Update (current|pending)RemoteDescription SDP after addIceCandidate

peerconnection.go

@@ -2070,8 +2070,15 @@ func (pc *PeerConnection) RemoteDescription() *SessionDescription {
 
 // AddICECandidate accepts an ICE candidate string and adds it
 // to the existing set of candidates.
-func (pc *PeerConnection) AddICECandidate(candidate ICECandidateInit) error {
-	remoteDesc := pc.RemoteDescription()
+func (pc *PeerConnection) AddICECandidate(candidate ICECandidateInit) error { //nolint:cyclop
+	pc.mu.Lock()
+	defer pc.mu.Unlock()
+
+	remoteDesc := pc.currentRemoteDescription
+	if pc.pendingRemoteDescription != nil {
+		remoteDesc = pc.pendingRemoteDescription
+	}
+
 	if remoteDesc == nil {
 		return &rtcerr.InvalidStateError{Err: ErrNoRemoteDescription}
 	}
@@ -2112,7 +2119,23 @@ func (pc *PeerConnection) AddICECandidate(candidate ICECandidateInit) error {
 		return err
 	}
 
-	return pc.iceTransport.AddRemoteCandidate(&c)
+	err = pc.iceTransport.AddRemoteCandidate(&c)
+	if err != nil {
+		return err
+	}
+
+	if pc.pendingRemoteDescription != nil {
+		if err := pc.pendingRemoteDescription.UpdateWithCandidate(candidate); err != nil {
+			return err
+		}
+	}
+	if pc.currentRemoteDescription != nil {
+		if err := pc.currentRemoteDescription.UpdateWithCandidate(candidate); err != nil {
+			return err
+		}
+	}
+
+	return nil
 }
 
 // Return true if the sdp contains a specific ufrag.

peerconnection_test.go

@@ -5,6 +5,7 @@ package webrtc
 
 import (
 	"runtime"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"testing"
@@ -968,3 +969,35 @@ func TestICETrickleCapabilityString(t *testing.T) {
 		assert.Equal(t, tt.expected, tt.value.String())
 	}
 }
+
+func TestPeerConnection_AddICECandidateUpdatesRemoteDescription(t *testing.T) {
+	pc, err := NewPeerConnection(Configuration{})
+	assert.NoError(t, err)
+
+	_, err = pc.AddTransceiverFromKind(RTPCodecTypeAudio)
+	assert.NoError(t, err)
+
+	_, err = pc.CreateDataChannel("test-channel", nil)
+	assert.NoError(t, err)
+
+	offer, err := pc.CreateOffer(nil)
+	assert.NoError(t, err)
+
+	assert.NoError(t, pc.SetRemoteDescription(offer))
+
+	candidateStr := "candidate:1 1 udp 1 1.2.3.4 1234 typ relay raddr 0.0.0.0 rport 0"
+	index := uint16(0)
+	assert.NoError(t, pc.AddICECandidate(ICECandidateInit{
+		Candidate:     candidateStr,
+		SDPMLineIndex: &index,
+	}))
+
+	remoteSDP := pc.RemoteDescription().SDP
+	assert.Contains(t, remoteSDP, candidateStr)
+
+	candidateIndex := strings.Index(remoteSDP, candidateStr)
+	dataChannelIndex := strings.Index(remoteSDP, "m=application")
+	assert.True(t, candidateIndex < dataChannelIndex, "Candidate should appear before the data channel m-line")
+
+	assert.NoError(t, pc.Close())
+}

sessiondescription.go

@@ -56,6 +56,47 @@ func (sd *SessionDescription) Unmarshal() (*sdp.SessionDescription, error) {
 	return sd.parsed, nil
 }
 
+// UpdateWithCandidate adds a candidate to the SDP.
+func (sd *SessionDescription) UpdateWithCandidate(candidate ICECandidateInit) error { //nolint:cyclop
+	// Work on a fresh parse rather than the cached sd.parsed: that structure
+	// may be concurrently read by the operations queue (e.g. dtlsRoleFromSDP,
+	// startRTP) so mutating its MediaDescriptions would race.
+	parsed := &sdp.SessionDescription{}
+	if err := parsed.UnmarshalString(sd.SDP); err != nil {
+		return fmt.Errorf("%w: %w", ErrSDPUnmarshalling, err)
+	}
+
+	var targetMedia *sdp.MediaDescription
+	if candidate.SDPMid != nil {
+		for _, m := range parsed.MediaDescriptions {
+			if mid, ok := m.Attribute(sdp.AttrKeyMID); ok && mid == *candidate.SDPMid {
+				targetMedia = m
+
+				break
+			}
+		}
+	} else if candidate.SDPMLineIndex != nil {
+		if int(*candidate.SDPMLineIndex) < len(parsed.MediaDescriptions) {
+			targetMedia = parsed.MediaDescriptions[*candidate.SDPMLineIndex]
+		}
+	}
+
+	if targetMedia == nil {
+		return nil
+	}
+
+	candidateValue := strings.TrimPrefix(candidate.Candidate, "candidate:")
+	targetMedia.WithValueAttribute("candidate", candidateValue)
+
+	marshaled, err := parsed.Marshal()
+	if err != nil {
+		return err
+	}
+	sd.SDP = string(marshaled)
+
+	return nil
+}
+
 func hasICETrickleOption(desc *sdp.SessionDescription) bool {
 	if value, ok := desc.Attribute(sdp.AttrKeyICEOptions); ok && hasTrickleOptionValue(value) {
 		return true