rtmp: support additional Enhanced-RTMP features (#3685) (#4088)

new features:
* support publishing Opus and AC-3 tracks
* support publishing more than 2 tracks. This is compatible with OBS multitrack video and OBS VOD audio track

internal/protocols/rtmp/message/msg_audio_ex_multitrack.go

@@ -0,0 +1,95 @@
+package message //nolint:dupl
+
+import (
+	"fmt"
+
+	"github.com/bluenviron/mediamtx/internal/protocols/rtmp/rawmessage"
+)
+
+// AudioExMultitrackType is a multitrack type.
+type AudioExMultitrackType uint8
+
+// multitrack types.
+const (
+	AudioExMultitrackTypeOneTrack             AudioExMultitrackType = 0
+	AudioExMultitrackTypeManyTracks           AudioExMultitrackType = 1
+	AudioExMultitrackTypeManyTracksManyCodecs AudioExMultitrackType = 2
+)
+
+// AudioExMultitrack is a multitrack extended message.
+type AudioExMultitrack struct {
+	MultitrackType AudioExMultitrackType
+	TrackID        uint8
+	Wrapped        Message
+}
+
+func (m *AudioExMultitrack) unmarshal(raw *rawmessage.Message) error { //nolint:dupl
+	if len(raw.Body) < 7 {
+		return fmt.Errorf("not enough bytes")
+	}
+
+	m.MultitrackType = AudioExMultitrackType(raw.Body[1] >> 4)
+	switch m.MultitrackType {
+	case AudioExMultitrackTypeOneTrack:
+	default:
+		return fmt.Errorf("unsupported multitrack type: %v", m.MultitrackType)
+	}
+
+	packetType := AudioExType(raw.Body[1] & 0b1111)
+	switch packetType {
+	case AudioExTypeSequenceStart:
+		m.Wrapped = &AudioExSequenceStart{}
+
+	case AudioExTypeSequenceEnd:
+		m.Wrapped = &AudioExSequenceEnd{}
+
+	case AudioExTypeMultichannelConfig:
+		m.Wrapped = &AudioExMultichannelConfig{}
+
+	case AudioExTypeCodedFrames:
+		m.Wrapped = &AudioExCodedFrames{}
+
+	default:
+		return fmt.Errorf("unsupported audio multitrack packet type: %v", packetType)
+	}
+
+	m.TrackID = raw.Body[6]
+
+	wrappedBody := make([]byte, 5+len(raw.Body[7:]))
+	copy(wrappedBody[1:], raw.Body[2:]) // fourCC
+	copy(wrappedBody[5:], raw.Body[7:]) // body
+	err := m.Wrapped.unmarshal(&rawmessage.Message{
+		ChunkStreamID:   raw.ChunkStreamID,
+		MessageStreamID: raw.MessageStreamID,
+		Timestamp:       raw.Timestamp,
+		Body:            wrappedBody,
+	})
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (m AudioExMultitrack) marshal() (*rawmessage.Message, error) {
+	wrappedEnc, err := m.Wrapped.marshal()
+	if err != nil {
+		return nil, err
+	}
+
+	body := make([]byte, 7+len(wrappedEnc.Body)-5)
+
+	body[0] = (9 << 4) | byte(AudioExTypeMultitrack)
+	body[1] = wrappedEnc.Body[0] & 0b1111
+	copy(body[2:], wrappedEnc.Body[1:])
+	body[6] = m.TrackID
+	copy(body[7:], wrappedEnc.Body[5:])
+
+	return &rawmessage.Message{
+		ChunkStreamID:   wrappedEnc.ChunkStreamID,
+		MessageStreamID: wrappedEnc.MessageStreamID,
+		Timestamp:       wrappedEnc.Timestamp,
+		Type:            uint8(TypeAudio),
+		Body:            body,
+	}, nil
+}