fix(hls): Fixed buffering issue with live HLS (shaka-project#4002)

HLS now appends segments in sequence mode. In order to handle seeks,
we set the timestampOffset property on SourceBuffer to the startTime of
the segment. This is done after every seek, or on startup.

For non-sequence-mode content (DASH), we normally set timestampOffset
based on the Period structure.  This should be suppressed in sequence
mode, though, where only the reference startTime matters.

The buffering issue was caused by two things in combination:

1. The HLS parser set meaningless timestampOffset values that would
change when a playlist was updated
2. We would use those timestampOffset values in setStreamProperties,
even though this should be skipped in sequence mode

These two things in combination would lead MediaSourceEngine to start
inserting segments near the start of the presentation, rather than at
the live edge.

This changes MediaSourceEngine so that in sequence mode, timestampOffset
is ignored in setStreamProperties.  This also cleans up the HLS parser
to set each reference's timestampOffset to 0, since they should be
ignored anyway.

lib/hls/hls_parser.js

@@ -1704,7 +1704,6 @@ shaka.hls.HlsParser = class {
    * @param {shaka.media.SegmentReference} previousReference
    * @param {!shaka.hls.Segment} hlsSegment
    * @param {number} startTime
-   * @param {number} timestampOffset
    * @param {!Map.<string, string>} variables
    * @param {string} absoluteMediaPlaylistUri
    * @return {!shaka.media.SegmentReference}
@@ -1713,7 +1712,7 @@ shaka.hls.HlsParser = class {
    */
   createSegmentReference_(
       initSegmentReference, previousReference, hlsSegment, startTime,
-      timestampOffset, variables, absoluteMediaPlaylistUri, type) {
+      variables, absoluteMediaPlaylistUri, type) {
     const tags = hlsSegment.tags;
     const absoluteSegmentUri = this.variableSubstitution_(
         hlsSegment.absoluteUri, variables);
@@ -1766,7 +1765,7 @@ shaka.hls.HlsParser = class {
             pStartByte,
             pEndByte,
             initSegmentReference,
-            timestampOffset,
+            /* timestampOffset= */ 0, // This value is ignored in sequence mode.
             /* appendWindowStart= */ 0,
             /* appendWindowEnd= */ Infinity);
         partialSegmentRefs.push(partial);
@@ -1834,7 +1833,7 @@ shaka.hls.HlsParser = class {
         startByte,
         endByte,
         initSegmentReference,
-        timestampOffset,
+        /* timestampOffset= */ 0, // This value is ignored in sequence mode.
         /* appendWindowStart= */ 0,
         /* appendWindowEnd= */ Infinity,
         partialSegmentRefs,
@@ -1960,7 +1959,6 @@ shaka.hls.HlsParser = class {
           previousReference,
           item,
           startTime,
-          firstStartTime,
           variables,
           playlist.absoluteUri,
           type);

lib/media/media_source_engine.js

@@ -691,13 +691,18 @@ shaka.media.MediaSourceEngine = class {
    * @param {number} appendWindowEnd The timestamp to set the append window end
    *   to.  For future appends, frames/samples with timestamps greater than this
    *   value will be dropped.
+   * @param {boolean} sequenceMode  If true, the timestampOffset will not be
+   *   applied in this step.
    * @return {!Promise}
    */
   async setStreamProperties(
-      contentType, timestampOffset, appendWindowStart, appendWindowEnd) {
+      contentType, timestampOffset, appendWindowStart, appendWindowEnd,
+      sequenceMode) {
     const ContentType = shaka.util.ManifestParserUtils.ContentType;
     if (contentType == ContentType.TEXT) {
-      this.textEngine_.setTimestampOffset(timestampOffset);
+      if (!sequenceMode) {
+        this.textEngine_.setTimestampOffset(timestampOffset);
+      }
       this.textEngine_.setAppendWindow(appendWindowStart, appendWindowEnd);
       return;
     }
@@ -714,7 +719,10 @@ shaka.media.MediaSourceEngine = class {
       this.enqueueOperation_(
           contentType,
           () => this.abort_(contentType)),
-      this.enqueueOperation_(
+      // Don't set the timestampOffset here when in sequenceMode, since we
+      // use timestampOffset for a different purpose in that mode (e.g. to
+      // indicate where the current segment is).
+      sequenceMode ? Promise.resolve() : this.enqueueOperation_(
           contentType,
           () => this.setTimestampOffset_(contentType, timestampOffset)),
       this.enqueueOperation_(

lib/media/streaming_engine.js

@@ -1520,7 +1520,7 @@ shaka.media.StreamingEngine = class {
 
           await this.playerInterface_.mediaSourceEngine.setStreamProperties(
               mediaState.type, timestampOffset, appendWindowStart,
-              appendWindowEnd);
+              appendWindowEnd, this.manifest_.sequenceMode);
         } catch (error) {
           mediaState.lastAppendWindowStart = null;
           mediaState.lastAppendWindowEnd = null;

