Avoid non-primary playlists continuously reloading for LL-HLS streams

For LL-HLS, the non-primary playlists originally keep reloading even after the primary playlist has been changed to another one. The reason being this is to check if the hinted(#EXT-X-PRELOAD-HINT) resource has been published or removed. If removed, the loading of it should be canceled, per the suggestion in the HLS spec:

"A Client SHOULD cancel a request for a hinted resource if it is not present in a subsequent Playlist update, such as in an EXT-X-PRELOAD-HINT tag or as part of another tag such as EXT-X-PART.  The client SHOULD ignore the results of such requests."

However, keeping the non-primary playlists reloading is not optimal. As a solution, we trigger the playlist reloading only when there is a preload chunk loading instead of every time after we have processed the playlist. Compared to the original implementation, this will save the requests of reloading non-primary playlist after we have taken action upon the preload chunk being published or removed.

Issue: androidx/media#1240
PiperOrigin-RevId: 626038032

RELEASENOTES.md

@@ -43,6 +43,8 @@
         delegated in `HlsSampleStreamWrapper` with an incorrect offset causing
         an `IndexOutOfBoundsException` or an `IllegalArgumentException`
         ([#1002](https://github.com/androidx/media/issues/1002)).
+    *   Fix bug where non-primary playlists keep reloading for LL-HLS streams
+        ([#1240](https://github.com/androidx/media/issues/1240)).
 *   DASH Extension:
 *   Smooth Streaming Extension:
 *   RTSP Extension:

libraries/exoplayer_hls/src/main/java/androidx/media3/exoplayer/hls/HlsSampleStreamWrapper.java

@@ -15,6 +15,7 @@
  */
 package androidx.media3.exoplayer.hls;
 
+import static androidx.media3.exoplayer.hls.HlsChunkSource.CHUNK_PUBLICATION_STATE_PRELOAD;
 import static androidx.media3.exoplayer.hls.HlsChunkSource.CHUNK_PUBLICATION_STATE_PUBLISHED;
 import static androidx.media3.exoplayer.hls.HlsChunkSource.CHUNK_PUBLICATION_STATE_REMOVED;
 import static androidx.media3.exoplayer.trackselection.TrackSelectionUtil.createFallbackOptions;
@@ -111,7 +112,11 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
 
     /**
      * Called to schedule a {@link #continueLoading(LoadingInfo)} call when the playlist referred by
-     * the given url changes.
+     * the given url changes, or it requires a refresh to check whether the hinted resource has been
+     * published or removed.
+     *
+     * <p>Note: This method will be called on a later handler loop than the one on which {@link
+     * #onPlaylistUpdated()} is invoked.
      */
     void onPlaylistRefreshRequired(Uri playlistUrl);
   }
@@ -543,6 +548,8 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
     int chunkState = chunkSource.getChunkPublicationState(lastMediaChunk);
     if (chunkState == CHUNK_PUBLICATION_STATE_PUBLISHED) {
       lastMediaChunk.publish();
+    } else if (chunkState == CHUNK_PUBLICATION_STATE_PRELOAD) {
+      handler.post(() -> callback.onPlaylistRefreshRequired(lastMediaChunk.playlistUrl));
     } else if (chunkState == CHUNK_PUBLICATION_STATE_REMOVED
         && !loadingFinished
         && loader.isLoading()) {

libraries/exoplayer_hls/src/main/java/androidx/media3/exoplayer/hls/playlist/DefaultHlsPlaylistTracker.java

@@ -763,13 +763,10 @@ public final class DefaultHlsPlaylistTracker
       }
       earliestNextLoadTimeMs =
           currentTimeMs + Util.usToMs(durationUntilNextLoadUs) - loadEventInfo.loadDurationMs;
-      // Schedule a load if this is the primary playlist or a playlist of a low-latency stream and
+      // Schedule a load if this is the primary playlist and it doesn't have an end tag. Else the
-      // it doesn't have an end tag. Else the next load will be scheduled when refreshPlaylist is
+      // next load will be scheduled when refreshPlaylist is called, or when this playlist becomes
-      // called, or when this playlist becomes the primary.
+      // the primary.
-      boolean scheduleLoad =
+      if (playlistUrl.equals(primaryMediaPlaylistUrl) && !playlistSnapshot.hasEndTag) {
-          playlistSnapshot.partTargetDurationUs != C.TIME_UNSET
-              || playlistUrl.equals(primaryMediaPlaylistUrl);
-      if (scheduleLoad && !playlistSnapshot.hasEndTag) {
         loadPlaylistInternal(getMediaPlaylistUriForReload());
       }
     }

libraries/exoplayer_hls/src/test/java/androidx/media3/exoplayer/hls/playlist/DefaultHlsPlaylistTrackerTest.java

@@ -22,6 +22,7 @@ import androidx.media3.datasource.DataSource;
 import androidx.media3.datasource.DefaultHttpDataSource;
 import androidx.media3.exoplayer.source.MediaSourceEventListener;
 import androidx.media3.exoplayer.upstream.DefaultLoadErrorHandlingPolicy;
+import androidx.media3.exoplayer.upstream.LoadErrorHandlingPolicy;
 import androidx.media3.test.utils.TestUtil;
 import androidx.media3.test.utils.robolectric.RobolectricUtil;
 import androidx.test.core.app.ApplicationProvider;
@@ -31,6 +32,7 @@ import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.atomic.AtomicReference;
 import okhttp3.HttpUrl;
 import okhttp3.mockwebserver.MockResponse;
 import okhttp3.mockwebserver.MockWebServer;
@@ -365,6 +367,85 @@ public class DefaultHlsPlaylistTrackerTest {
     assertThat(mediaPlaylists.get(1).trailingParts).hasSize(2);
   }
 
