WIP(HLS): Fix duplicate hinted segments (alternate fix)

lib/media/streaming_engine.js

@@ -1123,16 +1123,39 @@ shaka.media.StreamingEngine = class {
         mediaState.stream.segmentIndex,
         'segment index should have been generated already');
 
+    // This scenario requires special handling.  In LL-HLS and sequence mode, we
+    // need to be careful with preload-hinted partial segments, which don't have
+    // a known duration at the time we append them.  If we aren't careful, we
+    // can end up appending the same partial segment twice, since based on the
+    // last segment reference alone, we might think we need the same timestamp
+    // again.  If we make that mistake in sequence mode, we end up with repeated
+    // content and possible A/V desync.
+    // https://github.com/shaka-project/shaka-player/issues/4223#issuecomment-1138910167
+    const lastReference = mediaState.lastSegmentReference;
+    const lastReferenceHadUnknownDuration =
+        lastReference &&
+        lastReference.startTime == lastReference.endTime;
+
+    const isRepeatedSegment = (ref) => {
+      // Sometimes, the new ref's start time is a fraction of a second earlier,
+      // due to rounding errors.  We should still detect that as a repeat.
+      return ref &&
+          ref.startTime <= lastReference.startTime;
+    };
+
     if (mediaState.segmentIterator) {
       // Something is buffered from the same Stream.  Use the current position
       // in the segment index.  This is updated via next() after each segment is
       // appended.
-      return mediaState.segmentIterator.current();
-    } else if (mediaState.lastSegmentReference || bufferEnd) {
+      const currentRef = mediaState.segmentIterator.current();
+      if (lastReferenceHadUnknownDuration && isRepeatedSegment(currentRef)) {
+        // Advance one more time.
+        return mediaState.segmentIterator.next().value;
+      }
+      return currentRef;
+    } else if (lastReference || bufferEnd) {
       // Something is buffered from another Stream.
-      const time = mediaState.lastSegmentReference ?
-          mediaState.lastSegmentReference.endTime :
-          bufferEnd;
+      const time = lastReference ? lastReference.endTime : bufferEnd;
       goog.asserts.assert(time != null, 'Should have a time to search');
       shaka.log.v1(
           logPrefix, 'looking up segment from new stream endTime:', time);
@@ -1144,6 +1167,10 @@ shaka.media.StreamingEngine = class {
       if (ref == null) {
         shaka.log.warning(logPrefix, 'cannot find segment', 'endTime:', time);
       }
+      if (lastReferenceHadUnknownDuration && isRepeatedSegment(ref)) {
+        // Advance one more time.
+        return mediaState.segmentIterator.next().value;
+      }
       return ref;
     } else {
       // Nothing is buffered.  Start at the playhead time.