Fix missing RTX codec using SetCodecPreferences

Author: stephanrotolante

rtpcodec.go

@@ -143,7 +143,7 @@ func codecParametersFuzzySearch(
 	return RTPCodecParameters{}, codecMatchNone
 }
 
-// Given a CodecParameters find the RTX CodecParameters if one exists.
+// Given a CodecParameters find the RTX Payload if one exists.
 func findRTXPayloadType(needle PayloadType, haystack []RTPCodecParameters) PayloadType {
 	aptStr := fmt.Sprintf("apt=%d", needle)
 	for _, c := range haystack {
@@ -155,6 +155,18 @@ func findRTXPayloadType(needle PayloadType, haystack []RTPCodecParameters) Paylo
 	return PayloadType(0)
 }
 
+// Given a CodecParameters find the RTX CodecParameters if one exists.
+func findRTXPCodec(needle PayloadType, haystack []RTPCodecParameters) *RTPCodecParameters {
+	aptStr := fmt.Sprintf("apt=%d", needle)
+	for _, c := range haystack {
+		if aptStr == c.SDPFmtpLine {
+			return &c
+		}
+	}
+
+	return nil
+}
+
 func rtcpFeedbackIntersection(a, b []RTCPFeedback) (out []RTCPFeedback) {
 	for _, aFeedback := range a {
 		for _, bFeeback := range b {

rtptransceiver.go

@@ -83,6 +83,10 @@ func (t *RTPTransceiver) getCodecs() []RTPCodecParameters {
 			}
 			codec.RTCPFeedback = rtcpFeedbackIntersection(codec.RTCPFeedback, c.RTCPFeedback)
 			filteredCodecs = append(filteredCodecs, codec)
+
+			if rtxCodec := findRTXPCodec(c.PayloadType, mediaEngineCodecs); rtxCodec != nil {
+				filteredCodecs = append(filteredCodecs, *rtxCodec)
+			}
 		}
 	}
 

rtptransceiver_test.go

@@ -135,3 +135,53 @@ func Test_RTPTransceiver_SetCodecPreferences_PayloadType(t *testing.T) {
 
 	closePairNow(t, offerPC, answerPC)
 }
+
+func Test_RTPTransciever_SDP_Codec(t *testing.T) {
+	tests := []struct {
+		Label          string
+		setPreferences bool
+	}{
+		{
+			Label:          "",
+			setPreferences: false,
+		},
+		{
+			Label:          "",
+			setPreferences: true,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.Label, func(t *testing.T) {
+			pc, err := NewPeerConnection(Configuration{})
+			assert.NoError(t, err)
+
+			transceiver, err := pc.AddTransceiverFromKind(
+				RTPCodecTypeVideo,
+				RTPTransceiverInit{
+					Direction: RTPTransceiverDirectionRecvonly,
+				},
+			)
+			assert.NoError(t, err)
+
+			if test.setPreferences {
+				codec := RTPCodecCapability{
+					"video/vp8", 90000, 0, "", nil,
+				}
+
+				assert.NoError(t, transceiver.SetCodecPreferences(
+					[]RTPCodecParameters{
+						{
+							RTPCodecCapability: codec,
+						},
+					},
+				))
+			}
+
+			offer, err := pc.CreateOffer(nil)
+			assert.NoError(t, err)
+
+			assert.Equal(t, true, strings.Contains(offer.SDP, "apt=96"))
+		})
+	}
+}