pkg/types: return error instead of crashing when code supplies an incompatible type to types.LoadArgs()

pkg/types/args.go

@@ -63,6 +63,12 @@ func GetKeyField(keyString string, v reflect.Value) reflect.Value {
 	return v.Elem().FieldByName(keyString)
 }
 
+// UnmarshalableArgsError is used to indicate error unmarshalling args
+// from the args-string in the form "K=V;K2=V2;..."
+type UnmarshalableArgsError struct {
+	error
+}
+
 // LoadArgs parses args from a string in the form "K=V;K2=V2;..."
 func LoadArgs(args string, container interface{}) error {
 	if args == "" {
@@ -85,8 +91,13 @@ func LoadArgs(args string, container interface{}) error {
 			unknownArgs = append(unknownArgs, pair)
 			continue
 		}
-
-		u := keyField.Addr().Interface().(encoding.TextUnmarshaler)
+		keyFieldIface := keyField.Addr().Interface()
+		u, ok := keyFieldIface.(encoding.TextUnmarshaler)
+		if !ok {
+			return UnmarshalableArgsError{fmt.Errorf(
+				"ARGS: cannot unmarshal into field '%s' - type '%s' does not implement encoding.TextUnmarshaler",
+				keyString, reflect.TypeOf(keyFieldIface))}
+		}
 		err := u.UnmarshalText([]byte(valueString))
 		if err != nil {
 			return fmt.Errorf("ARGS: error parsing value of pair %q: %v)", pair, err)

pkg/types/args_test.go

@@ -118,4 +118,15 @@ var _ = Describe("LoadArgs", func() {
 			Expect(err).NotTo(HaveOccurred())
 		})
 	})
+
+	Context("When known arguments are passed and cannot be unmarshalled", func() {
+		It("LoadArgs should fail", func() {
+			conf := struct {
+				IP IPNet
+			}{}
+			err := LoadArgs("IP=10.0.0.0/24", &conf)
+			Expect(err).To(HaveOccurred())
+
+		})
+	})
 })