Fix index out of bounds exception when a Subtitle is empty

Issue: androidx/media#1516

#cherrypick

PiperOrigin-RevId: 648416119

RELEASENOTES.md

@@ -24,6 +24,9 @@
 *   Audio:
 *   Video:
 *   Text:
+    *   Fix an `IllegalArgumentException` from `LegacySubtitleUtil` when a
+        WebVTT subtitle sample contains no cues, e.g. as part of a DASH stream
+        ([#1516](https://github.com/androidx/media/issues/1516)).
 *   Metadata:
 *   Image:
 *   DRM:

libraries/extractor/src/main/java/androidx/media3/extractor/text/LegacySubtitleUtil.java

@@ -32,11 +32,14 @@ public class LegacySubtitleUtil {
    * Converts a {@link Subtitle} to a list of {@link CuesWithTiming} representing it, emitted to
    * {@code output}.
    *
-   * <p>This may only be called with {@link Subtitle} instances where the first event is non-empty
+   * <p>This may only be called with empty {@link Subtitle} instances, or those where the first
-   * and the last event is an empty cue list.
+   * event is non-empty and the last event is an empty cue list.
    */
   public static void toCuesWithTiming(
       Subtitle subtitle, OutputOptions outputOptions, Consumer<CuesWithTiming> output) {
+    if (subtitle.getEventTimeCount() == 0) {
+      return;
+    }
     int startIndex = getStartIndex(subtitle, outputOptions);
     boolean startedInMiddleOfCue = false;
     if (outputOptions.startTimeUs != C.TIME_UNSET) {

libraries/extractor/src/test/java/androidx/media3/extractor/text/webvtt/LegacySubtitleUtilWebvttTest.java

@@ -39,6 +39,8 @@ public class LegacySubtitleUtilWebvttTest {
   private static final String FIRST_SUBTITLE_STRING = "This is the first subtitle.";
   private static final String SECOND_SUBTITLE_STRING = "This is the second subtitle.";
 
+  private static final WebvttSubtitle EMPTY_SUBTITLE = new WebvttSubtitle(ImmutableList.of());
+
   private static final WebvttSubtitle SIMPLE_SUBTITLE =
       new WebvttSubtitle(
           Arrays.asList(
@@ -135,6 +137,14 @@ public class LegacySubtitleUtilWebvttTest {
         .containsExactly(SECOND_SUBTITLE_STRING);
   }
 
+  @Test
+  public void toCuesWithTiming_allCues_emptySubtitle() {
+    ImmutableList<CuesWithTiming> cuesWithTimingsList =
+        toCuesWithTimingList(EMPTY_SUBTITLE, SubtitleParser.OutputOptions.allCues());
+
+    assertThat(cuesWithTimingsList).isEmpty();
+  }
+
   @Test
   public void toCuesWithTiming_onlyEmitCuesAfterStartTime_startBetweenCues_simpleSubtitle() {
     ImmutableList<CuesWithTiming> cuesWithTimingsList =
@@ -252,6 +262,14 @@ public class LegacySubtitleUtilWebvttTest {
         .containsExactly(SECOND_SUBTITLE_STRING);
   }
 
+  @Test
+  public void toCuesWithTiming_onlyEmitCuesAfterStartTime_emptySubtitle() {
+    ImmutableList<CuesWithTiming> cuesWithTimingsList =
+        toCuesWithTimingList(EMPTY_SUBTITLE, SubtitleParser.OutputOptions.onlyCuesAfter(0));
+
+    assertThat(cuesWithTimingsList).isEmpty();
+  }
+
   @Test
   public void
       toCuesWithTiming_emitCuesAfterStartTimeThenThoseBefore_startAtStartOfCue_simpleSubtitle() {
@@ -405,6 +423,14 @@ public class LegacySubtitleUtilWebvttTest {
         .inOrder();
   }
 
+  @Test
+  public void toCuesWithTiming_emitCuesAfterStartTimeThenThoseBefore_emptySubtitle() {
+    ImmutableList<CuesWithTiming> cuesWithTimingsList =
+        toCuesWithTimingList(EMPTY_SUBTITLE, SubtitleParser.OutputOptions.onlyCuesAfter(0));
+
+    assertThat(cuesWithTimingsList).isEmpty();
+  }
+
   private static ImmutableList<CuesWithTiming> toCuesWithTimingList(
       Subtitle subtitle, SubtitleParser.OutputOptions outputOptions) {
     ImmutableList.Builder<CuesWithTiming> result = ImmutableList.builder();