rtph264: add error in case of a fragmented NALU with two starting packets

pkg/rtph264/decoder.go

@@ -15,8 +15,10 @@ import (
 var ErrMorePacketsNeeded = errors.New("need more packets")
 
 // ErrNonStartingPacketAndNoPrevious is returned when we decoded a non-starting
-// fragmented packet and we didn't received anything before.
-var ErrNonStartingPacketAndNoPrevious = errors.New("decoded a non-starting fragmented packet without any previous starting packets")
+// packet of a fragmented NALU and we didn't received anything before.
+// It's normal to receive this when we are decoding a stream that has been already
+// running for some time.
+var ErrNonStartingPacketAndNoPrevious = errors.New("decoded a non-starting fragmented packet without any previous starting packet")
 
 // PacketConnReader creates a io.Reader around a net.PacketConn.
 type PacketConnReader struct {
@@ -29,13 +31,6 @@ func (r PacketConnReader) Read(p []byte) (int, error) {
 	return n, err
 }
 
-type decoderState int
-
-const (
-	decoderStateInitial decoderState = iota
-	decoderStateReadingFragmented
-)
-
 // Decoder is a RTP/H264 decoder.
 type Decoder struct {
 	initialTs    uint32
@@ -43,7 +38,7 @@ type Decoder struct {
 
 	// for Decode()
 	startingPacketReceived bool
-	state                  decoderState
+	isReadingFragmented    bool
 	fragmentedBuf          []byte
 }
 
@@ -63,7 +58,7 @@ func (d *Decoder) Decode(byts []byte) ([][]byte, time.Duration, error) {
 	pkt := rtp.Packet{}
 	err := pkt.Unmarshal(byts)
 	if err != nil {
-		d.state = decoderStateInitial
+		d.isReadingFragmented = false
 		return nil, 0, err
 	}
 
@@ -72,8 +67,7 @@ func (d *Decoder) Decode(byts []byte) ([][]byte, time.Duration, error) {
 
 // DecodeRTP decodes NALUs from a rtp.Packet.
 func (d *Decoder) DecodeRTP(pkt *rtp.Packet) ([][]byte, time.Duration, error) {
-	switch d.state {
-	case decoderStateInitial:
+	if !d.isReadingFragmented {
 		if !d.initialTsSet {
 			d.initialTsSet = true
 			d.initialTs = pkt.Timestamp
@@ -135,7 +129,7 @@ func (d *Decoder) DecodeRTP(pkt *rtp.Packet) ([][]byte, time.Duration, error) {
 			typ := pkt.Payload[1] & 0x1F
 			d.fragmentedBuf = append([]byte{(nri << 5) | typ}, pkt.Payload[2:]...)
 
-			d.state = decoderStateReadingFragmented
+			d.isReadingFragmented = true
 			d.startingPacketReceived = true
 			return nil, 0, ErrMorePacketsNeeded
 
@@ -146,31 +140,38 @@ func (d *Decoder) DecodeRTP(pkt *rtp.Packet) ([][]byte, time.Duration, error) {
 
 		d.startingPacketReceived = true
 		return [][]byte{pkt.Payload}, d.decodeTimestamp(pkt.Timestamp), nil
+	}
+
+	// we are decoding a fragmented packet
 
-	default: // decoderStateReadingFragmented
 	if len(pkt.Payload) < 2 {
-			d.state = decoderStateInitial
+		d.isReadingFragmented = false
 		return nil, 0, fmt.Errorf("invalid FU-A packet (invalid size)")
 	}
 
 	typ := naluType(pkt.Payload[0] & 0x1F)
 	if typ != naluTypeFUA {
-			d.state = decoderStateInitial
+		d.isReadingFragmented = false
 		return nil, 0, fmt.Errorf("expected FU-A packet, got another type")
 	}
 
+	start := pkt.Payload[1] >> 7
 	end := (pkt.Payload[1] >> 6) & 0x01
 
+	if start == 1 {
+		d.isReadingFragmented = false
+		return nil, 0, fmt.Errorf("invalid FU-A packet (decoded two starting packets in a row)")
+	}
+
 	d.fragmentedBuf = append(d.fragmentedBuf, pkt.Payload[2:]...)
 
 	if end != 1 {
 		return nil, 0, ErrMorePacketsNeeded
 	}
 
-		d.state = decoderStateInitial
+	d.isReadingFragmented = false
 	d.startingPacketReceived = true
 	return [][]byte{d.fragmentedBuf}, d.decodeTimestamp(pkt.Timestamp), nil
-	}
 }
 
 // ReadSPSPPS reads RTP/H264 packets from a reader until SPS and PPS are

pkg/rtph264/rtph264_test.go

@@ -416,6 +416,28 @@ func TestDecodeErrors(t *testing.T) {
 			},
 			"expected FU-A packet, got another type",
 		},
+		{
+			"FU-A with two starting packets",
+			[][]byte{
+				mergeBytes(
+					[]byte{
+						0x80, 0x60, 0x44, 0xed, 0x88, 0x77, 0x79, 0xab,
+						0x9d, 0xbb, 0x78, 0x12, 0x1c, 0x85,
+					},
+					bytes.Repeat([]byte{0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07}, 182),
+					[]byte{0x00, 0x01},
+				),
+				mergeBytes(
+					[]byte{
+						0x80, 0x60, 0x44, 0xed, 0x88, 0x77, 0x79, 0xab,
+						0x9d, 0xbb, 0x78, 0x12, 0x1c, 0x85,
+					},
+					bytes.Repeat([]byte{0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07}, 182),
+					[]byte{0x00, 0x01},
+				),
+			},
+			"invalid FU-A packet (decoded two starting packets in a row)",
+		},
 		{
 			"MTAP",
 			[][]byte{{