feat(HLS): Optimize LL-HLS with byterange (shaka-project#5342)

- Mark the the full segment as preload if it has some partial preload
segment
- Request _HLS_part=0 when using LL-HLS byterange optimization

lib/hls/hls_parser.js

@@ -2360,6 +2360,9 @@ shaka.hls.HlsParser = class {
     const partialReferences = lastSegment.partialReferences;
     if (!lastSegment.partialReferences.length) {
       nextMediaSequence++;
+      if (lastSegment.hasByterangeOptimization()) {
+        nextPart = 0;
+      }
       return {
         nextMediaSequence,
         nextPart,
@@ -2844,6 +2847,7 @@ shaka.hls.HlsParser = class {
     let segmentWithByteRangeOptimization = false;
     let getUrisOptimization = null;
     let somePartialSegmentWithGap = false;
+    let isPreloadSegment = false;
 
     if (this.lowLatencyMode_ && hlsSegment.partialSegments.length) {
       const byterangeOptimizationSupport = (mimeType == 'video/mp4' ||
@@ -2937,6 +2941,7 @@ shaka.hls.HlsParser = class {
             hlsAes128Key);
         if (item.name == 'EXT-X-PRELOAD-HINT') {
           partial.markAsPreload();
+          isPreloadSegment = true;
         }
         // The spec doesn't say that we can assume INDEPENDENT=YES for the
         // first partial segment. It does call the flag "optional", though, and
@@ -3043,6 +3048,10 @@ shaka.hls.HlsParser = class {
 
     if (segmentWithByteRangeOptimization) {
       reference.markAsByterangeOptimization();
+
+      if (isPreloadSegment) {
+        reference.markAsPreload();
+      }
     }
 
     return reference;

test/hls/hls_live_unit.js

@@ -722,6 +722,7 @@ describe('HlsParser live', () => {
             /* baseUri= */ '', /* startByte= */ 0, /* endByte= */ null,
             /* timestampOffset= */ 0);
         ref2.markAsByterangeOptimization();
+        ref2.markAsPreload();
 
         await testInitialManifest(master, mediaWithPartialSegments,
             [ref, ref2]);