shell: only one shell is allowed to connect to the cluster

fix https://github.com/chrislusf/seaweedfs/issues/1266

weed/server/master_grpc_server.go

@@ -190,6 +190,19 @@ func (ms *MasterServer) KeepConnected(stream master_pb.Seaweed_KeepConnectedServ
 
 	peerAddress := findClientAddress(stream.Context(), req.GrpcPort)
 
+	// only one shell can be connected at any time
+	if req.Name == "shell" {
+		if ms.currentAdminShellClient == ""{
+			ms.currentAdminShellClient = peerAddress
+			defer func() {
+				ms.currentAdminShellClient = ""
+			}()
+		} else {
+			return fmt.Errorf("only one concurrent shell allowed, but another shell is already connected from %s", peerAddress)
+		}
+	}
+
+
 	stopChan := make(chan bool)
 
 	clientName, messageChan := ms.addClient(req.Name, peerAddress)