set SSRC of outgoing packets (#803)

In client and server, each format now has a fixed, unique, known in
advance SSRC, that is applied to outgoing packets belonging to each
format.

This is needed to support SRTP/MIKEY, that require each format to have
a fixed, unique, and known in advance SSRC.

A secondary effect is that SETUP responses now always contain SSRCs of
each format, regardless of the fact that the first packet has been
produced or not (previously we needed at least one packet, from which
the SSRC was extracted).

server_stream_format.go

@@ -1,6 +1,7 @@
 package gortsplib
 
 import (
+	"crypto/rand"
 	"sync/atomic"
 	"time"
 
@@ -11,15 +12,55 @@ import (
 	"github.com/bluenviron/gortsplib/v4/pkg/rtcpsender"
 )
 
+func randUint32() (uint32, error) {
+	var b [4]byte
+	_, err := rand.Read(b[:])
+	if err != nil {
+		return 0, err
+	}
+	return uint32(b[0])<<24 | uint32(b[1])<<16 | uint32(b[2])<<8 | uint32(b[3]), nil
+}
+
+func isServerStreamLocalSSRCTaken(ssrc uint32, stream *ServerStream, exclude *serverStreamFormat) bool {
+	for _, sm := range stream.medias {
+		for _, sf := range sm.formats {
+			if sf != exclude && sf.localSSRC == ssrc {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+func serverStreamPickLocalSSRC(sf *serverStreamFormat) (uint32, error) {
+	for {
+		ssrc, err := randUint32()
+		if err != nil {
+			return 0, err
+		}
+
+		if ssrc != 0 && !isServerStreamLocalSSRCTaken(ssrc, sf.sm.st, sf) {
+			return ssrc, nil
+		}
+	}
+}
+
 type serverStreamFormat struct {
 	sm     *serverStreamMedia
 	format format.Format
 
+	localSSRC      uint32
 	rtcpSender     *rtcpsender.RTCPSender
 	rtpPacketsSent *uint64
 }
 
-func (sf *serverStreamFormat) initialize() {
+func (sf *serverStreamFormat) initialize() error {
+	var err error
+	sf.localSSRC, err = serverStreamPickLocalSSRC(sf)
+	if err != nil {
+		return err
+	}
+
 	sf.rtpPacketsSent = new(uint64)
 
 	sf.rtcpSender = &rtcpsender.RTCPSender{
@@ -33,18 +74,26 @@ func (sf *serverStreamFormat) initialize() {
 		},
 	}
 	sf.rtcpSender.Initialize()
+
+	return nil
 }
 
-func (sf *serverStreamFormat) localSSRC() (uint32, bool) {
-	stats := sf.rtcpSender.Stats()
-	if stats != nil {
-		return stats.LocalSSRC, true
+func (sf *serverStreamFormat) close() {
+	if sf.rtcpSender != nil {
+		sf.rtcpSender.Close()
 	}
-
-	return 0, false
 }
 
-func (sf *serverStreamFormat) writePacketRTP(byts []byte, pkt *rtp.Packet, ntp time.Time) error {
+func (sf *serverStreamFormat) writePacketRTP(pkt *rtp.Packet, ntp time.Time) error {
+	pkt.SSRC = sf.localSSRC
+
+	byts := make([]byte, sf.sm.st.Server.MaxPacketSize)
+	n, err := pkt.MarshalTo(byts)
+	if err != nil {
+		return err
+	}
+	byts = byts[:n]
+
 	sf.rtcpSender.ProcessPacket(pkt, ntp, sf.format.PTSEqualsDTS(pkt))
 
 	le := uint64(len(byts))
@@ -52,7 +101,7 @@ func (sf *serverStreamFormat) writePacketRTP(byts []byte, pkt *rtp.Packet, ntp t
 	// send unicast
 	for r := range sf.sm.st.activeUnicastReaders {
 		if _, ok := r.setuppedMedias[sf.sm.media]; ok {
-			err := r.writePacketRTP(sf.sm.media, pkt.PayloadType, byts)
+			err := r.writePacketRTPEncoded(sf.sm.media, pkt.PayloadType, byts)
 			if err != nil {
 				r.onStreamWriteError(err)
 				continue