Set correct track id when skipping empty tracks in Mp4Extractor

The track id must be the index in the list of published tracks
as it's used as such elsewhere. This is currently not true if we
skip an empty track as all subsequent tracks get a wrong or even
invalid id.

#minor-release

PiperOrigin-RevId: 604929178
(cherry picked from commit 5f9c96a)

Author: tonihei

libraries/extractor/src/main/java/androidx/media3/extractor/mp4/Mp4Extractor.java

@@ -585,8 +585,8 @@ private void processMoovAtom(ContainerAtom moov) throws ParserException {
             isQuickTime,
             /* modifyTrackFunction= */ track -> track);
 
-    int trackCount = trackSampleTables.size();
-    for (int i = 0; i < trackCount; i++) {
+    int trackIndex = 0;
+    for (int i = 0; i < trackSampleTables.size(); i++) {
       TrackSampleTable trackSampleTable = trackSampleTables.get(i);
       if (trackSampleTable.sampleCount == 0) {
         continue;
@@ -596,7 +596,7 @@ private void processMoovAtom(ContainerAtom moov) throws ParserException {
           track.durationUs != C.TIME_UNSET ? track.durationUs : trackSampleTable.durationUs;
       durationUs = max(durationUs, trackDurationUs);
       Mp4Track mp4Track =
-          new Mp4Track(track, trackSampleTable, extractorOutput.track(i, track.type));
+          new Mp4Track(track, trackSampleTable, extractorOutput.track(trackIndex++, track.type));
 
       int maxInputSize;
       if (MimeTypes.AUDIO_TRUEHD.equals(track.format.sampleMimeType)) {

libraries/extractor/src/test/java/androidx/media3/extractor/mp4/Mp4ExtractorTest.java

@@ -16,10 +16,19 @@
 package androidx.media3.extractor.mp4;
 
 import static androidx.media3.extractor.mp4.FragmentedMp4Extractor.FLAG_EMIT_RAW_SUBTITLE_DATA;
+import static com.google.common.truth.Truth.assertThat;
 
+import androidx.media3.extractor.Extractor;
+import androidx.media3.extractor.PositionHolder;
 import androidx.media3.extractor.text.DefaultSubtitleParserFactory;
 import androidx.media3.extractor.text.SubtitleParser;
 import androidx.media3.test.utils.ExtractorAsserts;
+import androidx.media3.test.utils.FakeExtractorInput;
+import androidx.media3.test.utils.FakeExtractorOutput;
+import androidx.media3.test.utils.FakeTrackOutput;
+import androidx.media3.test.utils.TestUtil;
+import androidx.test.core.app.ApplicationProvider;
+import com.google.common.collect.ImmutableList;
 import java.util.ArrayList;
 import java.util.List;
 import org.junit.Test;
@@ -226,6 +235,51 @@ public void mp4SampleWithEditList() throws Exception {
         simulationConfig);
   }
 
+  @Test
+  public void mp4SampleWithEmptyTrack() throws Exception {
+    ExtractorAsserts.assertBehavior(
+        getExtractorFactory(subtitlesParsedDuringExtraction),
+        "media/mp4/sample_empty_track.mp4",
+        simulationConfig);
+  }
+
+  @Test
+  public void getSeekPoints_withEmptyTracks_returnsValidInformation() throws Exception {
+    Mp4Extractor extractor =
+        (Mp4Extractor) getExtractorFactory(subtitlesParsedDuringExtraction).create();
+    FakeExtractorInput input =
+        new FakeExtractorInput.Builder()
+            .setData(
+                TestUtil.getByteArray(
+                    ApplicationProvider.getApplicationContext(),
+                    "media/mp4/sample_empty_track.mp4"))
+            .build();
+    FakeExtractorOutput output =
+        new FakeExtractorOutput(
+            (id, type) -> new FakeTrackOutput(/* deduplicateConsecutiveFormats= */ true));
+    PositionHolder seekPositionHolder = new PositionHolder();
+    extractor.init(output);
+    int readResult = Extractor.RESULT_CONTINUE;
+    while (readResult != Extractor.RESULT_END_OF_INPUT) {
+      readResult = extractor.read(input, seekPositionHolder);
+      if (readResult == Extractor.RESULT_SEEK) {
+        long seekPosition = seekPositionHolder.position;
+        input.setPosition((int) seekPosition);
+      }
+    }
+    ImmutableList.Builder<Long> trackSeekTimesUs = ImmutableList.builder();
+    long testPositionUs = output.seekMap.getDurationUs() / 2;
+
+    for (int i = 0; i < output.numberOfTracks; i++) {
+      int trackId = output.trackOutputs.keyAt(i);
+      trackSeekTimesUs.add(extractor.getSeekPoints(testPositionUs, trackId).first.timeUs);
+    }
+    long extractorSeekTimeUs = extractor.getSeekPoints(testPositionUs).first.timeUs;
+
+    assertThat(output.numberOfTracks).isEqualTo(2);
+    assertThat(extractorSeekTimeUs).isIn(trackSeekTimesUs.build());
+  }
+
   private static ExtractorAsserts.ExtractorFactory getExtractorFactory(
       boolean subtitlesParsedDuringExtraction) {
     SubtitleParser.Factory subtitleParserFactory;