test/media/media_source_engine_integration.js

@@ -295,7 +295,27 @@ describe('MediaSourceEngine', () => {
     await mediaSourceEngine.setStreamProperties(ContentType.VIDEO,
         /* timestampOffset= */ 0,
         /* appendWindowStart= */ 5,
-        /* appendWindowEnd= */ 18);
+        /* appendWindowEnd= */ 18,
+        /* sequenceMode= */ false);
+    expect(buffered(ContentType.VIDEO, 0)).toBe(0);
+    await append(ContentType.VIDEO, 0);
+    expect(bufferStart(ContentType.VIDEO)).toBeCloseTo(5, 1);
+    expect(buffered(ContentType.VIDEO, 5)).toBeCloseTo(5, 1);
+    await append(ContentType.VIDEO, 1);
+    expect(buffered(ContentType.VIDEO, 5)).toBeCloseTo(13, 1);
+  });
+
+  it('does not initialize timestamp offset in sequence mode', async () => {
+    const initObject = new Map();
+    initObject.set(ContentType.VIDEO, getFakeStream(metadata.video));
+    await mediaSourceEngine.init(initObject, false);
+    await mediaSourceEngine.setDuration(presentationDuration);
+    await appendInit(ContentType.VIDEO);
+    await mediaSourceEngine.setStreamProperties(ContentType.VIDEO,
+        /* timestampOffset= */ 100,
+        /* appendWindowStart= */ 5,
+        /* appendWindowEnd= */ 18,
+        /* sequenceMode= */ true);
     expect(buffered(ContentType.VIDEO, 0)).toBe(0);
     await append(ContentType.VIDEO, 0);
     expect(bufferStart(ContentType.VIDEO)).toBeCloseTo(5, 1);
@@ -314,7 +334,8 @@ describe('MediaSourceEngine', () => {
     await mediaSourceEngine.setStreamProperties(ContentType.VIDEO,
         /* timestampOffset= */ 0,
         /* appendWindowStart= */ 0,
-        /* appendWindowEnd= */ 20);
+        /* appendWindowEnd= */ 20,
+        /* sequenceMode= */ false);
     await append(ContentType.VIDEO, 0);
     await append(ContentType.VIDEO, 1);
     expect(bufferStart(ContentType.VIDEO)).toBeCloseTo(0, 1);
@@ -326,7 +347,8 @@ describe('MediaSourceEngine', () => {
     await mediaSourceEngine.setStreamProperties(ContentType.VIDEO,
         /* timestampOffset= */ 15,
         /* appendWindowStart= */ 20,
-        /* appendWindowEnd= */ 35);
+        /* appendWindowEnd= */ 35,
+        /* sequenceMode= */ false);
     await append(ContentType.VIDEO, 0);
     await append(ContentType.VIDEO, 1);
     expect(bufferStart(ContentType.VIDEO)).toBeCloseTo(0, 1);

test/media/media_source_engine_unit.js

@@ -825,7 +825,8 @@ describe('MediaSourceEngine', () => {
       await mediaSourceEngine.setStreamProperties(ContentType.TEXT,
           /* timestampOffset= */ 10,
           /* appendWindowStart= */ 0,
-          /* appendWindowEnd= */ 20);
+          /* appendWindowEnd= */ 20,
+          /* sequenceMode= */ false);
       expect(mockTextEngine.setTimestampOffset).toHaveBeenCalledWith(10);
       expect(mockTextEngine.setAppendWindow).toHaveBeenCalledWith(0, 20);
     });

test/media/streaming_engine_unit.js

@@ -826,7 +826,7 @@ describe('StreamingEngine', () => {
       asymmetricMatch: (val) => val > 40 && val <= 40.1,
     };
     expect(mediaSourceEngine.setStreamProperties)
-        .toHaveBeenCalledWith('video', 0, lt20, gt40);
+        .toHaveBeenCalledWith('video', 0, lt20, gt40, false);
   });
 
   // Regression test for https://github.com/google/shaka-player/issues/3717

test/test/util/fake_media_source_engine.js

@@ -114,8 +114,8 @@ shaka.test.FakeMediaSourceEngine = class {
 
     /** @type {!jasmine.Spy} */
     this.setStreamProperties = jasmine.createSpy('setStreamProperties')
-        .and.callFake((type, offset, end) =>
-          this.setStreamPropertiesImpl_(type, offset, end));
+        .and.callFake((type, offset, end, sequenceMode) =>
+          this.setStreamPropertiesImpl_(type, offset, end, sequenceMode));
 
     /** @type {!jasmine.Spy} */
     this.remove = jasmine.createSpy('remove')
@@ -362,14 +362,17 @@ shaka.test.FakeMediaSourceEngine = class {
    * @param {string} type
    * @param {number} offset
    * @param {number} appendWindowEnd
+   * @param {boolean} sequenceMode
    * @return {!Promise}
    * @private
    */
-  setStreamPropertiesImpl_(type, offset, appendWindowEnd) {
+  setStreamPropertiesImpl_(type, offset, appendWindowEnd, sequenceMode) {
     if (!this.segments[type]) {
       throw new Error('unexpected type');
     }
-    this.timestampOffsets_[type] = offset;
+    if (!sequenceMode) {
+      this.timestampOffsets_[type] = offset;
+    }
     // Don't use |appendWindowEnd|.
     return Promise.resolve();
   }