pkg/kubelet/rktshim: implement cli-flag reflection utility

pkg/kubelet/rktshim/cli.go

@@ -0,0 +1,82 @@
+/*
+Copyright 2016 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package rktshim
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+	"strings"
+)
+
+var (
+	errFlagTagNotFound           = errors.New("arg: given field doesn't have a `flag` tag")
+	errStructFieldNotInitialized = errors.New("arg: given field is unitialized")
+)
+
+// TODO(tmrts): refactor these into an util pkg
+// Uses reflection to retrieve the `flag` tag of a field.
+// The value of the `flag` field with the value of the field is
+// used to construct a POSIX long flag argument string.
+func getLongFlagFormOfField(fieldValue reflect.Value, fieldType reflect.StructField) (string, error) {
+	flagTag := fieldType.Tag.Get("flag")
+	if flagTag == "" {
+		return "", errFlagTagNotFound
+	}
+
+	if fieldValue.IsValid() {
+		return "", errStructFieldNotInitialized
+	}
+
+	switch fieldValue.Kind() {
+	case reflect.Array:
+		fallthrough
+	case reflect.Slice:
+		var args []string
+		for i := 0; i < fieldValue.Len(); i++ {
+			args = append(args, fieldValue.Index(i).String())
+		}
+
+		return fmt.Sprintf("--%v=%v", flagTag, strings.Join(args, ",")), nil
+	}
+
+	return fmt.Sprintf("--%v=%v", flagTag, fieldValue), nil
+}
+
+// Uses reflection to transform a struct containing fields with `flag` tags
+// to a string slice of POSIX compliant long form arguments.
+func getArgumentFormOfStruct(strt interface{}) (flags []string) {
+	numberOfFields := reflect.ValueOf(strt).NumField()
+
+	for i := 0; i < numberOfFields; i++ {
+		fieldValue := reflect.ValueOf(strt).Field(i)
+		fieldType := reflect.TypeOf(strt).Field(i)
+
+		flagFormOfField, err := getLongFlagFormOfField(fieldValue, fieldType)
+		if err != nil {
+			continue
+		}
+
+		flags = append(flags, flagFormOfField)
+	}
+
+	return
+}
+
+func getFlagFormOfStruct(strt interface{}) (flags []string) {
+	return getArgumentFormOfStruct(strt)
+}