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

@@ -1739,8 +1739,8 @@ shaka.hls.HlsParser = class {
    * @param {number} timestampOffset
    * @param {!Map.<string, string>} variables
    * @param {string} absoluteMediaPlaylistUri
-   * @return {!shaka.media.SegmentReference}
    * @param {string} type
+   * @return {shaka.media.SegmentReference}
    * @private
    */
   createSegmentReference_(
@@ -1756,9 +1756,23 @@ shaka.hls.HlsParser = class {
     let startByte = 0;
     let endByte = null;
 
+    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 ?
@@ -1799,14 +1813,6 @@ shaka.hls.HlsParser = class {
             /* appendWindowEnd= */ Infinity);
         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
@@ -1825,6 +1831,7 @@ 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
@@ -2054,7 +2061,9 @@ shaka.hls.HlsParser = class {
             playlist.absoluteUri,
             type);
 
-        references.push(reference);
+        if (reference) {
+          references.push(reference);
+        }
       } else if (!this.lowLatencyMode_) {
         // If a segment has no extinfTag, it must contain partial segments.
         shaka.log.alwaysWarn('Low-latency HLS live stream detected, but ' +
@@ -2233,6 +2242,9 @@ shaka.hls.HlsParser = class {
         variables,
         /* absoluteMediaPlaylistUri= */ '',
         type);
+    goog.asserts.assert(
+        segmentRef != null, 'Segment reference should not be null!');
+
     // If we are updating the manifest, we can usually skip fetching the segment
     // by examining the references we already have.  This won't be possible if
     // there was some kind of lag or delay updating the manifest on the server,

test/hls/hls_live_unit.js

@@ -699,6 +699,32 @@ 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',
+        '#EXT-X-MAP:URI="init.mp4",BYTERANGE="616@0"\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)
+          .setResponseValue('test:/init.mp4', initSegmentData)
+          .setResponseValue('test:/main.mp4', segmentData)
+          .setResponseValue('test:/partial.mp4', segmentData);
+
+      // 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 = ManifestParser.makeReference('test:/main.mp4', 2, 4);