Include pending migrate tracks in ParticipantInfo. (#2493)

It is possible that participant state and migration state updates are
racing. And a participant update could end up with no tracks when
migration is being processed.

pkg/rtc/participant.go

@@ -514,8 +514,37 @@ func (p *ParticipantImpl) ToProtoWithVersion() (*livekit.ParticipantInfo, utils.
 		IsPublisher: p.IsPublisher(),
 	}
 	p.lock.RUnlock()
+
+	p.pendingTracksLock.RLock()
 	pi.Tracks = p.UpTrackManager.ToProto()
 
+	// add any pending migrating tracks, else an update could delete/unsubscribe from yet to be published, migrating tracks
+	maybeAdd := func(pti *pendingTrackInfo) {
+		if !pti.migrated {
+			return
+		}
+
+		found := false
+		for _, ti := range pi.Tracks {
+			if ti.Sid == pti.trackInfos[0].Sid {
+				found = true
+				break
+			}
+		}
+
+		if !found {
+			pi.Tracks = append(pi.Tracks, pti.trackInfos[0])
+		}
+	}
+
+	for _, pt := range p.pendingTracks {
+		maybeAdd(pt)
+	}
+	for _, ppt := range p.pendingPublishingTracks {
+		maybeAdd(ppt)
+	}
+	p.pendingTracksLock.RUnlock()
+
 	return pi, piv
 }
 
@@ -748,6 +777,10 @@ func (p *ParticipantImpl) SetMigrateInfo(
 	}
 	p.pendingTracksLock.Unlock()
 
+	if len(mediaTracks) != 0 {
+		p.setIsPublisher(true)
+	}
+
 	p.TransportManager.SetMigrateInfo(previousOffer, previousAnswer, dataChannels)
 }