Improve error message if product set more than once

On registration and deregistration the product flag (-p/--product)
is only supposed to be used once. But this is not enforced and the
error message from the server is not clear about what the user did
wrong. This patch enforces the one product rule at flag parse time
and prints a better message.

SUSE#171

suseconnect/suseconnect.go

@@ -32,6 +32,26 @@ func main() {
 	}
 }
 
+// singleStringFlag cannot be set more than once.
+// e.g. `cmd -p abc -p def` will give a parse error.
+type singleStringFlag struct {
+	value string
+	isSet bool
+}
+
+func (p *singleStringFlag) String() string {
+	return p.value
+}
+
+func (p *singleStringFlag) Set(value string) error {
+	if p.isSet {
+		return fmt.Errorf("this flag can only be specified once\n")
+	}
+	p.value = value
+	p.isSet = true
+	return nil
+}
+
 func connectMain() {
 	var (
 		status           bool
@@ -46,7 +66,7 @@ func connectMain() {
 		fsRoot           string
 		namespace        string
 		token            string
-		product          string
+		product          singleStringFlag
 		instanceDataFile string
 		listExtensions   bool
 		email            string
@@ -78,11 +98,11 @@ func connectMain() {
 	flag.StringVar(&namespace, "namespace", "", "")
 	flag.StringVar(&token, "regcode", "", "")
 	flag.StringVar(&token, "r", "", "")
-	flag.StringVar(&product, "product", "", "")
-	flag.StringVar(&product, "p", "", "")
 	flag.StringVar(&instanceDataFile, "instance-data", "", "")
 	flag.StringVar(&email, "email", "", "")
 	flag.StringVar(&email, "e", "", "")
+	flag.Var(&product, "product", "")
+	flag.Var(&product, "p", "")
 
 	flag.Parse()
 	if version {
@@ -121,8 +141,8 @@ func connectMain() {
 	if token != "" {
 		connect.CFG.Token = token
 	}
-	if product != "" {
-		if p, err := connect.SplitTriplet(product); err != nil {
+	if product.isSet {
+		if p, err := connect.SplitTriplet(product.value); err != nil {
 			fmt.Print("Please provide the product identifier in this format: ")
 			fmt.Print("<internal name>/<version>/<architecture>. You can find ")
 			fmt.Print("these values by calling: 'SUSEConnect --list-extensions'\n")
@@ -174,7 +194,7 @@ func connectMain() {
 			fmt.Print("Please use --instance-data only in combination ")
 			fmt.Print("with --url pointing to your RMT or SMT server\n")
 			os.Exit(1)
-		} else if connect.URLDefault() && token == "" && product == "" {
+		} else if connect.URLDefault() && token == "" && product.value == "" {
 			flag.Usage()
 			os.Exit(1)
 		} else if isSumaManaged() {