Bug 1222851 - Keep the one dropped frame prior the first one to prime the decoder. r=bryce

Some audio decoders, such as AAC and Opus have a need for a pre-roll content. As such, in order to be able to fully get the content of the first frame we keep the frame just prior that would have normally been dropped.

We set this frame to have a duration of 1us so that it will be dropped later by the decoding pipeline. The starting time of the first frame is adjusted so that we have continuous data, without gap in the buffered range.

Differential Revision: https://phabricator.services.mozilla.com/D20322

--HG--
extra : moz-landing-system : lando

dom/media/mediasource/TrackBuffersManager.cpp

@@ -1630,6 +1630,24 @@ void TrackBuffersManager::ProcessFrames(TrackBuffer& aSamples,
     aTrackData.mLastParsedEndTime = TimeUnit();
   }
 
+  auto addToSamples = [&](MediaRawData* aSample,
+                          const TimeInterval& aInterval) {
+    aSample->mTime = aInterval.mStart;
+    aSample->mDuration = aInterval.Length();
+    aSample->mTrackInfo = trackBuffer.mLastInfo;
+    samplesRange += aInterval;
+    sizeNewSamples += aSample->ComputedSizeOfIncludingThis();
+    samples.AppendElement(aSample);
+  };
+
+  // Will be set to the last frame dropped due to being outside mAppendWindow.
+  // It will be added prior the first following frame which can be added to the
+  // track buffer.
+  // This sample will be set with a duration of only 1us which will cause it to
+  // be dropped once returned by the decoder.
+  // This sample is required to "prime" the decoder so that the following frame
+  // can be fully decoded.
+  RefPtr<MediaRawData> previouslyDroppedSample;
   for (auto& sample : aSamples) {
     const TimeUnit sampleEndTime = sample->GetEndTime();
     if (sampleEndTime > aTrackData.mLastParsedEndTime) {
@@ -1646,6 +1664,7 @@ void TrackBuffersManager::ProcessFrames(TrackBuffer& aSamples,
       // frame and jump to the top of the loop to start processing the next
       // coded frame.
       if (!sample->mKeyframe) {
+        previouslyDroppedSample = nullptr;
         continue;
       }
       // 2. Set the need random access point flag on track buffer to false.
@@ -1734,6 +1753,7 @@ void TrackBuffersManager::ProcessFrames(TrackBuffer& aSamples,
       CheckSequenceDiscontinuity(presentationTimestamp);
 
       if (!sample->mKeyframe) {
+        previouslyDroppedSample = nullptr;
         continue;
       }
       if (appendMode == SourceBufferAppendMode::Sequence) {
@@ -1793,8 +1813,10 @@ void TrackBuffersManager::ProcessFrames(TrackBuffer& aSamples,
                    intersection.mStart.ToMicroseconds(),
                    intersection.mEnd.ToMicroseconds());
         sampleInterval = intersection;
-        sample->mDuration = intersection.Length();
       } else {
+        sample->mOriginalPresentationWindow = Some(sampleInterval);
+        sample->mTimecode = decodeTimestamp;
+        previouslyDroppedSample = sample;
         MSE_DEBUGV("frame [%" PRId64 ",%" PRId64
                    "] outside appendWindow [%" PRId64 ",%" PRId64 "] dropping",
                    sampleInterval.mStart.ToMicroseconds(),
@@ -1816,13 +1838,23 @@ void TrackBuffersManager::ProcessFrames(TrackBuffer& aSamples,
         continue;
       }
     }
+    if (previouslyDroppedSample) {
+      MSE_DEBUGV("Adding silent frame");
+      // This "silent" sample will be added so that it starts exactly before the
+      // first usable one. The duration of the actual sample will be adjusted so
+      // that the total duration staty the same.
+      // Setting a dummy presentation window of 1us will cause this sample to be
+      // dropped after decoding by the AudioTrimmer (if audio).
+      TimeInterval previouslyDroppedSampleInterval =
+          TimeInterval(sampleInterval.mStart,
+                       sampleInterval.mStart + TimeUnit::FromMicroseconds(1));
+      addToSamples(previouslyDroppedSample, previouslyDroppedSampleInterval);
+      previouslyDroppedSample = nullptr;
+      sampleInterval.mStart += previouslyDroppedSampleInterval.Length();
+    }
 
-    samplesRange += sampleInterval;
-    sizeNewSamples += sample->ComputedSizeOfIncludingThis();
-    sample->mTime = sampleInterval.mStart;
     sample->mTimecode = decodeTimestamp;
-    sample->mTrackInfo = trackBuffer.mLastInfo;
-    samples.AppendElement(sample);
+    addToSamples(sample, sampleInterval);
 
     // Steps 11,12,13,14, 15 and 16 will be done in one block in InsertFrames.