feat(ll-hls): only request delta updates when server supports it

We should only request playlist delta updates when the lowLatencyMode is
enabled, and when the server supports the feature. We can check if the
playlist has a "CAN-SKIP-UNTIL" tag to know if the server supports that.

Issue shaka-project#1525

Change-Id: I304a008aeb3a9e019f27304ba836d31538c94fbf

Author: michellezhuogg

lib/hls/hls_parser.js

@@ -302,7 +302,7 @@ shaka.hls.HlsParser = class {
     const PresentationType = shaka.hls.HlsParser.PresentationType_;
     const manifestUri = streamInfo.absoluteMediaPlaylistUri;
     const uriObj = new goog.Uri(manifestUri);
-    if (this.lowLatencyMode_) {
+    if (this.lowLatencyMode_ && streamInfo.canSkipSegments) {
       // Enable delta updates. This will replace older segments with
       // 'EXT-X-SKIP' tag in the media playlist.
       uriObj.setQueryData(new goog.Uri.QueryData('_HLS_skip=YES'));
@@ -1383,6 +1383,11 @@ shaka.hls.HlsParser = class {
       }
     }
 
+    const serverControlTag = shaka.hls.Utils.getFirstTagWithName(
+        playlist.tags, 'EXT-X-SERVER-CONTROL');
+    const canSkipSegments = serverControlTag ?
+          serverControlTag.getAttribute('CAN-SKIP-UNTIL') != null : false;
+
     /** @type {shaka.extern.Stream} */
     const stream = {
       id: this.globalId_++,
@@ -1425,6 +1430,7 @@ shaka.hls.HlsParser = class {
       maxTimestamp: lastEndTime,
       mediaSequenceToStartTime,
       discontinuityToMediaSequence,
+      canSkipSegments,
     };
   }
 
@@ -2658,7 +2664,8 @@ shaka.hls.HlsParser = class {
  *   minTimestamp: number,
  *   maxTimestamp: number,
  *   mediaSequenceToStartTime: !Map.<number, number>,
- *   discontinuityToMediaSequence: !Map.<number, number>
+ *   discontinuityToMediaSequence: !Map.<number, number>,
+ *   canSkipSegments: boolean
  * }}
  *
  * @description
@@ -2683,6 +2690,9 @@ shaka.hls.HlsParser = class {
  * @property  {!Map.<number, number>} discontinuityToMediaSequence
  *   A map of discontinuity sequence numbers to the media sequence number of the
  *   segment starting with that discontinuity sequence number.
+ * @property {boolean} canSkipSegments
+ *  True if the server supports delta playlist updates, and we can send a
+ *  request for a playlist that can skip older media segments.
  */
 shaka.hls.HlsParser.StreamInfo;
 

test/hls/hls_live_unit.js

@@ -907,6 +907,56 @@ describe('HlsParser live', () => {
             partialEndByte);  // partial segment request
       });
 
+      it('request playlist delta updates to skip segments', async () => {
+        const mediaWithDeltaUpdates = [
+          '#EXTM3U\n',
+          '#EXT-X-PLAYLIST-TYPE:LIVE\n',
+          '#EXT-X-TARGETDURATION:5\n',
+          '#EXT-X-MEDIA-SEQUENCE:0\n',
+          '#EXT-X-MAP:URI="init.mp4",BYTERANGE="616@0"\n',
+          '#EXT-X-SERVER-CONTROL:CAN-SKIP-UNTIL=60.0\n',
+          '#EXTINF:2,\n',
+          'main.mp4\n',
+          '#EXTINF:2,\n',
+          'main2.mp4\n',
+        ].join('');
+
+        const mediaWithSkippedSegments = [
+          '#EXTM3U\n',
+          '#EXT-X-TARGETDURATION:5\n',
+          '#EXT-X-MAP:URI="init.mp4",BYTERANGE="616@0"\n',
+          '#EXT-X-MEDIA-SEQUENCE:0\n',
+          '#EXT-X-SERVER-CONTROL:CAN-SKIP-UNTIL=60.0\n',
+          '#EXT-X-SKIP:SKIPPED-SEGMENTS=1\n',
+          '#EXTINF:2,\n',
+          'main2.mp4\n',
+          '#EXTINF:2,\n',
+          'main3.mp4\n',
+        ].join('');
+
+        fakeNetEngine
+            .setResponseText('test:/master', master)
+            .setResponseText('test:/video', mediaWithDeltaUpdates)
+            .setResponseText('test:/video?_HLS_skip=YES',
+                mediaWithSkippedSegments)
+            .setResponseValue('test:/init.mp4', initSegmentData)
+            .setResponseValue('test:/main.mp4', segmentData)
+            .setResponseValue('test:/main2.mp4', segmentData)
+            .setResponseValue('test:/main3.mp4', segmentData);
+
+        playerInterface.isLowLatencyMode = () => true;
+        await parser.start('test:/master', playerInterface);
+        // Replace the entries with the updated values.
+
+        fakeNetEngine.request.calls.reset();
+        await delayForUpdatePeriod();
+
+        fakeNetEngine.expectRequest(
+            'test:/video?_HLS_skip=YES',
+            shaka.net.NetworkingEngine.RequestType.MANIFEST);
+      });
+
+
       it('skips older segments', async () => {
         const mediaWithSkippedSegments = [
           '#EXTM3U\n',
@@ -935,7 +985,7 @@ describe('HlsParser live', () => {
       it('skips older segments with discontinuity', async () => {
         const mediaWithDiscontinuity2 = [
           '#EXTM3U\n',
-          '#EXT-X-PLAYLIST-TYPE:EVENT\n',
+          '#EXT-X-PLAYLIST-TYPE:LIVE\n',
           '#EXT-X-TARGETDURATION:5\n',
           '#EXT-X-MAP:URI="init.mp4",BYTERANGE="616@0"\n',
           '#EXT-X-DISCONTINUITY-SEQUENCE:30\n',