Refactor utils/flags.go, fixes #11892

Signed-off-by: Antonio Murdaca <me@runcom.ninja>

mflag/flag.go

@@ -1054,6 +1054,42 @@ func (f *FlagSet) Parse(arguments []string) error {
 	return nil
 }
 
+// ParseFlags is a utility function that adds a help flag if withHelp is true,
+// calls cmd.Parse(args) and prints a relevant error message if there are
+// incorrect number of arguments. It returns error only if error handling is
+// set to ContinueOnError and parsing fails. If error handling is set to
+// ExitOnError, it's safe to ignore the return value.
+func (cmd *FlagSet) ParseFlags(args []string, withHelp bool) error {
+	var help *bool
+	if withHelp {
+		help = cmd.Bool([]string{"#help", "-help"}, false, "Print usage")
+	}
+	if err := cmd.Parse(args); err != nil {
+		return err
+	}
+	if help != nil && *help {
+		cmd.Usage()
+		// just in case Usage does not exit
+		os.Exit(0)
+	}
+	if str := cmd.CheckArgs(); str != "" {
+		cmd.ReportError(str, withHelp)
+	}
+	return nil
+}
+
+func (cmd *FlagSet) ReportError(str string, withHelp bool) {
+	if withHelp {
+		if os.Args[0] == cmd.Name() {
+			str += ". See '" + os.Args[0] + " --help'"
+		} else {
+			str += ". See '" + os.Args[0] + " " + cmd.Name() + " --help'"
+		}
+	}
+	fmt.Fprintf(cmd.Out(), "docker: %s.\n", str)
+	os.Exit(1)
+}
+
 // Parsed reports whether f.Parse has been called.
 func (f *FlagSet) Parsed() bool {
 	return f.parsed