DASH: Output Format before InitializationChunk load completes

This optimization allows a ChunkSampleStream to output track
formats as soon as they're parsed during an InitializationChunk
load, rather than waiting until after the InitializationChunk
load is completed.

In DASH VOD, a single InitializationChunk typically loads the
moov and sidx atoms in that order. Hence for long form content
where the sidx is a non-trivial size, this may result in the
track formats being output a non-negligible period of time
sooner than was previously the case. This allows downstream
renderers to start codec initialization sooner, potentially
decreasing startup latency.

For a single test stream on a fast & stable network, this
pretty consistently reduced elapsed time until both audio and
video codecs have been initialized from ~0.5s to ~0.3 seconds
on a Galaxy S8. For 5 test runs without and with these patches,
the eventTime logged by EventLogger for the second decoder
init were:

Without (secs): 0.47 0.47 0.45 0.48 0.46
With (secs)   : 0.32 0.33 0.34 0.31 0.40

PiperOrigin-RevId: 289845089

RELEASENOTES.md

@@ -65,11 +65,12 @@
   Matroska or MP4.
 * Javadocs: Add favicon for easier identification in browser tabs
 * FMP4: Add support for encrypted AC-4 tracks.
-* Reduce startup latency for DASH and SmoothStreaming adaptive playbacks.
-  In previous versions, codec initialization would only occur after the network
-  connection for requesting the first media segment had been established. Codec
-  initialization can now occur before this network connection being established,
-  reducing startup latency.
+* Startup latency optimizations:
+  * Reduce startup latency for DASH and SmoothStreaming playbacks by allowing
+    codec initialization to occur before the network connection for the first
+    media segment has been established.
+  * Reduce startup latency for on-demand DASH playbacks by allowing codec
+    initialization to occur before the sidx box has been loaded.
 
 ### 2.11.1 (2019-12-20) ###
 

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

@@ -21,7 +21,10 @@ import com.google.android.exoplayer2.source.SampleQueue;
 import com.google.android.exoplayer2.source.chunk.ChunkExtractorWrapper.TrackOutputProvider;
 import com.google.android.exoplayer2.util.Log;
 
-/** An output for {@link BaseMediaChunk}s. */
+/**
+ * A {@link TrackOutputProvider} that provides {@link TrackOutput TrackOutputs} based on a
+ * predefined mapping from track type to output.
+ */
 public final class BaseMediaChunkOutput implements TrackOutputProvider {
 
   private static final String TAG = "BaseMediaChunkOutput";

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

@@ -74,7 +74,7 @@ public class ChunkSampleStream<T extends ChunkSource> implements SampleStream, S
   private final List<BaseMediaChunk> readOnlyMediaChunks;
   private final SampleQueue primarySampleQueue;
   private final SampleQueue[] embeddedSampleQueues;
-  private final BaseMediaChunkOutput mediaChunkOutput;
+  private final BaseMediaChunkOutput chunkOutput;
 
   private Format primaryDownstreamTrackFormat;
   @Nullable private ReleaseCallback<T> releaseCallback;
@@ -142,7 +142,7 @@ public class ChunkSampleStream<T extends ChunkSource> implements SampleStream, S
       trackTypes[i + 1] = embeddedTrackTypes[i];
     }
 
-    mediaChunkOutput = new BaseMediaChunkOutput(trackTypes, sampleQueues);
+    chunkOutput = new BaseMediaChunkOutput(trackTypes, sampleQueues);
     pendingResetPositionUs = positionUs;
     lastSeekPositionUs = positionUs;
   }
@@ -554,8 +554,10 @@ public class ChunkSampleStream<T extends ChunkSource> implements SampleStream, S
         decodeOnlyUntilPositionUs = resetToMediaChunk ? 0 : pendingResetPositionUs;
         pendingResetPositionUs = C.TIME_UNSET;
       }
-      mediaChunk.init(mediaChunkOutput);
+      mediaChunk.init(chunkOutput);
       mediaChunks.add(mediaChunk);
+    } else if (loadable instanceof InitializationChunk) {
+      ((InitializationChunk) loadable).init(chunkOutput);
     }
     long elapsedRealtimeMs =
         loader.startLoading(

