fixed the synchronized-audio corruption

internal/source/source.go

@@ -598,8 +598,10 @@ func (s *SyncSource) Close() error {
 	return s.inst.Close()
 }
 
-// NextSync reads both at a synced timestamp. Returns video Frame + audio AudioFrame
-func (s *SyncSource) NextSync(ctx context.Context, audioBatch uint64, timeout time.Duration) (Frame, AudioFrame, error) {
+// NextSync reads one video frame and the audio interval between this video
+// timestamp and the next. Deriving the interval for every frame preserves
+// exact long-term timing for fractional video rates.
+func (s *SyncSource) NextSync(ctx context.Context, timeout time.Duration) (Frame, AudioFrame, error) {
 	var timeouts int
 	for {
 		select {
@@ -635,7 +637,30 @@ func (s *SyncSource) NextSync(ctx context.Context, audioBatch uint64, timeout ti
 			}
 			// read audio at the same timestamp
 			aIdx := mxl.TimestampToIndex(s.aRate, ts)
+			nextTimestamp := mxl.IndexToTimestamp(s.rate, s.idx+1)
+			nextAudioIndex := mxl.TimestampToIndex(s.aRate, nextTimestamp)
+			if nextAudioIndex <= aIdx {
+				return Frame{}, AudioFrame{}, wrapError(
+					"calculate synchronized audio interval",
+					ErrorKindInvalidConfig,
+					fmt.Errorf("invalid audio interval: %d..%d", aIdx, nextAudioIndex),
+				)
+			}
+			audioBatch := nextAudioIndex - aIdx
 			av, aerr := s.ar.GetSamples(aIdx, int(audioBatch), 50*time.Millisecond)
+			if aerr != nil {
+				kind := ErrorKindUnavailable
+				if errors.Is(aerr, mxl.ErrTimeout) ||
+					errors.Is(aerr, mxl.ErrOutOfRangeEarly) ||
+					errors.Is(aerr, mxl.ErrOutOfRangeLate) {
+					kind = ErrorKindTemporary
+				}
+				return Frame{}, AudioFrame{}, wrapError(
+					"read synchronized audio",
+					kind,
+					aerr,
+				)
+			}
 			vFrame := Frame{
 				Index: g.Index, Width: s.width, Height: s.height,
 				Stride: s.stride, Size: g.GrainSize,
@@ -643,23 +668,19 @@ func (s *SyncSource) NextSync(ctx context.Context, audioBatch uint64, timeout ti
 			}
 			s.idx++
 
-			var aFrame AudioFrame
-			if aerr == nil {
-				samples := make([][]byte, s.chans)
-				for ch := uint64(0); ch < s.chans; ch++ {
-					f1, f2, _ := av.ChannelFragments(ch)
-					if len(f2) > 0 {
-						samples[ch] = append(f1, f2...)
-					} else {
-						samples[ch] = f1
-					}
-				}
-				aFrame = AudioFrame{
-					Index: aIdx, SampleCount: audioBatch,
-					Channels: s.chans, Samples: samples,
+			samples := make([][]byte, s.chans)
+			for ch := uint64(0); ch < s.chans; ch++ {
+				f1, f2, _ := av.ChannelFragments(ch)
+				if len(f2) > 0 {
+					samples[ch] = append(f1, f2...)
+				} else {
+					samples[ch] = f1
 				}
 			}
-			// even if audio failed, video returns
+			aFrame := AudioFrame{
+				Index: aIdx, SampleCount: audioBatch,
+				Channels: s.chans, Samples: samples,
+			}
 			return vFrame, aFrame, nil
 		case errors.Is(err, mxl.ErrTimeout), errors.Is(err, mxl.ErrOutOfRangeEarly), errors.Is(err, mxl.ErrOutOfRangeLate):
 			timeouts++