dmctl/:Clear workers cache before we run a command (pingcap#365)

* clear workers and new rootCmd every time before we start a subCommand

dm/ctl/ctl.go

@@ -28,21 +28,32 @@ import (
 
 var (
 	commandMasterFlags = CommandMasterFlags{}
-	rootCmd            = &cobra.Command{
-		Use:   "dmctl",
-		Short: "DM control",
-	}
+	rootCmd            *cobra.Command
 )
 
 // CommandMasterFlags are flags that used in all commands for dm-master
 type CommandMasterFlags struct {
 	workers []string // specify workers to control on these dm-workers
 }
 
+// Reset clears cache of CommandMasterFlags
+func (c CommandMasterFlags) Reset() {
+	c.workers = c.workers[:0]
+}
+
 func init() {
+	rootCmd = NewRootCmd()
+}
+
+// NewRootCmd generates a new rootCmd
+func NewRootCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "dmctl",
+		Short: "DM control",
+	}
 	// --worker worker1 -w worker2 --worker=worker3,worker4 -w=worker5,worker6
-	rootCmd.PersistentFlags().StringSliceVarP(&commandMasterFlags.workers, "worker", "w", []string{}, "DM-worker ID")
-	rootCmd.AddCommand(
+	cmd.PersistentFlags().StringSliceVarP(&commandMasterFlags.workers, "worker", "w", []string{}, "DM-worker ID")
+	cmd.AddCommand(
 		master.NewStartTaskCmd(),
 		master.NewStopTaskCmd(),
 		master.NewPauseTaskCmd(),
@@ -66,6 +77,7 @@ func init() {
 		master.NewPurgeRelayCmd(),
 		master.NewMigrateRelayCmd(),
 	)
+	return cmd
 }
 
 // Init initializes dm-control
@@ -116,6 +128,8 @@ func PrintHelp(args []string) {
 
 // Start starts running a command
 func Start(args []string) {
+	commandMasterFlags.Reset()
+	rootCmd = NewRootCmd()
 	rootCmd.SetArgs(args)
 	if err := rootCmd.Execute(); err != nil {
 		fmt.Println(rootCmd.UsageString())