*: support getting role in auth v3

This commit implements RoleGet() RPC of etcdserver and adds a new
subcommand "role get" to etcdctl v3. It will list up permissions that
are granted to a given role.

$ ETCDCTL_API=3 bin/etcdctl role get r1
Role r1
KV Read:
        b
        d
KV Write:
        a
        c
        d

etcdctl/ctlv3/command/role_command.go

@@ -31,6 +31,7 @@ func NewRoleCommand() *cobra.Command {
 
 	ac.AddCommand(newRoleAddCommand())
 	ac.AddCommand(newRoleGrantCommand())
+	ac.AddCommand(newRoleGetCommand())
 
 	return ac
 }
@@ -51,6 +52,14 @@ func newRoleGrantCommand() *cobra.Command {
 	}
 }
 
+func newRoleGetCommand() *cobra.Command {
+	return &cobra.Command{
+		Use:   "get <role name>",
+		Short: "get detailed information of a role",
+		Run:   roleGetCommandFunc,
+	}
+}
+
 // roleAddCommandFunc executes the "role add" command.
 func roleAddCommandFunc(cmd *cobra.Command, args []string) {
 	if len(args) != 1 {
@@ -83,3 +92,29 @@ func roleGrantCommandFunc(cmd *cobra.Command, args []string) {
 
 	fmt.Printf("Role %s updated\n", args[0])
 }
+
+// roleGetCommandFunc executes the "role get" command.
+func roleGetCommandFunc(cmd *cobra.Command, args []string) {
+	if len(args) != 1 {
+		ExitWithError(ExitBadArgs, fmt.Errorf("role get command requires role name as its argument."))
+	}
+
+	resp, err := mustClientFromCmd(cmd).Auth.RoleGet(context.TODO(), args[0])
+	if err != nil {
+		ExitWithError(ExitError, err)
+	}
+
+	fmt.Printf("Role %s\n", args[0])
+	fmt.Println("KV Read:")
+	for _, perm := range resp.Perm {
+		if perm.PermType == clientv3.PermRead || perm.PermType == clientv3.PermReadWrite {
+			fmt.Printf("\t%s\n", string(perm.Key))
+		}
+	}
+	fmt.Println("KV Write:")
+	for _, perm := range resp.Perm {
+		if perm.PermType == clientv3.PermWrite || perm.PermType == clientv3.PermReadWrite {
+			fmt.Printf("\t%s\n", string(perm.Key))
+		}
+	}
+}