Reuse passive created sendonly transceiver

When peerconnection create a transceiver by remote's recvonly
offer, the transceiver will have nil sender but can't be reused
since its currentDirection is set to sendonly. This pr change
the currentDirection to inactive so it can be reused by AddTrack.

peerconnection.go

@@ -1270,16 +1270,18 @@ func setRTPTransceiverCurrentDirection(answer *SessionDescription, currentTransc
 			case RTPTransceiverDirectionSendonly:
 				direction = RTPTransceiverDirectionRecvonly
 			case RTPTransceiverDirectionRecvonly:
-				// Pion will answer recvonly with a offer recvonly transceiver, so we should
-				// not change the direction to sendonly if we are the offerer, otherwise this
-				// tranceiver can't be reuse for AddTrack
-				if t.Direction() != RTPTransceiverDirectionRecvonly {
-					direction = RTPTransceiverDirectionSendonly
-				}
+				direction = RTPTransceiverDirectionSendonly
 			default:
 			}
 		}
 
+		// If a transceiver is created by applying a remote description that has recvonly transceiver,
+		// it will have no sender. In this case, the transceiver's current direction is set to inactive so
+		// that the transceiver can be reused by next AddTrack.
+		if direction == RTPTransceiverDirectionSendonly && t.Sender() == nil {
+			direction = RTPTransceiverDirectionInactive
+		}
+
 		t.setCurrentDirection(direction)
 	}
 	return nil

peerconnection_renegotiation_test.go

@@ -1233,3 +1233,55 @@ func TestPeerConnection_Renegotiation_MidConflict(t *testing.T) {
 	assert.NoError(t, offerPC.Close())
 	assert.NoError(t, answerPC.Close())
 }
+
+func TestPeerConnection_Regegotiation_AnswerAddsTrack(t *testing.T) {
+	lim := test.TimeOut(time.Second * 30)
+	defer lim.Stop()
+
+	report := test.CheckRoutines(t)
+	defer report()
+
+	pcOffer, pcAnswer, err := newPair()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	tracksCh := make(chan *TrackRemote)
+	pcOffer.OnTrack(func(track *TrackRemote, r *RTPReceiver) {
+		tracksCh <- track
+		for {
+			if _, _, readErr := track.ReadRTP(); errors.Is(readErr, io.EOF) {
+				return
+			}
+		}
+	})
+
+	vp8Track, err := NewTrackLocalStaticSample(RTPCodecCapability{MimeType: MimeTypeVP8}, "foo", "bar")
+	assert.NoError(t, err)
+	assert.NoError(t, signalPair(pcOffer, pcAnswer))
+
+	_, err = pcOffer.AddTransceiverFromKind(RTPCodecTypeVideo, RtpTransceiverInit{
+		Direction: RTPTransceiverDirectionRecvonly,
+	})
+	assert.NoError(t, err)
+	assert.NoError(t, signalPair(pcOffer, pcAnswer))
+
+	_, err = pcAnswer.AddTransceiverFromKind(RTPCodecTypeVideo, RtpTransceiverInit{
+		Direction: RTPTransceiverDirectionSendonly,
+	})
+	assert.NoError(t, err)
+
+	assert.NoError(t, err)
+	_, err = pcAnswer.AddTrack(vp8Track)
+	assert.NoError(t, err)
+	assert.NoError(t, signalPair(pcOffer, pcAnswer))
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	go sendVideoUntilDone(ctx.Done(), t, []*TrackLocalStaticSample{vp8Track})
+
+	<-tracksCh
+	cancel()
+
+	closePairNow(t, pcOffer, pcAnswer)
+}