Use MaskingMediaSource for AdsMediaSource content source

This means the content source is 'prepared' instantly with a
placeholder, enabling all further preparation steps (e.g. loading
preroll ads) while the actual content is still preparing. This
improvement can speed up the start time for prerolls in  manifest-based
content that doesn't have a zero-time preparation step like progressive
media.

Issue: androidx/media#1358
PiperOrigin-RevId: 633640746

RELEASENOTES.md

@@ -30,6 +30,9 @@
     *   Add new error code
         `PlaybackException.ERROR_CODE_DECODING_RESOURCES_RECLAIMED` that is used
         when codec resources are reclaimed for higher priority tasks.
+    *   Let `AdsMediaSource` load preroll ads before initial content media
+        preparation completes
+        ([#1358](https://github.com/androidx/media/issues/1358)).
 *   Transformer:
     *   Work around a decoder bug where the number of audio channels was capped
         at stereo when handling PCM input.

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/source/ads/AdsMediaSource.java

@@ -38,6 +38,7 @@ import androidx.media3.datasource.TransferListener;
 import androidx.media3.exoplayer.source.CompositeMediaSource;
 import androidx.media3.exoplayer.source.LoadEventInfo;
 import androidx.media3.exoplayer.source.MaskingMediaPeriod;
+import androidx.media3.exoplayer.source.MaskingMediaSource;
 import androidx.media3.exoplayer.source.MediaLoadData;
 import androidx.media3.exoplayer.source.MediaPeriod;
 import androidx.media3.exoplayer.source.MediaSource;
@@ -134,7 +135,7 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
   private static final MediaPeriodId CHILD_SOURCE_MEDIA_PERIOD_ID =
       new MediaPeriodId(/* periodUid= */ new Object());
 
-  private final MediaSource contentMediaSource;
+  private final MaskingMediaSource contentMediaSource;
   @Nullable final MediaItem.DrmConfiguration contentDrmConfiguration;
   private final MediaSource.Factory adMediaSourceFactory;
   private final AdsLoader adsLoader;
@@ -171,7 +172,8 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
       MediaSource.Factory adMediaSourceFactory,
       AdsLoader adsLoader,
       AdViewProvider adViewProvider) {
-    this.contentMediaSource = contentMediaSource;
+    this.contentMediaSource =
+        new MaskingMediaSource(contentMediaSource, /* useLazyPreparation= */ true);
     this.contentDrmConfiguration =
         checkNotNull(contentMediaSource.getMediaItem().localConfiguration).drmConfiguration;
     this.adMediaSourceFactory = adMediaSourceFactory;
@@ -206,6 +208,7 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
     super.prepareSourceInternal(mediaTransferListener);
     ComponentListener componentListener = new ComponentListener();
     this.componentListener = componentListener;
+    contentTimeline = contentMediaSource.getTimeline();
     prepareChildSource(CHILD_SOURCE_MEDIA_PERIOD_ID, contentMediaSource);
     mainHandler.post(
         () ->

libraries/exoplayer/src/test/java/androidx/media3/exoplayer/source/ads/AdsMediaSourceTest.java

@@ -20,6 +20,7 @@ import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 import static org.mockito.Mockito.when;
 import static org.robolectric.Shadows.shadowOf;
@@ -35,6 +36,7 @@ import androidx.media3.common.Timeline;
 import androidx.media3.datasource.DataSpec;
 import androidx.media3.exoplayer.analytics.PlayerId;
 import androidx.media3.exoplayer.source.DefaultMediaSourceFactory;
+import androidx.media3.exoplayer.source.MaskingMediaSource;
 import androidx.media3.exoplayer.source.MediaPeriod;
 import androidx.media3.exoplayer.source.MediaSource;
 import androidx.media3.exoplayer.source.MediaSource.MediaPeriodId;
@@ -81,7 +83,9 @@ public final class AdsMediaSourceTest {
           /* isDynamic= */ false,
           /* useLiveConfiguration= */ false,
           /* manifest= */ null,
-          MediaItem.fromUri(Uri.parse("https://google.com/empty")));
+          FakeMediaSource.FAKE_MEDIA_ITEM);
+  private static final Timeline PLACEHOLDER_CONTENT_TIMELINE =
+      new MaskingMediaSource.PlaceholderTimeline(FakeMediaSource.FAKE_MEDIA_ITEM);
   private static final Object CONTENT_PERIOD_UID =
       CONTENT_TIMELINE.getUidOfPeriod(/* periodIndex= */ 0);
 
