code: func (s *rpcServer) limitConnections(w http.ResponseWriter, remoteAddr string) bool {
  	if int(atomic.LoadInt32(&s.numClients)+1) > cfg.RPCMaxClients {
  		rpcsLog.Infof("Max RPC clients exceeded [%d] - "+
  			"disconnecting client %s", cfg.RPCMaxClients,
  			remoteAddr)
  		http.Error(w, "503 Too busy.  Try again later.",
  			http.StatusServiceUnavailable)
  		return true
  	}
  	return false
  }
docs: // limitConnections responds with a 503 service unavailable and returns true if
  // adding another client would exceed the maximum allow RPC clients.
  //
  // This function is safe for concurrent access.