fix panic 5435 (#5436)

weed/server/master_grpc_server_assign.go

@@ -97,6 +97,9 @@ func (ms *MasterServer) Assign(ctx context.Context, req *master_pb.AssignRequest
 			continue
 		}
 		dn := dnList.Head()
+		if dn == nil {
+			continue
+		}
 		var replicas []*master_pb.Location
 		for _, r := range dnList.Rest() {
 			replicas = append(replicas, &master_pb.Location{

weed/server/master_server_handlers.go

@@ -149,7 +149,9 @@ func (ms *MasterServer) dirAssignHandler(w http.ResponseWriter, r *http.Request)
 		} else {
 			ms.maybeAddJwtAuthorization(w, fid, true)
 			dn := dnList.Head()
-
+			if dn == nil {
+				continue
+			}
 			writeJsonQuiet(w, r, http.StatusOK, operation.AssignResult{Fid: fid, Url: dn.Url(), PublicUrl: dn.PublicUrl, Count: count})
 			return
 		}

weed/topology/volume_layout.go

@@ -301,7 +301,7 @@ func (vl *VolumeLayout) PickForWrite(count uint64, option *VolumeGrowOption) (vi
 			if float64(info.Size) > float64(vl.volumeSizeLimit)*option.Threshold() {
 				shouldGrow = true
 			}
-			return vid, count, locationList, shouldGrow, nil
+			return vid, count, locationList.Copy(), shouldGrow, nil
 		}
 		return 0, 0, nil, shouldGrow, errors.New("Strangely vid " + vid.String() + " is on no machine!")
 	}

weed/topology/volume_location_list.go

@@ -28,6 +28,9 @@ func (dnll *VolumeLocationList) Copy() *VolumeLocationList {
 
 func (dnll *VolumeLocationList) Head() *DataNode {
 	//mark first node as master volume
+	if dnll.Length() == 0 {
+		return nil
+	}
 	return dnll.list[0]
 }