@@ -147,7 +151,8 @@ public final class AdsMediaSourceTest {
   }
 
   @Test
-  public void createPeriod_preparesChildAdMediaSourceAndRefreshesSourceInfo() {
+  public void createPeriod_forPreroll_preparesChildAdMediaSourceAndRefreshesSourceInfo() {
+    // This should be unused if we only create the preroll period.
     contentMediaSource.setNewSourceInfo(CONTENT_TIMELINE);
     adsMediaSource.createPeriod(
         new MediaPeriodId(
@@ -162,11 +167,14 @@ public final class AdsMediaSourceTest {
     assertThat(prerollAdMediaSource.isPrepared()).isTrue();
     verify(mockMediaSourceCaller)
         .onSourceInfoRefreshed(
-            adsMediaSource, new SinglePeriodAdTimeline(CONTENT_TIMELINE, AD_PLAYBACK_STATE));
+            adsMediaSource,
+            new SinglePeriodAdTimeline(PLACEHOLDER_CONTENT_TIMELINE, AD_PLAYBACK_STATE));
   }
 
   @Test
-  public void createPeriod_preparesChildAdMediaSourceAndRefreshesSourceInfoWithAdMediaSourceInfo() {
+  public void
+      createPeriod_forPreroll_preparesChildAdMediaSourceAndRefreshesSourceInfoWithAdMediaSourceInfo() {
+    // This should be unused if we only create the preroll period.
     contentMediaSource.setNewSourceInfo(CONTENT_TIMELINE);
     adsMediaSource.createPeriod(
         new MediaPeriodId(
@@ -183,13 +191,12 @@ public final class AdsMediaSourceTest {
         .onSourceInfoRefreshed(
             adsMediaSource,
             new SinglePeriodAdTimeline(
-                CONTENT_TIMELINE,
+                PLACEHOLDER_CONTENT_TIMELINE,
                 AD_PLAYBACK_STATE.withAdDurationsUs(new long[][] {{PREROLL_AD_DURATION_US}})));
   }
 
   @Test
-  public void createPeriod_createsChildPrerollAdMediaPeriod() {
-    contentMediaSource.setNewSourceInfo(CONTENT_TIMELINE);
+  public void createPeriod_forPreroll_createsChildPrerollAdMediaPeriod() {
     adsMediaSource.createPeriod(
         new MediaPeriodId(
             CONTENT_PERIOD_UID,
@@ -206,7 +213,7 @@ public final class AdsMediaSourceTest {
   }
 
   @Test
-  public void createPeriod_createsChildContentMediaPeriod() {
+  public void createPeriod_forContent_createsChildContentMediaPeriodAndLoadsContentTimeline() {
     contentMediaSource.setNewSourceInfo(CONTENT_TIMELINE);
     shadowOf(Looper.getMainLooper()).idle();
     adsMediaSource.createPeriod(
@@ -216,6 +223,12 @@ public final class AdsMediaSourceTest {
 
     contentMediaSource.assertMediaPeriodCreated(
         new MediaPeriodId(CONTENT_PERIOD_UID, /* windowSequenceNumber= */ 0));
+    ArgumentCaptor<Timeline> adsTimelineCaptor = ArgumentCaptor.forClass(Timeline.class);
+    verify(mockMediaSourceCaller, times(2))
+        .onSourceInfoRefreshed(eq(adsMediaSource), adsTimelineCaptor.capture());
+    TestUtil.timelinesAreSame(
+        adsTimelineCaptor.getValue(),
+        new SinglePeriodAdTimeline(CONTENT_TIMELINE, AD_PLAYBACK_STATE));
   }
 
   @Test