Fix config_diff panics when old config or new config is empty

Fixes #190

worker/config_diff.go

@@ -53,6 +53,7 @@ func diffMirrorConfig(oldList, newList []mirrorConfig) []mirrorCfgTrans {
 	sort.Sort(sortableMirrorList(oList))
 	sort.Sort(sortableMirrorList(nList))
 
+	if len(oList) != 0 && len(nList) != 0 {
 		// insert a tail node to both lists
 		// as the maximum node
 		lastOld, lastNew := oList[len(oList)-1], nList[len(nList)-1]
@@ -83,6 +84,14 @@ func diffMirrorConfig(oldList, newList []mirrorConfig) []mirrorCfgTrans {
 				j++
 			}
 		}
+	} else {
+		for i := 0; i < len(oList); i++ {
+			operations = append(operations, mirrorCfgTrans{diffDelete, oList[i]})
+		}
+		for i := 0; i < len(nList); i++ {
+			operations = append(operations, mirrorCfgTrans{diffAdd, nList[i]})
+		}
+	}
 
 	return operations
 }