have createSession fill the sessionInfo.cancel field, have Conn use Conn.session.cancel instead of storing its own cancellation, this should prevent any of these things from being both nil and reachable at the same time

src/yggdrasil/router.go

@@ -161,8 +161,7 @@ func (r *router) handleTraffic(packet []byte) {
 		return
 	}
 	sinfo, isIn := r.core.sessions.getSessionForHandle(&p.Handle)
-	if !isIn || sinfo.cancel == nil {
-		// FIXME make sure sinfo.cancel can never be nil
+	if !isIn {
 		util.PutBytes(p.Payload)
 		return
 	}