+  @Test
+  public void start_lowLatencyNotScheduleReloadForNonPrimaryPlaylist() throws Exception {
+    List<HttpUrl> httpUrls =
+        enqueueWebServerResponses(
+            new String[] {
+              "/multivariant.m3u8",
+              "/media0/playlist.m3u8",
+              "/media1/playlist.m3u8",
+              "/media1/playlist.m3u8",
+              "/media1/playlist.m3u8?_HLS_msn=14&_HLS_part=0",
+            },
+            getMockResponse(SAMPLE_M3U8_LIVE_MULTIVARIANT),
+            getMockResponse(
+                SAMPLE_M3U8_LIVE_MEDIA_CAN_BLOCK_RELOAD_LOW_LATENCY_FULL_SEGMENT_PRELOAD),
+            getMockResponse(
+                SAMPLE_M3U8_LIVE_MEDIA_CAN_BLOCK_RELOAD_LOW_LATENCY_FULL_SEGMENT_PRELOAD),
+            getMockResponse(
+                SAMPLE_M3U8_LIVE_MEDIA_CAN_BLOCK_RELOAD_LOW_LATENCY_FULL_SEGMENT_PRELOAD),
+            getMockResponse(
+                SAMPLE_M3U8_LIVE_MEDIA_CAN_BLOCK_RELOAD_LOW_LATENCY_FULL_SEGMENT_PRELOAD_NEXT));
+
+    DefaultHlsPlaylistTracker defaultHlsPlaylistTracker =
+        new DefaultHlsPlaylistTracker(
+            dataType -> new DefaultHttpDataSource.Factory().createDataSource(),
+            new DefaultLoadErrorHandlingPolicy(),
+            new DefaultHlsPlaylistParserFactory());
+    List<HlsMediaPlaylist> mediaPlaylists = new ArrayList<>();
+    AtomicInteger playlistCounter = new AtomicInteger();
+    AtomicReference<TimeoutException> primaryPlaylistChangeExceptionRef = new AtomicReference<>();
+    defaultHlsPlaylistTracker.addListener(
+        new HlsPlaylistTracker.PlaylistEventListener() {
+          @Override
+          public void onPlaylistChanged() {
+            // Upon the first call of onPlaylistChanged(), we simulate the situation that the
+            // primary playlist url changes.
+            Uri url = defaultHlsPlaylistTracker.getMultivariantPlaylist().mediaPlaylistUrls.get(1);
+            if (defaultHlsPlaylistTracker.isSnapshotValid(url)) {
+              return;
+            }
+            defaultHlsPlaylistTracker.refreshPlaylist(url);
+            try {
+              // Make sure that the playlist for the new url has been refreshed and set it as the
+              // current primary playlist, before this method returns.
+              RobolectricUtil.runMainLooperUntil(
+                  () ->
+                      defaultHlsPlaylistTracker.getPlaylistSnapshot(url, /* isForPlayback= */ true)
+                          != null);
+            } catch (TimeoutException e) {
+              primaryPlaylistChangeExceptionRef.set(e);
+            }
+          }
+
+          @Override
+          public boolean onPlaylistError(
+              Uri url, LoadErrorHandlingPolicy.LoadErrorInfo loadErrorInfo, boolean forceRetry) {
+            return false;
+          }
+        });
+
+    defaultHlsPlaylistTracker.start(
+        Uri.parse(mockWebServer.url("/multivariant.m3u8").toString()),
+        new MediaSourceEventListener.EventDispatcher(),
+        mediaPlaylist -> {
+          mediaPlaylists.add(mediaPlaylist);
+          playlistCounter.addAndGet(1);
+        });
+    RobolectricUtil.runMainLooperUntil(() -> playlistCounter.get() >= 2);
+    defaultHlsPlaylistTracker.stop();
+
+    assertThat(primaryPlaylistChangeExceptionRef.get()).isNull();
+    assertRequestUrlsCalled(httpUrls);
+    assertThat(mediaPlaylists.get(0).mediaSequence).isEqualTo(10);
+    assertThat(mediaPlaylists.get(0).segments).hasSize(4);
+    assertThat(mediaPlaylists.get(0).trailingParts).hasSize(1);
+    assertThat(mediaPlaylists.get(1).mediaSequence).isEqualTo(10);
+    assertThat(mediaPlaylists.get(1).segments).hasSize(4);
+    assertThat(mediaPlaylists.get(1).trailingParts).hasSize(2);
+  }
+
   @Test
   public void start_httpBadRequest_forcesFullNonBlockingPlaylistRequest()
       throws IOException, TimeoutException, InterruptedException {

libraries/test_data/src/test/assets/media/m3u8/live_low_latency_multivariant

@@ -3,3 +3,5 @@
 
 #EXT-X-STREAM-INF:BANDWIDTH=2000000,CODECS="avc1.640028,mp4a.40.2"
 media0/playlist.m3u8
+#EXT-X-STREAM-INF:BANDWIDTH=1000000,CODECS="avc1.640028,mp4a.40.2"
+media1/playlist.m3u8