Bug 1258922: [MSE] P4. Set draining flag to true when skip to next keyframe failed. r=gerald

data promise is only resolved once the decoder has been drained. It was possible for a promise to never be resolved if skipping to the next key frame failed.

MozReview-Commit-ID: GimbQTImH9e

dom/media/MediaFormatReader.cpp

@@ -1447,6 +1447,15 @@ MediaFormatReader::OnVideoSkipFailed(MediaTrackDemuxer::SkipFailureHolder aFailu
       NotifyEndOfStream(TrackType::kVideoTrack);
       break;
     case DemuxerFailureReason::WAITING_FOR_DATA:
+      // While there is nothing to drain considering the decoder has been
+      // flushed in SkipVideoDemuxToNextKeyFrame, we need to set mNeedDraining
+      // to true as the video MediaDataPromise will only be rejected once drain
+      // has completed.
+      MOZ_DIAGNOSTIC_ASSERT(!mVideo.mDecodingRequested,
+                            "Reset must have been called");
+      if (!mVideo.mWaitingForData) {
+        mVideo.mNeedDraining = true;
+      }
       NotifyWaitingForData(TrackType::kVideoTrack);
       break;
     case DemuxerFailureReason::CANCELED: