conference: handle metadata of streams properly

Don't include tracks in the metadata that are not yet published (for
which we don't have any remote streams available). Also, inform about
metadata changes once tracks get published and unpublished.

Author: daniel-abramov

pkg/conference/processor.go

@@ -49,7 +49,6 @@ func (c *Conference) processPeerMessage(message common.Message[ParticipantID, pe
 	switch msg := message.Content.(type) {
 	case peer.JoinedTheCall:
 		participant.logger.Info("Joined the call")
-		c.resendMetadataToAllExcept(participant.id)
 
 	case peer.LeftTheCall:
 		participant.logger.Info("Left the call")
@@ -69,6 +68,7 @@ func (c *Conference) processPeerMessage(message common.Message[ParticipantID, pe
 		}
 
 		participant.publishedTracks[key] = msg.Track
+		c.resendMetadataToAllExcept(participant.id)
 
 	case peer.PublishedTrackFailed:
 		participant.logger.Infof("Failed published track: %s", msg.Track.ID())
@@ -85,6 +85,8 @@ func (c *Conference) processPeerMessage(message common.Message[ParticipantID, pe
 			otherParticipant.peer.UnsubscribeFrom([]*webrtc.TrackLocalStaticRTP{msg.Track})
 		}
 
+		c.resendMetadataToAllExcept(participant.id)
+
 	case peer.NewICECandidate:
 		participant.logger.Debug("Received a new local ICE candidate")
 

pkg/conference/state.go

@@ -63,13 +63,24 @@ func (c *Conference) removeParticipant(participantID ParticipantID) {
 }
 
 // Helper to get the list of available streams for a given participant, i.e. the list of streams
-// that a given participant **can subscribe to**.
+// that a given participant **can subscribe to**. Each stream may have multiple tracks.
 func (c *Conference) getAvailableStreamsFor(forParticipant ParticipantID) event.CallSDPStreamMetadata {
 	streamsMetadata := make(event.CallSDPStreamMetadata)
 	for id, participant := range c.participants {
+		// Skip us. As we know about our own tracks.
 		if forParticipant != id {
-			for streamID, metadata := range participant.streamMetadata {
-				streamsMetadata[streamID] = metadata
+			// Now, find out which of published tracks belong to the streams for which we have metadata
+			// available and construct a metadata map for a given participant based on that.
+			for _, track := range participant.publishedTracks {
+				trackID, streamID := track.ID(), track.StreamID()
+
+				if metadata, ok := streamsMetadata[track.StreamID()]; ok {
+					metadata.Tracks[trackID] = event.CallSDPStreamMetadataTrack{}
+					streamsMetadata[streamID] = metadata
+				} else if metadata, ok := participant.streamMetadata[streamID]; ok {
+					metadata.Tracks = event.CallSDPStreamMetadataTracks{trackID: event.CallSDPStreamMetadataTrack{}}
+					streamsMetadata[streamID] = metadata
+				}
 			}
 		}
 	}