fix(HLS): Require SegmentIndex to return independent segments only

lib/media/segment_index.js

@@ -9,6 +9,7 @@ goog.provide('shaka.media.SegmentIndex');
 goog.provide('shaka.media.SegmentIterator');
 
 goog.require('goog.asserts');
+goog.require('shaka.log');
 goog.require('shaka.media.SegmentReference');
 goog.require('shaka.util.IReleasable');
 goog.require('shaka.util.Timer');
@@ -424,10 +425,17 @@ shaka.media.SegmentIndex = class {
 
   /**
    * Returns a new iterator that initially points to the segment that contains
-   * the given time.  Like the normal iterator, next() must be called first to
-   * get to the first element. Returns null if we do not find a segment at the
+   * the given time, or the nearest independent segment before it.
+   *
+   * Like the normal iterator, next() must be called first to get to the first
+   * element. Returns null if we do not find a segment at the
    * requested time.
    *
+   * The first segment returned by the iterator _MUST_ be an independent
+   * segment.  Assumes that only partial references can be dependent, based on
+   * RFC 8216 rev 13, section 8.1: "Each (non-Partial) Media Segment in a Media
+   * Playlist will contain at least one independent frame."
+   *
    * @param {number} time
    * @return {?shaka.media.SegmentIterator}
    * @export
@@ -447,9 +455,18 @@ shaka.media.SegmentIndex = class {
     if (ref && ref.hasPartialSegments()) {
       // Look for a partial SegmentReference.
       for (let i = ref.partialReferences.length - 1; i >= 0; --i) {
-        const r = ref.partialReferences[i];
+        let r = ref.partialReferences[i];
         // Note that a segment ends immediately before the end time.
         if ((time >= r.startTime) && (time < r.endTime)) {
+          // Find an independent partial segment by moving backwards.
+          while (i && !r.isIndependent()) {
+            i--;
+            r = ref.partialReferences[i];
+          }
+          if (!r.isIndependent()) {
+            shaka.log.alwaysError('No independent partial segment found!');
+            return null;
+          }
           // Call to next() should move the partial segment, not the full
           // segment.
           index++;

lib/media/streaming_engine.js

@@ -1262,17 +1262,6 @@ shaka.media.StreamingEngine = class {
             mediaState.stream.segmentIndex.getIteratorForTime(presentationTime);
         ref = mediaState.segmentIterator &&
             mediaState.segmentIterator.next().value;
-        // For LL-HLS we need to find the nearest INDEPENDENT segment
-        let attempt = 0;
-        const maxAttempts = 5;
-        while (ref && !ref.isIndependent() && attempt < maxAttempts) {
-          const newTime = ref.startTime - 0.1;
-          mediaState.segmentIterator =
-              mediaState.stream.segmentIndex.getIteratorForTime(newTime);
-          ref = mediaState.segmentIterator &&
-              mediaState.segmentIterator.next().value;
-          attempt++;
-        }
       }
       if (ref == null) {
         shaka.log.warning(logPrefix, 'cannot find segment',

test/media/segment_index_unit.js

@@ -740,6 +740,58 @@ describe('SegmentIndex', /** @suppress {accessControls} */ () => {
       expect(iterator.current()).toBe(null);
     });
 
+    describe('getIteratorForTime', () => {
+      it('begins with an independent partial segment', () => {
+        // This test contains its own segment refs, which are manipulated to
+        // look a little different from the more general ones used elsewhere.
+        // In particular, we mark some partial refs as dependent, and we make
+        // the second partial list much longer.
+        const partialRefs1 = [
+          makeReference(uri(10.15), 10, 15),
+          makeReference(uri(15.20), 15, 20),
+        ];
+        partialRefs1[1].markAsNonIndependent();
+
+        const partialRefs2 = [
+          makeReference(uri(20.25), 20, 25),
+          makeReference(uri(25.30), 25, 30),
+          makeReference(uri(30.35), 30, 35),
+          makeReference(uri(35.40), 35, 40),
+          makeReference(uri(40.45), 40, 45),
+          makeReference(uri(45.50), 45, 50),
+          makeReference(uri(50.55), 50, 55),
+          makeReference(uri(55.60), 55, 60),
+          makeReference(uri(60.65), 60, 65),
+          makeReference(uri(65.70), 65, 70),
+        ];
+        // All but the first:
+        for (const r of partialRefs2.slice(1)) {
+          r.markAsNonIndependent();
+        }
+
+        const localInputRefs = [
+          makeReference(uri(0.10), 0, 10),
+          makeReference(uri(10.20), 10, 20, partialRefs1),
+          makeReference(uri(20.70), 20, 70, partialRefs2),
+        ];
+        const index = new shaka.media.SegmentIndex(localInputRefs);
+
+        // This time points to partialRefs1[0], which is independent.
+        const iterator1 = index.getIteratorForTime(11);
+        expect(iterator1.next().value).toBe(partialRefs1[0]);
+
+        // Even though the time would point to partialRefs1[1], that is not
+        // independent.  So it walks back to partialRefs1[0].
+        const iterator2 = index.getIteratorForTime(16);
+        expect(iterator2.next().value).toBe(partialRefs1[0]);
+
+        // Even though the time would point to partialRefs2[9], that is not
+        // independent.  So it walks all the way back to partialRefs2[0].
+        const iterator3 = index.getIteratorForTime(69);
+        expect(iterator3.next().value).toBe(partialRefs2[0]);
+      });
+    });
+
     describe('next', () => {
       it('starts with the first segment', () => {
         const index = new shaka.media.SegmentIndex(inputRefs);