fix: Fix buffering due to re-fetch (SegmentIterator) (shaka-project#3419

)

In multi-period live DASH streams, the SegmentIterator would sometimes
get reset to a value pointing to the beginning of the segment
references for the period, causing StreamingEngine to re-fetch
segments it already had.

This fixes SegmentIterator's getIteratorForTime() to return null when
a reference can't be found, and fixes StreamingEngine to handle null
returns by waiting and checking again.

Closes shaka-project#3354

b/186457868

Change-Id: I2b34ee52dd12b59e1c1237258050b50e3189bee3

lib/media/segment_index.js

@@ -383,22 +383,25 @@ shaka.media.SegmentIndex = class {
 
   /** @return {!shaka.media.SegmentIterator} */
   [Symbol.iterator]() {
-    return this.getIteratorForTime(0);
+    const iter = this.getIteratorForTime(0);
+    goog.asserts.assert(iter != null, 'Iterator for 0 should never be null!');
+    return iter;
   }
 
   /**
    * 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.
+   * get to the first element. Returns null if we do not find a segment at the
+   * requested time.
    *
    * @param {number} time
-   * @return {!shaka.media.SegmentIterator}
+   * @return {?shaka.media.SegmentIterator}
    * @export
    */
   getIteratorForTime(time) {
     let index = this.find(time);
     if (index == null) {
-      index = -1;
+      return null;
     } else {
       index--;
     }
@@ -517,8 +520,13 @@ shaka.media.SegmentIterator = class {
         'Please use SegmentIndex.getIteratorForTime instead of seek().');
 
     const iter = this.segmentIndex_.getIteratorForTime(time);
-    this.currentPosition_ = iter.currentPosition_;
-    this.currentPartialPosition_ = iter.currentPartialPosition_;
+    if (iter) {
+      this.currentPosition_ = iter.currentPosition_;
+      this.currentPartialPosition_ = iter.currentPartialPosition_;
+    } else {
+      this.currentPosition_ = Number.MAX_VALUE;
+      this.currentPartialPosition_ = 0;
+    }
     return this.next().value;
   }
 

lib/media/streaming_engine.js

@@ -1113,7 +1113,8 @@ shaka.media.StreamingEngine = class {
 
       mediaState.segmentIterator =
           mediaState.stream.segmentIndex.getIteratorForTime(time);
-      const ref = mediaState.segmentIterator.next().value;
+      const ref = mediaState.segmentIterator &&
+          mediaState.segmentIterator.next().value;
       if (ref == null) {
         shaka.log.warning(logPrefix, 'cannot find segment', 'endTime:', time);
       }
@@ -1135,14 +1136,16 @@ shaka.media.StreamingEngine = class {
       if (inaccurateTolerance) {
         mediaState.segmentIterator =
             mediaState.stream.segmentIndex.getIteratorForTime(lookupTime);
-        ref = mediaState.segmentIterator.next().value;
+        ref = mediaState.segmentIterator &&
+            mediaState.segmentIterator.next().value;
       }
       if (!ref) {
         // If we can't find a valid segment with the drifted time, look for a
         // segment with the presentation time.
         mediaState.segmentIterator =
             mediaState.stream.segmentIndex.getIteratorForTime(presentationTime);
-        ref = mediaState.segmentIterator.next().value;
+        ref = mediaState.segmentIterator &&
+            mediaState.segmentIterator.next().value;
       }
       if (ref == null) {
         shaka.log.warning(logPrefix, 'cannot find segment',