webrtc: switch to recvonly transceivers (#4059)

This fixes compatibility with devices that support decoding AV1 but
doesn't support encoding it.

This was previously impossible to achieve due to a bug that prevented
video from being displayed when recvonly transceivers were in use and
audio was not present.

internal/protocols/webrtc/peer_connection.go

@@ -131,15 +131,31 @@ func (co *PeerConnection) Start() error {
 	if co.Publish {
 		videoSetupped := false
 		audioSetupped := false
+		for _, tr := range co.OutgoingTracks {
+			if tr.isVideo() {
+				videoSetupped = true
+			} else {
+				audioSetupped = true
+			}
+		}
+
+		// When audio is not used, a track has to be present anyway,
+		// otherwise video is not displayed on Firefox and Chrome.
+		if !audioSetupped {
+			co.OutgoingTracks = append(co.OutgoingTracks, &OutgoingTrack{
+				Caps: webrtc.RTPCodecCapability{
+					MimeType:  webrtc.MimeTypePCMU,
+					ClockRate: 8000,
+				},
+			})
+		}
 
 		for _, tr := range co.OutgoingTracks {
 			var codecType webrtc.RTPCodecType
 			if tr.isVideo() {
 				codecType = webrtc.RTPCodecTypeVideo
-				videoSetupped = true
 			} else {
 				codecType = webrtc.RTPCodecTypeAudio
-				audioSetupped = true
 			}
 
 			err := mediaEngine.RegisterCodec(webrtc.RTPCodecParameters{
@@ -151,8 +167,8 @@ func (co *PeerConnection) Start() error {
 			}
 		}
 
-		// always register at least a video and a audio codec
-		// otherwise handshake will fail or audio will be muted on some clients (like Firefox)
+		// When video is not used, a track must not be added but a codec has to present.
+		// Otherwise audio is muted on Firefox and Chrome.
 		if !videoSetupped {
 			err := mediaEngine.RegisterCodec(webrtc.RTPCodecParameters{
 				RTPCodecCapability: webrtc.RTPCodecCapability{
@@ -165,18 +181,6 @@ func (co *PeerConnection) Start() error {
 				return err
 			}
 		}
-		if !audioSetupped {
-			err := mediaEngine.RegisterCodec(webrtc.RTPCodecParameters{
-				RTPCodecCapability: webrtc.RTPCodecCapability{
-					MimeType:  webrtc.MimeTypePCMU,
-					ClockRate: 8000,
-				},
-				PayloadType: 0,
-			}, webrtc.RTPCodecTypeAudio)
-			if err != nil {
-				return err
-			}
-		}
 	} else {
 		for _, codec := range incomingVideoCodecs {
 			err := mediaEngine.RegisterCodec(codec, webrtc.RTPCodecTypeVideo)

internal/servers/webrtc/reader.js

@@ -366,7 +366,7 @@
         sdpSemantics: 'unified-plan',
       });
 
-      const direction = 'sendrecv';
+      const direction = 'recvonly';
       this.pc.addTransceiver('video', { direction });
       this.pc.addTransceiver('audio', { direction });