Rework handling of buffered keyframes.

Instead of buffering the last keyframe, we merely keep track of its
seqno, and use the main cache for recovering.  We also send the whole
sequence of packets rather than just the keyframe itself.

rtpconn/rtpwriter.go

@@ -206,9 +206,10 @@ func (writer *rtpWriter) add(track conn.DownTrack, add bool, max int) error {
 	}
 }
 
-func sendKeyframe(kf []uint16, track conn.DownTrack, cache *packetcache.Cache) {
+func sendSequence(kf, last uint16, track conn.DownTrack, cache *packetcache.Cache) {
 	buf := make([]byte, packetcache.BufSize)
-	for _, seqno := range kf {
+	seqno := kf
+	for ((last - seqno) & 0x8000) == 0 {
 		bytes := cache.Get(seqno, buf)
 		if bytes == 0 {
 			return
@@ -218,6 +219,7 @@ func sendKeyframe(kf []uint16, track conn.DownTrack, cache *packetcache.Cache) {
 		if err != nil {
 			return
 		}
+		seqno++
 	}
 }
 
@@ -253,14 +255,12 @@ func rtpWriterLoop(writer *rtpWriter, track *rtpUpTrack) {
 					action.track.SetCname(cname)
 				}
 
-				found, _, lts := track.cache.Last()
-				kts, _, kf := track.cache.Keyframe()
-				if found && len(kf) > 0 {
-					if ((lts-kts)&0x80000000) != 0 ||
-						lts-kts < 2*90000 {
-						// we got a recent keyframe
-						go sendKeyframe(
-							kf,
+				last, foundLast := track.cache.Last()
+				kf, foundKf := track.cache.Keyframe()
+				if foundLast && foundKf {
+					if last-kf < 40 { // modulo 2^16
+						go sendSequence(
+							kf, last,
 							action.track,
 							track.cache,
 						)
@@ -331,11 +331,11 @@ func nackWriter(track *rtpUpTrack) {
 
 	// drop any nacks before the last keyframe
 	var cutoff uint16
-	found, seqno, _ := track.cache.KeyframeSeqno()
+	seqno, found := track.cache.Keyframe()
 	if found {
 		cutoff = seqno
 	} else {
-		last, lastSeqno, _ := track.cache.Last()
+		lastSeqno, last := track.cache.Last()
 		if !last {
 			// NACK on a fresh track?  Give up.
 			return