fix(HLS): Fix AV sync over ad boundaries (#4824)

If server-side ad segments aren't aligned, AV could get out of sync by
accumulating errors in the timestampOffset of the SourceBuffers.

This improves the issue by tracking discontinuity boundaries and
resetting timestampOffset to theoretical segment start times when a
boundary is crossed.

Issue #4589

lib/hls/hls_parser.js

@@ -2603,12 +2603,12 @@ shaka.hls.HlsParser = class {
     /** @type {shaka.extern.HlsAes128Key|undefined} */
     let hlsAes128Key = undefined;
 
-    // We may need to look at the media itself to determine a segment start
-    // time.
+    let discontinuitySequence = shaka.hls.Utils.getFirstTagWithNameAsNumber(
+        playlist.tags, 'EXT-X-DISCONTINUITY-SEQUENCE', 0);
     const mediaSequenceNumber = shaka.hls.Utils.getFirstTagWithNameAsNumber(
         playlist.tags, 'EXT-X-MEDIA-SEQUENCE', 0);
-    const skipTag = shaka.hls.Utils.getFirstTagWithName(playlist.tags,
-        'EXT-X-SKIP');
+    const skipTag = shaka.hls.Utils.getFirstTagWithName(
+        playlist.tags, 'EXT-X-SKIP');
     const skippedSegments =
         skipTag ? Number(skipTag.getAttributeValue('SKIPPED-SEGMENTS')) : 0;
     let position = mediaSequenceNumber + skippedSegments;
@@ -2631,6 +2631,12 @@ shaka.hls.HlsParser = class {
           (i == 0) ? firstStartTime : previousReference.endTime;
       position = mediaSequenceNumber + skippedSegments + i;
 
+      const discontinuityTag = shaka.hls.Utils.getFirstTagWithName(
+          playlist.tags, 'EXT-X-DISCONTINUITY');
+      if (discontinuityTag) {
+        discontinuitySequence++;
+      }
+
       // Apply new AES-128 tags as you see them, keeping a running total.
       for (const drmTag of item.tags) {
         if (drmTag.name == 'EXT-X-KEY' &&
@@ -2668,6 +2674,8 @@ shaka.hls.HlsParser = class {
       previousReference = reference;
 
       if (reference) {
+        reference.discontinuitySequence = discontinuitySequence;
+
         if (this.config_.hls.ignoreManifestProgramDateTime &&
             this.minSequenceNumber_ != null &&
             position < this.minSequenceNumber_) {

lib/media/media_source_engine.js

@@ -896,6 +896,22 @@ shaka.media.MediaSourceEngine = class {
     ]);
   }
 
+  /**
+   * Adjust timestamp offset to maintain AV sync across discontinuities.
+   * Only used in sequence mode.
+   *
+   * @param {shaka.util.ManifestParserUtils.ContentType} contentType
+   * @param {number} timestampOffset
+   * @return {!Promise}
+   */
+  async resync(contentType, timestampOffset) {
+    goog.asserts.assert(this.sequenceMode_,
+        'resyncAudio only used with sequence mode!');
+    await this.enqueueOperation_(
+        contentType,
+        () => this.setTimestampOffset_(contentType, timestampOffset));
+  }
+
   /**
    * @param {string=} reason Valid reasons are 'network' and 'decode'.
    * @return {!Promise}

lib/media/segment_reference.js

@@ -246,6 +246,9 @@ shaka.media.SegmentReference = class {
 
     /** @type {?shaka.media.SegmentReference.ThumbnailSprite} */
     this.thumbnailSprite = null;
+
+    /** @type {number} */
+    this.discontinuitySequence = 0;
   }
 
   /**

lib/media/streaming_engine.js

@@ -1659,6 +1659,19 @@ shaka.media.StreamingEngine = class {
       }
     }
 
+    if (this.manifest_.sequenceMode) {
+      // Across discontinuity bounds, we should resync timestamps for
+      // sequence mode playbacks.  The next segment appended should
+      // land at its theoretical timestamp from the segment index.
+      const lastDiscontinuitySequence =
+          mediaState.lastSegmentReference ?
+              mediaState.lastSegmentReference.discontinuitySequence : null;
+      if (reference.discontinuitySequence != lastDiscontinuitySequence) {
+        operations.push(this.playerInterface_.mediaSourceEngine.resync(
+            mediaState.type, reference.startTime));
+      }
+    }
+
     await Promise.all(operations);
   }
 
@@ -1713,8 +1726,6 @@ shaka.media.StreamingEngine = class {
 
     await this.evict_(mediaState, presentationTime);
     this.destroyer_.ensureNotDestroyed();
-    shaka.log.v1(logPrefix, 'appending media segment at',
-        (reference.syncTime == null ? 'unknown' : reference.syncTime));
 
     // 'seeked' or 'adaptation' triggered logic applies only to this
     // appendBuffer() call.

test/test/util/manifest_parser_util.js

@@ -73,7 +73,7 @@ shaka.test.ManifestParser = class {
     const appendWindowStart = /** @type {?} */(jasmine.any(Number));
     const appendWindowEnd = /** @type {?} */(jasmine.any(Number));
 
-    return new shaka.media.SegmentReference(
+    const ref = new shaka.media.SegmentReference(
         start, end, getUris, startByte, endByte,
         initSegmentReference,
         timestampOffset,
@@ -82,7 +82,8 @@ shaka.test.ManifestParser = class {
         partialReferences,
         tilesLayout,
         /* tileDuration= */ undefined,
-        syncTime,
-    );
+        syncTime);
+    ref.discontinuitySequence = /** @type {?} */(jasmine.any(Number));
+    return ref;
   }
 };