fix(hls): Fix X-PRELOAD-HINT failure with LL mode off (#4212)

When LL mode was off, the HLS parser would throw on X-PRELOAD-HINT
segments.  The logic for requiring EXTINF has been corrected so that
this will not happen.

Closes #4185

lib/hls/hls_parser.js

@@ -1826,7 +1826,7 @@ shaka.hls.HlsParser = class {
    * @param {!Map.<string, string>} variables
    * @param {string} absoluteMediaPlaylistUri
    * @param {string} type
-   * @return {!shaka.media.SegmentReference}
+   * @return {shaka.media.SegmentReference}
    * @private
    */
   createSegmentReference_(
@@ -1871,9 +1871,23 @@ shaka.hls.HlsParser = class {
       status = shaka.media.SegmentReference.Status.MISSING;
     }
 
+    if (!extinfTag) {
+      if (hlsSegment.partialSegments.length == 0) {
+        // EXTINF tag must be available if the segment has no partial segments.
+        throw new shaka.util.Error(
+            shaka.util.Error.Severity.CRITICAL,
+            shaka.util.Error.Category.MANIFEST,
+            shaka.util.Error.Code.HLS_REQUIRED_TAG_MISSING, 'EXTINF');
+      } else if (!this.lowLatencyMode_) {
+        // Without EXTINF and without low-latency mode, partial segments get
+        // ignored.
+        return null;
+      }
+    }
+
     // Create SegmentReferences for the partial segments.
     const partialSegmentRefs = [];
-    if (this.lowLatencyMode_ && hlsSegment.partialSegments.length) {
+    if (this.lowLatencyMode_) {
       for (let i = 0; i < hlsSegment.partialSegments.length; i++) {
         const item = hlsSegment.partialSegments[i];
         const pPreviousReference = i == 0 ?
@@ -1924,14 +1938,6 @@ shaka.hls.HlsParser = class {
             partialStatus);
         partialSegmentRefs.push(partial);
       } // for-loop of hlsSegment.partialSegments
-    } else {
-      // EXTINF tag must be available if the segment has no partial segments.
-      if (!extinfTag) {
-        throw new shaka.util.Error(
-            shaka.util.Error.Severity.CRITICAL,
-            shaka.util.Error.Category.MANIFEST,
-            shaka.util.Error.Code.HLS_REQUIRED_TAG_MISSING, 'EXTINF');
-      }
     }
 
     // If the segment has EXTINF tag, set the segment's end time, start byte
@@ -1950,6 +1956,7 @@ shaka.hls.HlsParser = class {
     } else {
       endTime = partialSegmentRefs[partialSegmentRefs.length - 1].endTime;
     }
+
     // If the segment has EXT-X-BYTERANGE tag, set the start byte and end byte
     // base on the byterange information. If segment has no EXT-X-BYTERANGE tag
     // and has partial segments, set the start byte and end byte base on the
@@ -2118,7 +2125,9 @@ shaka.hls.HlsParser = class {
           playlist.absoluteUri,
           type);
 
-      references.push(reference);
+      if (reference) {
+        references.push(reference);
+      }
     }
 
     // If some segments have sync times, but not all, extrapolate the sync

test/hls/hls_live_unit.js

@@ -603,6 +603,28 @@ describe('HlsParser live', () => {
       ManifestParser.verifySegmentIndex(video, [ref, ref2]);
     });
 
+    // Test for https://github.com/shaka-project/shaka-player/issues/4185
+    it('does not fail on preload hints with LL mode off', async () => {
+      // LL mode must be off for this test!
+      playerInterface.isLowLatencyMode = () => false;
+
+      const mediaWithPartialSegments = [
+        '#EXTM3U\n',
+        '#EXT-X-TARGETDURATION:5\n',
+        '#EXTINF:4,\n',
+        'main.mp4\n',
+        '#EXT-X-PART:DURATION=2,URI="partial.mp4",BYTERANGE=210@0\n',
+        '#EXT-X-PRELOAD-HINT:TYPE=PART,URI="partial.mp4",BYTERANGE-START=210\n',
+      ].join('');
+
+      fakeNetEngine
+          .setResponseText('test:/master', master)
+          .setResponseText('test:/video', mediaWithPartialSegments);
+
+      // If this throws, the test fails.  Otherwise, it passes.
+      await parser.start('test:/master', playerInterface);
+    });
+
     describe('update', () => {
       it('adds new segments when they appear', async () => {
         const ref1 = makeReference(