stream/flv: write first previous tag size

stream/flv/encoder.go

@@ -57,12 +57,11 @@ var (
 type Encoder struct {
 	dst io.Writer
 
-	fps         int
-	audio       bool
-	video       bool
-	lastTagSize int
-	header      Header
-	start       time.Time
+	fps    int
+	audio  bool
+	video  bool
+	header Header
+	start  time.Time
 }
 
 // NewEncoder retuns a new FLV encoder.
@@ -193,6 +192,17 @@ func (s *frameScanner) readByte() (b byte, ok bool) {
 func (e *Encoder) Encode(frame []byte) error {
 	var frameType byte
 	var packetType byte
+	if e.start.IsZero() {
+		// This is the first frame, so write the PreviousTagSize0.
+		//
+		// See https://download.macromedia.com/f4v/video_file_format_spec_v10_1.pdf
+		// section E.3.
+		var zero [4]byte
+		_, err := e.dst.Write(zero[:])
+		if err != nil {
+			return err
+		}
+	}
 	timeStamp := e.getNextTimestamp()
 	// Do we have video to send off?
 	if e.video {

stream/flv/flv.go

@@ -79,6 +79,8 @@ type Header struct {
 }
 
 func (h *Header) Bytes() []byte {
+	// See https://download.macromedia.com/f4v/video_file_format_spec_v10_1.pdf
+	// section E.2.
 	const headerLength = 9
 	b := [headerLength]byte{
 		0: 'F', 1: 'L', 2: 'V', 3: version,