Adjust appendWindowStart to help rounding errors.

Since we set the appendWindowStart to exactly the start of the Period,
we can get rounding issues where the first frame of the next Period
is removed.  If we remove the first frame, then MSE will remove up to
the next keyframe, causing a large gap.  Avoid slight rounding errors
by adjusting appendWindowStart back a little.

Bug: 71808910
Change-Id: I933352441c34046db7bf777264eb1ee4225d55e1

lib/media/streaming_engine.js

@@ -282,11 +282,13 @@ shaka.media.StreamingEngine.MediaState_;
 
 
 /**
- * The minimum number seconds that will remain buffered after evicting media.
- *
+ * The fudge factor for the appendWindowStart.  By adjusting the window
+ * backward, we avoid rounding errors that could cause us to remove the keyframe
+ * at the start of the Period.
  * @const {number}
+ * @private
  */
-shaka.media.StreamingEngine.prototype.MIN_BUFFER_LENGTH = 2;
+shaka.media.StreamingEngine.APPEND_WINDOW_START_FUDGE_ = 0.00001;
 
 
 /** @override */
@@ -1306,7 +1308,10 @@ shaka.media.StreamingEngine.prototype.fetchAndAppend_ = function(
   // Compute the append window.
   var duration = this.manifest_.presentationTimeline.getDuration();
   var followingPeriod = this.manifest_.periods[currentPeriodIndex + 1];
-  var appendWindowStart = currentPeriod.startTime;
+  // Rounding issues can cause us to remove the first frame of the Period, so
+  // reduce the start time slightly.
+  var windowFudge = shaka.media.StreamingEngine.APPEND_WINDOW_START_FUDGE_;
+  var appendWindowStart = Math.max(0, currentPeriod.startTime - windowFudge);
   var appendWindowEnd = followingPeriod ? followingPeriod.startTime : duration;
   goog.asserts.assert(
       reference.startTime <= appendWindowEnd,

test/media/streaming_engine_unit.js

@@ -922,6 +922,30 @@ describe('StreamingEngine', function() {
     expect(timeline.setDuration).toHaveBeenCalledWith(35);
   });
 
+  it('applies fudge factor for appendWindowStart', function() {
+    setupVod();
+    mediaSourceEngine = new shaka.test.FakeMediaSourceEngine(segmentData);
+    createStreamingEngine();
+
+    playhead.getTime.and.returnValue(0);
+    onStartupComplete.and.callFake(setupFakeGetTime.bind(null, 0));
+    onChooseStreams.and.callFake(defaultOnChooseStreams);
+
+    // Here we go!
+    streamingEngine.init();
+    runTest();
+
+    // The second Period starts at 20, so we should set the appendWindowStart to
+    // 20, but reduced by a small fudge factor.
+    var lt20 = {
+      asymmetricMatch: function(val) {
+        return val > 19.9 && val < 20;
+      }
+    };
+    expect(mediaSourceEngine.setStreamProperties)
+        .toHaveBeenCalledWith('video', 20, lt20, 40);
+  });
+
   describe('switchVariant/switchTextStream', function() {
     var initialVariant;
     var sameAudioVariant;