Fix flake in FFmpegDemuxer::SeekOnVideoTrackChange

There is a small number of crashes in m70 caused by video-only
src= players being disabled when put in the background. The check for
track count size in FFmpegDemuxer::OnSelectedVideoTrackChanged wasn't
sufficient to ensure that the vector index lookup in
SeekOnVideoTrackChange() would succede, as FindAndEnableProperTracks()
can sometimes not find the tracks it's looking for.

Bug: 887648
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;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I23f9457da5f67887f6e7b1a63699c9c7fa0107ed
Reviewed-on: https://chromium-review.googlesource.com/1238994
Commit-Queue: Ted Meyer <tmathmeyer@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593389}

media/filters/ffmpeg_demuxer.cc

@@ -1765,8 +1765,15 @@ void FFmpegDemuxer::SeekOnVideoTrackChange(
     TrackChangeCB seek_completed_cb,
     DemuxerStream::Type stream_type,
     const std::vector<DemuxerStream*>& streams) {
-  DCHECK_EQ(streams.size(), 1u);
   DCHECK_EQ(stream_type, DemuxerStream::VIDEO);
+  if (streams.size() != 1u) {
+    // If FFmpegDemuxer::FindAndEnableProperTracks() was not able to find the
+    // selected streams in the ID->DemuxerStream map, then its possible for
+    // this vector to be empty. If that's the case, we don't want to bother
+    // with seeking, and just call the callback immediately.
+    std::move(seek_completed_cb).Run(stream_type, streams);
+    return;
+  }
   SeekInternal(seek_to_time,
                base::BindOnce(&FFmpegDemuxer::OnVideoSeekedForTrackChange,
                               weak_factory_.GetWeakPtr(), streams[0],

media/filters/ffmpeg_demuxer_unittest.cc

@@ -160,6 +160,17 @@ class FFmpegDemuxerTest : public testing::Test {
     InitializeDemuxerInternal(expected_pipeline_status, base::Time());
   }
 
+  void SeekOnVideoTrackChangePassthrough(
+      base::TimeDelta time,
+      base::OnceCallback<void(DemuxerStream::Type,
+                              const std::vector<DemuxerStream*>&)> cb,
+      DemuxerStream::Type type,
+      const std::vector<DemuxerStream*>& streams) {
+    // The tests can't access private methods directly because gtest uses
+    // some magic macros that break the 'friend' declaration.
+    demuxer_->SeekOnVideoTrackChange(time, std::move(cb), type, streams);
+  }
+
   MOCK_METHOD2(OnReadDoneCalled, void(int, int64_t));
 
   struct ReadExpectation {
@@ -1714,4 +1725,18 @@ TEST_F(FFmpegDemuxerTest, MultitrackMemoryUsage) {
   EXPECT_EQ(156011, demuxer_->GetMemoryUsage());
 }
 
+TEST_F(FFmpegDemuxerTest, SeekOnVideoTrackChangeWontSeekIfEmpty) {
+  // We only care about video tracks.
+  CreateDemuxer("bear-320x240-video-only.webm");
+  InitializeDemuxer();
+  std::vector<DemuxerStream*> streams;
+  base::RunLoop loop;
+
+  SeekOnVideoTrackChangePassthrough(
+      base::TimeDelta(), base::BindOnce(QuitLoop, loop.QuitClosure()),
+      DemuxerStream::VIDEO, streams);
+
+  loop.Run();
+}
+
 }  // namespace media