Bug 1528265 - P2. Adjust time of last block if needed. r=chunmin

The produce the same outcome as originally done in bug 1487797. However, we only do so on the last block ever retrieved.

Ideally, we should calculate the start time based on the number of frames previously returned. It would avoid such inconsistency and while we had one file presenting such symptom, I'm not sure we can always rely for the frame_or_sample_num value to be valid in reference to the first frame block size.

Depends on D66989

Differential Revision: https://phabricator.services.mozilla.com/D66990

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

dom/media/flac/FlacDemuxer.cpp

@@ -109,6 +109,7 @@ class FrameHeader {
     // A frame is made of Blocksize sample.
     mIndex = mVariableBlockSize ? frame_or_sample_num
                                 : frame_or_sample_num * mBlocksize;
+    mFrameOrSampleNum = frame_or_sample_num;
 
     // Sample rate.
     if (sr_code < 12) {
@@ -154,6 +155,11 @@ class FrameHeader {
     FLAC_CHMODE_MID_SIDE,
   };
   AudioInfo mInfo;
+  // mFrameOrSampleNum is either:
+  // 1- coded sample number if blocksize is variable or
+  // 2- coded frame number if blocksize is fixed.
+  // A frame is made of Blocksize sample.
+  uint64_t mFrameOrSampleNum = 0;
   // Index in samples from start;
   int64_t mIndex = 0;
   bool mVariableBlockSize = false;
@@ -306,6 +312,17 @@ class Frame {
     }
   }
 
+  void ResetStartTimeIfNeeded(const Frame& aReferenceFrame) {
+    if (Header().mVariableBlockSize ||
+        aReferenceFrame.Header().mVariableBlockSize ||
+        aReferenceFrame.Header().mBlocksize <= Header().mBlocksize) {
+      // Not a fixed size frame, or nothing to adjust.
+      return;
+    }
+    mHeader.mIndex =
+        Header().mFrameOrSampleNum * aReferenceFrame.Header().mBlocksize;
+  }
+
   uint32_t Size() const { return mSize; }
 
   TimeUnit Time() const {
@@ -384,6 +401,13 @@ class FrameParser {
     if (mFrame.IsValid()) {
       if (mNextFrame.EOS()) {
         mFrame.SetEndOffset(aResource.Tell());
+        // If the blocksize is fixed, the frame's starting sample number will be
+        // the frame number times the blocksize. However, the last block may
+        // have been incorrectly set as shorter than the stream blocksize.
+        // We recalculate the start time of this last sample using the first
+        // frame blocksize.
+        // TODO: should we use an overall counter of frames instead?
+        mFrame.ResetStartTimeIfNeeded(mFirstFrame);
       } else if (mNextFrame.IsValid()) {
         mFrame.SetEndOffset(mNextFrame.Offset());
         mFrame.SetEndTime(mNextFrame.Header().Index());