MSE: Drop empty AV buffers emitted by Stream Parser

Since FFmpeg decode of 0-byte audio and video buffers is now disallowed,
and FFmpegDemuxer skips enqueuing such packets into its DemuxerStream,
this change does similar for MSE:

Any 0-byte |data| audio or video buffers are dropped during initial
processing in FrameProcessor::ProcessFrames(). No 0-byte |data| text
buffers are dropped, because:
* We don't use FFmpeg to decode in-band MSE text, and
* In-band MSE text buffers' |side_data| may contain meaningful
  information. (This assumption isn't asserted by this change).

Note, this change takes the simpler route of dropping 0-byte AV buffers
before they're buffered by the coded frame processing algorithm. A more
complex alternative to skip them when reading from SourceBufferStream
was rejected in hopes this simpler change is sufficient.

Includes ability in ChunkDemuxerTest to vary |block_size_| instead of
always using |kBlockSize|. While the product change is closer to
FrameProcessor, FrameProcessorTests always encode the coded frame's
timestamp as the coded frame contents to enable precise verification of
processing results, so the test of this change is done via
ChunkDemuxerTest instead.

BUG=823375,663438
TEST=ChunkDemuxerTest.ZeroLengthFramesDropped

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ie6a802ba64a576f0219ca7c400fc7dec3a45fbf6
Reviewed-on: https://chromium-review.googlesource.com/974427
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546159}

media/base/test_helpers.h

@@ -381,6 +381,13 @@ MATCHER_P3(NegativeDtsFailureWhenByDts, frame_type, pts_us, dts_us, "") {
                "and filtering against append window");
 }
 
+MATCHER_P2(DiscardingEmptyFrame, pts_us, dts_us, "") {
+  return CONTAINS_STRING(arg,
+                         "Discarding empty audio or video coded frame, PTS=" +
+                             base::IntToString(pts_us) +
+                             "us, DTS=" + base::IntToString(dts_us) + "us");
+}
+
 }  // namespace media
 
 #endif  // MEDIA_BASE_TEST_HELPERS_H_

media/filters/frame_processor.cc

@@ -20,6 +20,7 @@ const int kMaxDtsBeyondPtsWarnings = 10;
 const int kMaxAudioNonKeyframeWarnings = 10;
 const int kMaxNumKeyframeTimeGreaterThanDependantWarnings = 1;
 const int kMaxMuxedSequenceModeWarnings = 1;
+const int kMaxSkippedEmptyFrameWarnings = 5;
 
 // Helper class to capture per-track details needed by a frame processor. Some
 // of this information may be duplicated in the short-term in the associated
@@ -370,9 +371,21 @@ bool FrameProcessor::ProcessFrames(
   // https://rawgit.com/w3c/media-source/d8f901f22/
   //     index.html#sourcebuffer-coded-frame-processing
   // 1. For each coded frame in the media segment run the following steps:
-  for (StreamParser::BufferQueue::const_iterator frames_itr = frames.begin();
-       frames_itr != frames.end(); ++frames_itr) {
-    if (!ProcessFrame(*frames_itr, append_window_start, append_window_end,
+  for (const auto& frame : frames) {
+    // Skip any 0-byte audio or video buffers, since they cannot produce any
+    // valid decode output (and are rejected by FFmpeg A/V decode.) Retain
+    // 0-byte text buffers because their |side_data| just might be useful, and
+    // we don't feed them to FFmpeg later.
+    if (!frame->data_size() && frame->type() != DemuxerStream::TEXT) {
+      LIMITED_MEDIA_LOG(DEBUG, media_log_, num_skipped_empty_frame_warnings_,
+                        kMaxSkippedEmptyFrameWarnings)
+          << "Discarding empty audio or video coded frame, PTS="
+          << frame->timestamp().InMicroseconds()
+          << "us, DTS=" << frame->GetDecodeTimestamp().InMicroseconds() << "us";
+      continue;
+    }
+
+    if (!ProcessFrame(frame, append_window_start, append_window_end,
                       timestamp_offset)) {
       FlushProcessedFrames();
       return false;

media/filters/frame_processor.h

@@ -190,6 +190,7 @@ class MEDIA_EXPORT FrameProcessor {
   int num_dts_beyond_pts_warnings_ = 0;
   int num_audio_non_keyframe_warnings_ = 0;
   int num_muxed_sequence_mode_warnings_ = 0;
+  int num_skipped_empty_frame_warnings_ = 0;
 
   DISALLOW_COPY_AND_ASSIGN(FrameProcessor);
 };

media/formats/webm/cluster_builder.cc

@@ -191,7 +191,7 @@ void ClusterBuilder::WriteBlock(uint8_t* buf,
   DCHECK_GE(flags, 0);
   DCHECK_LE(flags, 0xff);
   DCHECK(data);
-  DCHECK_GT(size, 0);
+  DCHECK_GE(size, 0);  // For testing, allow 0-byte coded frames.
   DCHECK_NE(cluster_timecode_, -1);
 
   int64_t timecode_delta = timecode - cluster_timecode_;