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

@@ -21,6 +21,7 @@ import com.google.android.exoplayer2.extractor.DefaultExtractorInput;
 import com.google.android.exoplayer2.extractor.Extractor;
 import com.google.android.exoplayer2.extractor.ExtractorInput;
 import com.google.android.exoplayer2.extractor.PositionHolder;
+import com.google.android.exoplayer2.source.chunk.ChunkExtractorWrapper.TrackOutputProvider;
 import com.google.android.exoplayer2.upstream.DataSource;
 import com.google.android.exoplayer2.upstream.DataSpec;
 import com.google.android.exoplayer2.util.Assertions;
@@ -144,16 +145,13 @@ public class ContainerMediaChunk extends BaseMediaChunk {
   }
 
   /**
-   * Returns the {@link ChunkExtractorWrapper.TrackOutputProvider} to be used by the wrapped
-   * extractor.
+   * Returns the {@link TrackOutputProvider} to be used by the wrapped extractor.
    *
    * @param baseMediaChunkOutput The {@link BaseMediaChunkOutput} most recently passed to {@link
    *     #init(BaseMediaChunkOutput)}.
-   * @return A {@link ChunkExtractorWrapper.TrackOutputProvider} to be used by the wrapped
-   *     extractor.
+   * @return A {@link TrackOutputProvider} to be used by the wrapped extractor.
    */
-  protected ChunkExtractorWrapper.TrackOutputProvider getTrackOutputProvider(
-      BaseMediaChunkOutput baseMediaChunkOutput) {
+  protected TrackOutputProvider getTrackOutputProvider(BaseMediaChunkOutput baseMediaChunkOutput) {
     return baseMediaChunkOutput;
   }
 }

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

@@ -22,11 +22,13 @@ import com.google.android.exoplayer2.extractor.DefaultExtractorInput;
 import com.google.android.exoplayer2.extractor.Extractor;
 import com.google.android.exoplayer2.extractor.ExtractorInput;
 import com.google.android.exoplayer2.extractor.PositionHolder;
+import com.google.android.exoplayer2.source.chunk.ChunkExtractorWrapper.TrackOutputProvider;
 import com.google.android.exoplayer2.upstream.DataSource;
 import com.google.android.exoplayer2.upstream.DataSpec;
 import com.google.android.exoplayer2.util.Assertions;
 import com.google.android.exoplayer2.util.Util;
 import java.io.IOException;
+import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
 
 /**
  * A {@link Chunk} that uses an {@link Extractor} to decode initialization data for single track.
@@ -37,6 +39,7 @@ public final class InitializationChunk extends Chunk {
 
   private final ChunkExtractorWrapper extractorWrapper;
 
+  @MonotonicNonNull private TrackOutputProvider trackOutputProvider;
   private long nextLoadPosition;
   private volatile boolean loadCanceled;
 
@@ -60,6 +63,17 @@ public final class InitializationChunk extends Chunk {
     this.extractorWrapper = extractorWrapper;
   }
 
+  /**
+   * Initializes the chunk for loading, setting a {@link TrackOutputProvider} for track outputs to
+   * which formats will be written as they are loaded.
+   *
+   * @param trackOutputProvider The {@link TrackOutputProvider} for track outputs to which formats
+   *     will be written as they are loaded.
+   */
+  public void init(TrackOutputProvider trackOutputProvider) {
+    this.trackOutputProvider = trackOutputProvider;
+  }
+
   // Loadable implementation.
 
   @Override
@@ -72,9 +86,7 @@ public final class InitializationChunk extends Chunk {
   public void load() throws IOException, InterruptedException {
     if (nextLoadPosition == 0) {
       extractorWrapper.init(
-          /* trackOutputProvider= */ null,
-          /* startTimeUs= */ C.TIME_UNSET,
-          /* endTimeUs= */ C.TIME_UNSET);
+          trackOutputProvider, /* startTimeUs= */ C.TIME_UNSET, /* endTimeUs= */ C.TIME_UNSET);
     }
     try {
       // Create and open the input.