Fix resetting resumeAt in StreamingEngine.

Based on PR #839

We use |resumeAt| to handle switching between Periods that do and do not
have text streams.  It is used to override what time we need to start
buffering from.  Once we have used it (i.e. started streaming), we
should reset it (to ensure it doesn't break seeking).

There was a bug where if the stream wasn't available yet, it we
would reset |resumeAt| but not start streaming.  Meaning on the next
update, we would check at the playhead time instead of the next Period
time.  Now we reset |resumeAt| only after we know we have started
streaming.

Closes #839

Change-Id: Ibd3ce680cec129719869c8d4a7dda409b573a17f

lib/media/streaming_engine.js

@@ -927,7 +927,6 @@ shaka.media.StreamingEngine.prototype.update_ = function(mediaState) {
   // Get the next timestamp we need.
   var timeNeeded = this.getTimeNeeded_(mediaState, playheadTime);
   shaka.log.v2(logPrefix, 'timeNeeded=' + timeNeeded);
-  mediaState.resumeAt = 0;
 
   var currentPeriodIndex = this.findPeriodContainingStream_(mediaState.stream);
   var needPeriodIndex = this.findPeriodContainingTime_(timeNeeded);
@@ -992,6 +991,7 @@ shaka.media.StreamingEngine.prototype.update_ = function(mediaState) {
     return 1;
   }
 
+  mediaState.resumeAt = 0;
   this.fetchAndAppend_(mediaState, playheadTime, currentPeriodIndex, reference);
   return null;
 };

test/media/streaming_engine_unit.js

@@ -754,6 +754,46 @@ describe('StreamingEngine', function() {
     });
   });
 
+  it('doesn\'t get stuck when 2nd Period isn\'t available yet', function() {
+    // See: https://github.com/google/shaka-player/pull/839
+    setupVod();
+    manifest.periods[0].textStreams = [];
+
+    // For the first update, indicate the segment isn't available.  This should
+    // not cause us to fallback to the Playhead time to determine which segment
+    // to start streaming.
+    var oldGet = textStream2.getSegmentReference;
+    textStream2.getSegmentReference = function(idx) {
+      if (idx == 1) {
+        textStream2.getSegmentReference = oldGet;
+        return null;
+      }
+      return oldGet(idx);
+    };
+
+    mediaSourceEngine = new shaka.test.FakeMediaSourceEngine(segmentData);
+    createStreamingEngine();
+
+    playhead.getTime.and.returnValue(0);
+    onStartupComplete.and.callFake(setupFakeGetTime.bind(null, 0));
+    onChooseStreams.and.callFake(function(period) {
+      var chosen = defaultOnChooseStreams(period);
+      if (period == manifest.periods[0])
+        delete chosen[ContentType.TEXT];
+      return chosen;
+    });
+
+    // Here we go!
+    streamingEngine.init();
+    runTest();
+
+    expect(mediaSourceEngine.segments).toEqual({
+      audio: [true, true, true, true],
+      video: [true, true, true, true],
+      text: [false, false, true, true]
+    });
+  });
+
   it('only reinitializes text when switching streams', function() {
     // See: https://github.com/google/shaka-player/issues/910
     setupVod();