Add fail-early checks for TrackSelectorResult correctness

The two arrays need to have the same length and the selection
must match in their nullness (unless for TYPE_NONE
renderers). Clarify this more clearly in the docs and add
new asssertions for it. This avoids that the player is failing
in obscure ways much later.

Issue: #1473
#cherrypick
PiperOrigin-RevId: 646086833

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/MediaPeriodHolder.java

@@ -15,14 +15,14 @@
  */
 package androidx.media3.exoplayer;
 
+import static androidx.media3.common.util.Assertions.checkState;
 import static androidx.media3.exoplayer.MediaPeriodQueue.areDurationsCompatible;
 import static java.lang.Math.max;
 
 import androidx.annotation.Nullable;
 import androidx.media3.common.C;
 import androidx.media3.common.Format;
 import androidx.media3.common.Timeline;
-import androidx.media3.common.util.Assertions;
 import androidx.media3.common.util.Log;
 import androidx.media3.common.util.NullableType;
 import androidx.media3.exoplayer.source.ClippingMediaPeriod;
@@ -212,7 +212,7 @@ public void handlePrepared(float playbackSpeed, Timeline timeline) throws ExoPla
    * @param rendererPositionUs The playing position in renderer time, in microseconds.
    */
   public void reevaluateBuffer(long rendererPositionUs) {
-    Assertions.checkState(isLoadingMediaPeriod());
+    checkState(isLoadingMediaPeriod());
     if (prepared) {
       mediaPeriod.reevaluateBuffer(toPeriodTime(rendererPositionUs));
     }
@@ -230,7 +230,7 @@ public void reevaluateBuffer(long rendererPositionUs) {
    */
   public void continueLoading(
       long rendererPositionUs, float playbackSpeed, long lastRebufferRealtimeMs) {
-    Assertions.checkState(isLoadingMediaPeriod());
+    checkState(isLoadingMediaPeriod());
     long loadingPeriodPositionUs = toPeriodTime(rendererPositionUs);
     mediaPeriod.continueLoading(
         new LoadingInfo.Builder()
@@ -255,6 +255,15 @@ public TrackSelectorResult selectTracks(float playbackSpeed, Timeline timeline)
       throws ExoPlaybackException {
     TrackSelectorResult selectorResult =
         trackSelector.selectTracks(rendererCapabilities, getTrackGroups(), info.id, timeline);
+    for (int i = 0; i < selectorResult.length; i++) {
+      if (selectorResult.isRendererEnabled(i)) {
+        checkState(
+            selectorResult.selections[i] != null
+                || rendererCapabilities[i].getTrackType() == C.TRACK_TYPE_NONE);
+      } else {
+        checkState(selectorResult.selections[i] == null);
+      }
+    }
     for (ExoTrackSelection trackSelection : selectorResult.selections) {
       if (trackSelection != null) {
         trackSelection.onPlaybackSpeed(playbackSpeed);
@@ -324,13 +333,13 @@ public long applyTrackSelection(
     hasEnabledTracks = false;
     for (int i = 0; i < sampleStreams.length; i++) {
       if (sampleStreams[i] != null) {
-        Assertions.checkState(newTrackSelectorResult.isRendererEnabled(i));
+        checkState(newTrackSelectorResult.isRendererEnabled(i));
         // hasEnabledTracks should be true only when non-empty streams exists.
         if (rendererCapabilities[i].getTrackType() != C.TRACK_TYPE_NONE) {
           hasEnabledTracks = true;
         }
       } else {
-        Assertions.checkState(newTrackSelectorResult.selections[i] == null);
+        checkState(newTrackSelectorResult.selections[i] == null);
       }
     }
     return positionUs;

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/trackselection/MappingTrackSelector.java

@@ -450,7 +450,7 @@ public final TrackSelectorResult selectTracks(
    * @param timeline The {@link Timeline} holding the period for which tracks are to be selected.
    * @return A pair consisting of the track selections and configurations for each renderer. A null
    *     configuration indicates the renderer should be disabled, in which case the track selection
-   *     will also be null. A track selection may also be null for a non-disabled renderer if {@link
+   *     must also be null. A track selection may also be null for a non-disabled renderer if {@link
    *     RendererCapabilities#getTrackType()} is {@link C#TRACK_TYPE_NONE}.
    * @throws ExoPlaybackException If an error occurs while selecting the tracks.
    */

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/trackselection/TrackSelectorResult.java

@@ -15,11 +15,15 @@
  */
 package androidx.media3.exoplayer.trackselection;
 
+import static androidx.media3.common.util.Assertions.checkArgument;
+
 import androidx.annotation.Nullable;
+import androidx.media3.common.C;
 import androidx.media3.common.Tracks;
 import androidx.media3.common.util.NullableType;
 import androidx.media3.common.util.UnstableApi;
 import androidx.media3.common.util.Util;
+import androidx.media3.exoplayer.RendererCapabilities;
 import androidx.media3.exoplayer.RendererConfiguration;
 
 /** The result of a {@link TrackSelector} operation. */
@@ -69,6 +73,9 @@ public TrackSelectorResult(
    * @param rendererConfigurations A {@link RendererConfiguration} for each renderer. A null entry
    *     indicates the corresponding renderer should be disabled.
    * @param selections A {@link ExoTrackSelection} array containing the selection for each renderer.
+   *     If the renderer is disabled with a null {@link RendererConfiguration}, then it must have a
+   *     null selection too. It can also have a null selection if it has a {@linkplain
+   *     RendererCapabilities#getTrackType() track type} of {@link C#TRACK_TYPE_NONE}.
    * @param tracks Description of the available tracks and which one were selected.
    * @param info An opaque object that will be returned to {@link
    *     TrackSelector#onSelectionActivated(Object)} should the selection be activated. May be
@@ -79,6 +86,7 @@ public TrackSelectorResult(
       @NullableType ExoTrackSelection[] selections,
       Tracks tracks,
       @Nullable Object info) {
+    checkArgument(rendererConfigurations.length == selections.length);
     this.rendererConfigurations = rendererConfigurations;
     this.selections = selections.clone();
     this.tracks = tracks;