+        // If there's still a chance of avoiding a seek, try and seek within the sample queue.
+        if (!seekRequired) {
+          SampleQueue sampleQueue = sampleQueues.valueAt(i);
+          sampleQueue.rewind();
+          seekRequired = !sampleQueue.advanceTo(positionUs, true, true)
+              && sampleQueue.getReadIndex() != 0;
         }
       }
     }
@@ -224,7 +229,7 @@ import java.io.IOException;
       if (loader.isLoading()) {
         loader.cancelLoading();
       }
-    } else if (seenFirstTrackSelection ? selectedNewTracks : positionUs != 0) {
+    } else if (seekRequired) {
       positionUs = seekToUs(positionUs);
       // We'll need to reset renderers consuming from all streams due to the seek.
       for (int i = 0; i < streams.length; i++) {
@@ -239,7 +244,10 @@ import java.io.IOException;
 
   @Override
   public void discardBuffer(long positionUs) {
-    // Do nothing.
+    int trackCount = sampleQueues.size();
+    for (int i = 0; i < trackCount; i++) {
+      sampleQueues.valueAt(i).discardTo(positionUs, false, trackEnabledStates[i]);
+    }
   }
 
   @Override
@@ -303,9 +311,13 @@ import java.io.IOException;
     // If we're not pending a reset, see if we can seek within the sample queues.
     boolean seekInsideBuffer = !isPendingReset();
     for (int i = 0; seekInsideBuffer && i < trackCount; i++) {
-      if (trackEnabledStates[i]) {
-        seekInsideBuffer = sampleQueues.valueAt(i).skipToKeyframeBefore(positionUs, false);
-      }
+      SampleQueue sampleQueue = sampleQueues.valueAt(i);
+      sampleQueue.rewind();
+      // TODO: For sparse tracks (e.g. text, metadata) this may return false when an in-buffer
+      // seek should be allowed. If there are non-sparse tracks (e.g. video, audio) for which
+      // in-buffer seeking is successful, we should perform an in-buffer seek unconditionally.
+      seekInsideBuffer = sampleQueue.advanceTo(positionUs, true, false);
+      sampleQueue.discardToRead();
     }
     // If we failed to seek within the sample queues, we need to restart.
     if (!seekInsideBuffer) {
@@ -338,17 +350,16 @@ import java.io.IOException;
     if (notifyReset || isPendingReset()) {
       return C.RESULT_NOTHING_READ;
     }
-
-    return sampleQueues.valueAt(track).readData(formatHolder, buffer, formatRequired,
+    return sampleQueues.valueAt(track).read(formatHolder, buffer, formatRequired,
         loadingFinished, lastSeekPositionUs);
   }
 
   /* package */ void skipData(int track, long positionUs) {
     SampleQueue sampleQueue = sampleQueues.valueAt(track);
     if (loadingFinished && positionUs > sampleQueue.getLargestQueuedTimestampUs()) {
-      sampleQueue.skipAll();
+      sampleQueue.advanceToEnd();
     } else {
-      sampleQueue.skipToKeyframeBefore(positionUs, true);
+      sampleQueue.advanceTo(positionUs, true, true);
     }
   }
 
@@ -372,12 +383,15 @@ import java.io.IOException;
   @Override
   public void onLoadCanceled(ExtractingLoadable loadable, long elapsedRealtimeMs,
       long loadDurationMs, boolean released) {
+    if (released) {
+      return;
+    }
     copyLengthFromLoader(loadable);
-    if (!released && enabledTrackCount > 0) {
-      int trackCount = sampleQueues.size();
-      for (int i = 0; i < trackCount; i++) {
-        sampleQueues.valueAt(i).reset(trackEnabledStates[i]);
-      }
+    int trackCount = sampleQueues.size();
+    for (int i = 0; i < trackCount; i++) {
+      sampleQueues.valueAt(i).reset(true);
+    }
+    if (enabledTrackCount > 0) {
       callback.onContinueLoadingRequested(this);
     }
   }
@@ -508,7 +522,7 @@ import java.io.IOException;
       notifyReset = prepared;
       int trackCount = sampleQueues.size();
       for (int i = 0; i < trackCount; i++) {
-        sampleQueues.valueAt(i).reset(!prepared || trackEnabledStates[i]);
+        sampleQueues.valueAt(i).reset(true);
       }
       loadable.setLoadPosition(0, 0);
     }

