Compare buffering goals to the difference between next segment's start

time and playhead time.

This prevents streaming engine from requesting more segments
if it encounters a gap.

Related to #472.

Change-Id: Ie9fac1b427b631ac467656c15fd8608e12c07efc

Author: ismena

lib/media/streaming_engine.js

@@ -795,8 +795,24 @@ shaka.media.StreamingEngine.prototype.update_ = function(mediaState) {
 
   // Compute how far we've buffered ahead of the playhead.
   var playheadTime = this.playhead_.getTime();
-  var bufferedAhead = this.mediaSourceEngine_.bufferedAheadOf(
-      mediaState.type, playheadTime, 0.1);
+
+  // Get the next timestamp we need.
+  var bufferEnd = this.mediaSourceEngine_.bufferEnd(mediaState.type);
+  var timeNeeded = this.getTimeNeeded_(mediaState, playheadTime);
+  shaka.log.v2(logPrefix, 'timeNeeded=' + timeNeeded);
+
+  var needPeriodIndex = this.findPeriodContainingTime_(timeNeeded);
+  var needPeriod = this.manifest_.periods[needPeriodIndex];
+  var reference = this.getSegmentReferenceNeeded_(
+      mediaState, playheadTime, bufferEnd, needPeriodIndex);
+
+  var bufferedAhead;
+  if (reference) {
+    bufferedAhead = needPeriod.startTime + reference.startTime - playheadTime;
+  } else {
+    bufferedAhead = this.mediaSourceEngine_.bufferedAheadOf(
+        mediaState.type, playheadTime, 0.1);
+  }
 
   goog.asserts.assert(this.manifest_, 'manifest_ should not be null');
   goog.asserts.assert(this.config_, 'config_ should not be null');
@@ -822,12 +838,6 @@ shaka.media.StreamingEngine.prototype.update_ = function(mediaState) {
     return 0.5;
   }
 
-  // Get the next timestamp we need.
-  var bufferEnd = this.mediaSourceEngine_.bufferEnd(mediaState.type);
-  var timeNeeded = this.getTimeNeeded_(
-      mediaState, playheadTime, bufferedAhead, bufferEnd);
-  shaka.log.v2(logPrefix, 'timeNeeded=' + timeNeeded);
-
   // Check if we've buffered to the end of the presentation.
   if (timeNeeded >= this.manifest_.presentationTimeline.getDuration()) {
     // We shouldn't rebuffer if the playhead is close to the end of the
@@ -855,7 +865,6 @@ shaka.media.StreamingEngine.prototype.update_ = function(mediaState) {
   // available once it's switched to. Note that we don't use the non-existence
   // of SegmentReferences as an indicator to determine Period boundaries
   // because SegmentIndexes can provide SegmentReferences outside its Period.
-  var needPeriodIndex = this.findPeriodContainingTime_(timeNeeded);
   if (needPeriodIndex != currentPeriodIndex) {
     shaka.log.debug(logPrefix,
                     'need Period ' + needPeriodIndex,
@@ -866,7 +875,7 @@ shaka.media.StreamingEngine.prototype.update_ = function(mediaState) {
     return null;
   }
 
-  var reference = this.getSegmentReferenceNeeded_(
+  reference = this.getSegmentReferenceNeeded_(
       mediaState, playheadTime, bufferEnd, currentPeriodIndex);
   if (!reference) {
     // The segment could not be found, does not exist, or is not available.  In
@@ -888,15 +897,13 @@ shaka.media.StreamingEngine.prototype.update_ = function(mediaState) {
  *
  * @param {shaka.media.StreamingEngine.MediaState_} mediaState
  * @param {number} playheadTime
- * @param {number} bufferedAhead
- * @param {?number} bufferEnd
  * @return {number} The next timestamp needed.
  * @throws {!shaka.util.Error} if the buffer is inconsistent with our
  *   expectations.
  * @private
  */
 shaka.media.StreamingEngine.prototype.getTimeNeeded_ = function(
-    mediaState, playheadTime, bufferedAhead, bufferEnd) {
+    mediaState, playheadTime) {
   // Get the next timestamp we need. We must use |lastSegmentReference|
   // to determine this and not the actual buffer for two reasons:
   //   1. actual segments end slightly before their advertised end times, so