fix(DASH): Fix creation of multiperiod trickmode streams (#7229)

lib/util/periods.js

@@ -890,26 +890,33 @@ shaka.util.PeriodCombiner = class {
         // Create a fresh output stream for trick-mode playback.
         output.trickModeVideo = shaka.util.PeriodCombiner.cloneStream_(
             input.trickModeVideo);
-        // TODO: fix the createSegmentIndex function for trickModeVideo.
-        // The trick-mode tracks in multi-period content should have trick-mode
-        // segment indexes whenever available, rather than only regular-mode
-        // segment indexes.
+        output.trickModeVideo.matchedStreams = [];
         output.trickModeVideo.createSegmentIndex = () => {
-          // Satisfy the compiler about the type.
-          goog.asserts.assert(
-              output.segmentIndex instanceof shaka.media.MetaSegmentIndex,
-              'The stream should have a MetaSegmentIndex.');
-          output.trickModeVideo.segmentIndex = output.segmentIndex.clone();
+          if (output.trickModeVideo.segmentIndex) {
+            return Promise.resolve();
+          }
+          const segmentIndex = new shaka.media.MetaSegmentIndex();
+          goog.asserts.assert(output.trickModeVideo.matchedStreams,
+              'trickmode matched streams should exist');
+          for (const stream of output.trickModeVideo.matchedStreams) {
+            goog.asserts.assert(stream.segmentIndex,
+                'trickmode segment index should exist');
+            segmentIndex.appendSegmentIndex(stream.segmentIndex);
+          }
+          output.trickModeVideo.segmentIndex = segmentIndex;
+
           return Promise.resolve();
         };
       }
 
       // Concatenate the trick mode input onto the trick mode output.
+      output.trickModeVideo.matchedStreams.push(input.trickModeVideo);
       shaka.util.PeriodCombiner.concatenateStreams_(
           output.trickModeVideo, input.trickModeVideo);
     } else if (output.trickModeVideo) {
       // We have a trick mode output, but no input from this Period.  Fill it in
       // from the standard input Stream.
+      output.trickModeVideo.matchedStreams.push(input);
       shaka.util.PeriodCombiner.concatenateStreams_(
           output.trickModeVideo, input);
     }