library/core/src/main/java/com/google/android/exoplayer2/upstream/Loader.java

@@ -119,17 +119,23 @@ public final class Loader implements LoaderErrorThrower {
 
   }
 
+  /**
+   * A callback to be notified when a {@link Loader} has finished being released.
+   */
+  public interface ReleaseCallback {
+
+    /**
+     * Called when the {@link Loader} has finished being released.
+     */
+    void onLoaderReleased();
+
+  }
+
   public static final int RETRY = 0;
   public static final int RETRY_RESET_ERROR_COUNT = 1;
   public static final int DONT_RETRY = 2;
   public static final int DONT_RETRY_FATAL = 3;
 
-  private static final int MSG_START = 0;
-  private static final int MSG_CANCEL = 1;
-  private static final int MSG_END_OF_SOURCE = 2;
-  private static final int MSG_IO_EXCEPTION = 3;
-  private static final int MSG_FATAL_ERROR = 4;
-
   private final ExecutorService downloadExecutorService;
 
   private LoadTask<? extends Loadable> currentTask;
@@ -150,7 +156,7 @@ public final class Loader implements LoaderErrorThrower {
    *
    * @param <T> The type of the loadable.
    * @param loadable The {@link Loadable} to load.
-   * @param callback A callback to called when the load ends.
+   * @param callback A callback to be called when the load ends.
    * @param defaultMinRetryCount The minimum number of times the load must be retried before
    *     {@link #maybeThrowError()} will propagate an error.
    * @throws IllegalStateException If the calling thread does not have an associated {@link Looper}.
@@ -188,20 +194,28 @@ public final class Loader implements LoaderErrorThrower {
   }
 
   /**
-   * Releases the {@link Loader}, running {@code postLoadAction} on its thread. This method should
-   * be called when the {@link Loader} is no longer required.
+   * Releases the {@link Loader}. This method should be called when the {@link Loader} is no longer
+   * required.
    *
-   * @param postLoadAction A {@link Runnable} to run on the loader's thread when
-   *     {@link Loadable#load()} is no longer running.
+   * @param callback A callback to be called when the release ends. Will be called synchronously
+   *     from this method if no load is in progress, or asynchronously once the load has been
+   *     canceled otherwise. May be null.
+   * @return True if {@code callback} was called synchronously. False if it will be called
+   *     asynchronously or if {@code callback} is null.
    */
-  public void release(Runnable postLoadAction) {
+  public boolean release(ReleaseCallback callback) {
+    boolean callbackInvoked = false;
     if (currentTask != null) {
       currentTask.cancel(true);
-    }
-    if (postLoadAction != null) {
-      downloadExecutorService.execute(postLoadAction);
+      if (callback != null) {
+        downloadExecutorService.execute(new ReleaseTask(callback));
+      }
+    } else if (callback != null) {
+      callback.onLoaderReleased();
+      callbackInvoked = true;
     }
     downloadExecutorService.shutdown();
+    return callbackInvoked;
   }
 
   // LoaderErrorThrower implementation.
@@ -228,6 +242,12 @@ public final class Loader implements LoaderErrorThrower {
 
     private static final String TAG = "LoadTask";
 
+    private static final int MSG_START = 0;
+    private static final int MSG_CANCEL = 1;
+    private static final int MSG_END_OF_SOURCE = 2;
+    private static final int MSG_IO_EXCEPTION = 3;
+    private static final int MSG_FATAL_ERROR = 4;
+
     private final T loadable;
     private final Loader.Callback<T> callback;
     public final int defaultMinRetryCount;
@@ -390,4 +410,24 @@ public final class Loader implements LoaderErrorThrower {
 
   }
 
+  private static final class ReleaseTask extends Handler implements Runnable {
+
+    private final ReleaseCallback callback;
+
+    public ReleaseTask(ReleaseCallback callback) {
+      this.callback = callback;
+    }
+
+    @Override
+    public void run() {
+      sendEmptyMessage(0);
+    }
+
+    @Override
+    public void handleMessage(Message msg) {
+      callback.onLoaderReleased();
+    }
+
+  }
+
 }