Fix merging of selected streams.

Playback would fail if a renderer is toggled from consuming from
one child to another in a single step.

Issue: #1900

-------------
Created by MOE: https:/google/moe
MOE_MIGRATED_REVID=135270356

RELEASENOTES.md

@@ -4,7 +4,8 @@
 
 * Fixes for MergingMediaSource and sideloaded subtitles.
  ([#1882](https:/google/ExoPlayer/issues/1882),
- [#1854](https:/google/ExoPlayer/issues/1854)).
+ [#1854](https:/google/ExoPlayer/issues/1854),
+ [#1900](https:/google/ExoPlayer/issues/1900)).
 * Reduced effect of application code leaking player references
  ([#1855](https:/google/ExoPlayer/issues/1855)).
 * Initial support for fragmented MP4 in HLS.

library/src/main/java/com/google/android/exoplayer2/source/MergingMediaPeriod.java

@@ -85,7 +85,8 @@ public long selectTracks(TrackSelection[] selections, boolean[] mayRetainStreamF
  }
  }
  streamPeriodIndices.clear();
- // Select tracks for each child, copying the resulting streams back into the streams array.
+ // Select tracks for each child, copying the resulting streams back into a new streams array.
+ SampleStream[] newStreams = new SampleStream[selections.length];
  SampleStream[] childStreams = new SampleStream[selections.length];
  TrackSelection[] childSelections = new TrackSelection[selections.length];
  ArrayList<MediaPeriod> enabledPeriodsList = new ArrayList<>(periods.length);
@@ -106,22 +107,20 @@ public long selectTracks(TrackSelection[] selections, boolean[] mayRetainStreamF
  if (selectionChildIndices[j] == i) {
  // Assert that the child provided a stream for the selection.
  Assertions.checkState(childStreams[j] != null);
- streams[j] = childStreams[j];
+ newStreams[j] = childStreams[j];
  periodEnabled = true;
  streamPeriodIndices.put(childStreams[j], i);
  } else if (streamChildIndices[j] == i) {
  // Assert that the child cleared any previous stream.
  Assertions.checkState(childStreams[j] == null);
- if (selectionChildIndices[j] == C.INDEX_UNSET) {
- // No other child will be setting the stream at index j, so clear it.
- streams[j] = null;
- }
  }
  }
  if (periodEnabled) {
  enabledPeriodsList.add(periods[i]);
  }
  }
+ // Copy the new streams back into the streams array.
+ System.arraycopy(newStreams, 0, streams, 0, newStreams.length);
  // Update the local state.
  enabledPeriods = new MediaPeriod[enabledPeriodsList.size()];
  enabledPeriodsList.toArray(enabledPeriods);

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

@@ -39,6 +39,7 @@
 import com.google.android.exoplayer2.upstream.DataSource;
 import com.google.android.exoplayer2.upstream.Loader;
 import com.google.android.exoplayer2.upstream.ParsingLoadable;
+import com.google.android.exoplayer2.util.Assertions;
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.IdentityHashMap;
@@ -154,7 +155,8 @@ public long selectTracks(TrackSelection[] selections, boolean[] mayRetainStreamF
  }
  boolean selectedNewTracks = false;
  streamWrapperIndices.clear();
- // Select tracks for each child, copying the resulting streams back into the streams array.
+ // Select tracks for each child, copying the resulting streams back into a new streams array.
+ SampleStream[] newStreams = new SampleStream[selections.length];
  SampleStream[] childStreams = new SampleStream[selections.length];
  TrackSelection[] childSelections = new TrackSelection[selections.length];
  ArrayList<HlsSampleStreamWrapper> enabledSampleStreamWrapperList = new ArrayList<>(
@@ -168,19 +170,23 @@ public long selectTracks(TrackSelection[] selections, boolean[] mayRetainStreamF
  mayRetainStreamFlags, childStreams, streamResetFlags, !seenFirstTrackSelection);
  boolean wrapperEnabled = false;
  for (int j = 0; j < selections.length; j++) {
- if (selectionChildIndices[j] == i
- || (selectionChildIndices[j] == C.INDEX_UNSET && streamChildIndices[j] == i)) {
- streams[j] = childStreams[j];
- if (childStreams[j] != null) {
- wrapperEnabled = true;
- streamWrapperIndices.put(childStreams[j], i);
- }
+ if (selectionChildIndices[j] == i) {
+ // Assert that the child provided a stream for the selection.
+ Assertions.checkState(childStreams[j] != null);
+ newStreams[j] = childStreams[j];
+ wrapperEnabled = true;
+ streamWrapperIndices.put(childStreams[j], i);
+ } else if (streamChildIndices[j] == i) {
+ // Assert that the child cleared any previous stream.
+ Assertions.checkState(childStreams[j] == null);
  }
  }
  if (wrapperEnabled) {
  enabledSampleStreamWrapperList.add(sampleStreamWrappers[i]);
  }
  }
+ // Copy the new streams back into the streams array.
+ System.arraycopy(newStreams, 0, streams, 0, newStreams.length);
  // Update the local state.
  enabledSampleStreamWrappers = new HlsSampleStreamWrapper[enabledSampleStreamWrapperList.size()];
  enabledSampleStreamWrapperList.toArray(enabledSampleStreamWrappers);