Output Empty WebVtt Segments

According to the HLS+WebVTT spec, if there is no text in a segment
a webvtt file with no cues can be added to the manifest. By outputting
empty segments it allows the accumulated duration in the Master Playlist
to better represent the duration of the text stream.

Spec Reference: //tools.ietf.org/html/draft-pantos-http-live-streaming-23
  Section 3.5. WebVTT

Bug: #205

Change-Id: I5de01200fd9fa99c57949c773e8ee926b0f6ba8a

packager/media/formats/webvtt/webvtt_pipeline_unittest.cc

@@ -26,6 +26,7 @@ TEST(WebVttTextPipelineTest, SegmentedOutput) {
   const char* kOutput1 = "memory://output/template-1.vtt";
   const char* kOutput2 = "memory://output/template-2.vtt";
   const char* kOutput3 = "memory://output/template-3.vtt";
+  const char* kOutput4 = "memory://output/template-4.vtt";
 
   const char* kInputFile = "memory://input.vtt";
   const char* kInput =
@@ -53,8 +54,14 @@ TEST(WebVttTextPipelineTest, SegmentedOutput) {
       "Thank you.\n";
 
   // Segment One
-  // 00:00:10.000 to 00:00:20.000
+  // 00:00:00.000 to 00:00:10.000
   const char* kExpectedOutput1 =
+      "WEBVTT\n"
+      "\n";
+
+  // Segment Two
+  // 00:00:10.000 to 00:00:20.000
+  const char* kExpectedOutput2 =
       "WEBVTT\n"
       "\n"
       "1\n"
@@ -62,9 +69,9 @@ TEST(WebVttTextPipelineTest, SegmentedOutput) {
       "This blade has a dark past.\n"
       "\n";
 
-  // Segment Two
+  // Segment Three
   // 00:00:20.000 to 00:00:30.000
-  const char* kExpectedOutput2 =
+  const char* kExpectedOutput3 =
       "WEBVTT\n"
       "\n"
       "1\n"
@@ -80,9 +87,9 @@ TEST(WebVttTextPipelineTest, SegmentedOutput) {
       "You're a fool for traveling alone,\nso completely unprepared.\n"
       "\n";
 
-  // Segment Three
+  // Segment Four
   // 00:00:30.000 to 00:00:40.000
-  const char* kExpectedOutput3 =
+  const char* kExpectedOutput4 =
       "WEBVTT\n"
       "\n"
       "3\n"
@@ -128,6 +135,7 @@ TEST(WebVttTextPipelineTest, SegmentedOutput) {
   ASSERT_FILE_STREQ(kOutput1, kExpectedOutput1);
   ASSERT_FILE_STREQ(kOutput2, kExpectedOutput2);
   ASSERT_FILE_STREQ(kOutput3, kExpectedOutput3);
+  ASSERT_FILE_STREQ(kOutput4, kExpectedOutput4);
 }
 }  // namespace media
 }  // namespace shaka

packager/media/formats/webvtt/webvtt_segmenter.cc

@@ -35,7 +35,10 @@ Status WebVttSegmenter::Process(std::unique_ptr<StreamData> stream_data) {
 Status WebVttSegmenter::OnFlushRequest(size_t input_stream_index) {
   Status status;
   while (status.ok() && samples_.size()) {
-    status.Update(OnSegmentEnd());
+    // It is not possible for there to be any gaps, or else we would have
+    // already ended the segments before them. So just close the last remaining
+    // open segments.
+    OnSegmentEnd(samples_.top().segment);
   }
   return status.ok() ? FlushAllDownstreams() : status;
 }
@@ -68,23 +71,19 @@ Status WebVttSegmenter::OnTextSample(std::shared_ptr<const TextSample> sample) {
     samples_.push(seg_sample);
   }
 
-  Status status;
-
-  while (status.ok() && samples_.size() &&
-         samples_.top().segment < start_segment) {
-    // WriteNextSegment will pop elements from |samples_| which will
-    // eventually allow the loop to exit.
-    status.Update(OnSegmentEnd());
+  // Output all the segments that come before the start of this cue's first
+  // segment.
+  for (; current_segment_ < start_segment; current_segment_++) {
+    Status status = OnSegmentEnd(current_segment_);
+    if (!status.ok()) {
+      return status;
+    }
   }
 
-  return status;
+  return Status::OK;
 }
 
-Status WebVttSegmenter::OnSegmentEnd() {
-  DCHECK(samples_.size());
-
-  const uint64_t segment = samples_.top().segment;
-
+Status WebVttSegmenter::OnSegmentEnd(uint64_t segment) {
   Status status;
   while (status.ok() && samples_.size() && samples_.top().segment == segment) {
     status.Update(

packager/media/formats/webvtt/webvtt_segmenter.h

@@ -56,8 +56,10 @@ class WebVttSegmenter : public MediaHandler {
   Status InitializeInternal() override;
 
   Status OnTextSample(std::shared_ptr<const TextSample> sample);
-  Status OnSegmentEnd();
 
+  Status OnSegmentEnd(uint64_t segment);
+
+  uint64_t current_segment_ = 0;
   uint64_t segment_duration_ms_;
   std::priority_queue<WebVttSegmentedTextSample,
                       std::vector<WebVttSegmentedTextSample>,

packager/media/formats/webvtt/webvtt_segmenter_unittest.cc

@@ -142,7 +142,7 @@ TEST_F(WebVttSegmenterTest, CreatesSegmentsForCues) {
 //           |          |
 //           |          | [---B---]
 //           |          |
-TEST_F(WebVttSegmenterTest, SkipsEmptySegments) {
+TEST_F(WebVttSegmenterTest, DoesntSkipsEmptySegments) {
   const uint64_t kSampleDuration = kSegmentDuration / 2;
 
   {
@@ -160,7 +160,11 @@ TEST_F(WebVttSegmenterTest, SkipsEmptySegments) {
         OnProcess(IsSegmentInfo(kStreamIndex, kStartTimeSigned,
                                 kSegmentDuration, !kSubSegment, !kEncrypted)));
 
-    // There is no segment two
+    // Segment two (empty)
+    EXPECT_CALL(*Output(kOutputIndex),
+                OnProcess(IsSegmentInfo(
+                    kStreamIndex, kStartTimeSigned + kSegmentDuration,
+                    kSegmentDuration, !kSubSegment, !kEncrypted)));
 
     // Segment Three
     EXPECT_CALL(*Output(kOutputIndex),