Broadcast cond var on RTX write. (#4038)

* Broadcast cond var on RTX write.

High forwarding latency logs all show high queuing delay so far. From
code inspection, RTX writes were not signaling the cond var. Not sure if
that is the reason, but adding a signal there for further tests.

* Remove return values from writeRTX as they are not used

pkg/sfu/buffer/buffer.go

@@ -455,7 +455,7 @@ func (b *Buffer) SetPrimaryBufferForRTX(primaryBuffer *Buffer) {
 	}
 }
 
-func (b *Buffer) writeRTX(rtxPkt *rtp.Packet, arrivalTime int64) (n int, err error) {
+func (b *Buffer) writeRTX(rtxPkt *rtp.Packet, arrivalTime int64) {
 	b.Lock()
 	defer b.Unlock()
 	if !b.bound {
@@ -481,14 +481,14 @@ func (b *Buffer) writeRTX(rtxPkt *rtp.Packet, arrivalTime int64) (n int, err err
 	repairedPkt.SequenceNumber = binary.BigEndian.Uint16(rtxPkt.Payload[:2])
 	repairedPkt.SSRC = b.mediaSSRC
 	repairedPkt.Payload = rtxPkt.Payload[2:]
-	n, err = repairedPkt.MarshalTo(b.rtxPktBuf)
+	n, err := repairedPkt.MarshalTo(b.rtxPktBuf)
 	if err != nil {
 		b.logger.Errorw("could not marshal repaired packet", err, "ssrc", b.mediaSSRC, "sn", repairedPkt.SequenceNumber)
 		return
 	}
 
 	b.calc(b.rtxPktBuf[:n], &repairedPkt, arrivalTime, true)
-	return
+	b.readCond.Broadcast()
 }
 
 func (b *Buffer) Read(buff []byte) (n int, err error) {