config/env.go

/*
 * Copyright (C) 2017 eschao <esc.chao@gmail.com>
 *
 * 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 config

import (
	"bytes"
	"fmt"
	"io"
	"os"
	"reflect"
	"strings"
)

// ParseValueEnvFunc func to parse env value
type ParseValueEnvFunc func(value reflect.Value, prefix string) error

// Parse parses given structure interface, extracts environment definitions
// from its tag and sets structure with defined environment variables
func ParseEnv(out interface{}) error {
	return ParseEnvWith(out, "", parseValueEnv)
}

// UsageEnv prints usage description of config i to out
func UsageEnv(out io.Writer, in interface{}) error {
	if _, err := out.Write([]byte("Environment Usage:\n")); err != nil {
		return fmt.Errorf("write usage: %w", err)
	}

	buf := bytes.NewBufferString("")

	if err := ParseEnvWith(in, "", func(value reflect.Value, prefix string) error {
		return usageValueEnv(buf, value, prefix)
	}); err != nil {
		return err
	}

	var tabPos int

	for _, line := range strings.Split(strings.TrimRight(buf.String(), "\n"), "\n") {
		if pos := strings.Index(line, "\t"); pos > tabPos {
			tabPos = pos
		}
	}

	const TabSize = 8

	for _, line := range strings.Split(strings.TrimRight(buf.String(), "\n"), "\n") {
		if pos := strings.Index(line, "\t"); pos >= 0 {
			if count := tabPos/TabSize - pos/TabSize; count > 0 {
				line = line[:pos] + strings.Repeat("\t", count) + line[pos:]
			}
		}

		if _, err := out.Write([]byte(line + "\n")); err != nil {
			return fmt.Errorf("write usage: %w", err)
		}
	}

	return nil
}

// ParseWith parses with given structure interface and environment name prefix
// It is normally used in nested structure.
// For example: we have such structure
// type Database struct {
//   Host string `env:"HOST"`
// }

//	type Server struct {
//	  Server string `env:"SERVER"`
//	  DB     Database `env:"DB_"`
//	}
//
// The Server.DB.Host will be mapped to environment variable: DB_HOST which is
// concatenated from DB tag in Server struct and Host tag in Database struct
func ParseEnvWith(out interface{}, prefix string, parser ParseValueEnvFunc) error {
	ptrRef := reflect.ValueOf(out)

	if ptrRef.IsNil() || ptrRef.Kind() != reflect.Ptr {
		return fmt.Errorf("%w: %s",
			errExpectStructPointerInsteadOf, ptrRef.Kind().String())
	}

	valueOfStruct := ptrRef.Elem()
	if valueOfStruct.Kind() != reflect.Struct {
		return fmt.Errorf("%w: %s",
			errExpectStructPointerInsteadOf, valueOfStruct.Kind().String())
	}

	return parser(valueOfStruct, prefix)
}

func usageValueEnv(out io.Writer, value reflect.Value, prefix string) error {
	var err error

	typeOfStruct := value.Type()

	for i := 0; i < value.NumField() && err == nil; i++ {
		valueOfField := value.Field(i)
		kindOfField := valueOfField.Kind()
		structOfField := typeOfStruct.Field(i)

		// Recursively unmarshal if value is ptr type
		if kindOfField == reflect.Ptr {
			if !valueOfField.IsNil() && valueOfField.CanSet() {
				err = ParseEnvWith(
					valueOfField.Interface(),
					prefix+structOfField.Tag.Get("env"),
					func(value reflect.Value, prefix string) error {
						return usageValueEnv(out, value, prefix)
					},
				)
			} else {
				continue
			}
		} else if kindOfField == reflect.Struct {
			err = usageValueEnv(out, valueOfField, prefix+structOfField.Tag.Get("env"))
		}

		if err != nil {
			return err
		}

		if kindOfField != reflect.Struct {
			err = usageFieldValueEnv(out, structOfField, prefix)
		}
	}

	return err
}

// parseValue parses a reflect.Value object
func parseValueEnv(value reflect.Value, prefix string) error {
	var err error

	typeOfStruct := value.Type()

	for i := 0; i < value.NumField() && err == nil; i++ {
		valueOfField := value.Field(i)
		kindOfField := valueOfField.Kind()
		structOfField := typeOfStruct.Field(i)

		// Recursively unmarshal if value is ptr type
		if kindOfField == reflect.Ptr {
			if !valueOfField.IsNil() && valueOfField.CanSet() {
				err = ParseEnvWith(
					valueOfField.Interface(),
					prefix+structOfField.Tag.Get("env"),
					parseValueEnv,
				)
			} else {
				continue
			}
		} else if kindOfField == reflect.Struct {
			err = parseValueEnv(valueOfField, prefix+structOfField.Tag.Get("env"))
		}

		if err != nil {
			return err
		}

		if kindOfField != reflect.Struct {
			err = setFieldValueEnv(valueOfField, structOfField, prefix)
		}
	}

	return err
}

// setFieldValue sets a reflect.Value with environment value
func setFieldValueEnv(value reflect.Value, field reflect.StructField, prefix string) error {
	envName := field.Tag.Get("env")
	if envName == "" {
		return nil
	}

	envValue, ok := os.LookupEnv(prefix + envName)
	if !ok {
		return nil
	}

	if !value.CanSet() {
		return fmt.Errorf("%w: %s", errValueCannotBeChanged, field.Name)
	}

	if setUnmarshalTextValue(value, envValue) {
		return nil
	}

	if ok, err := setDurationValue(value, envValue); ok {
		return err
	}

	return setSimpleEnvValue(value, field, envValue)
}

func setSimpleEnvValue(value reflect.Value, field reflect.StructField, str string) error {
	var err error

	kind := value.Kind()
	switch kind {
	case reflect.Bool:
		err = setValueWithBool(value, str)
	case reflect.String:
		value.SetString(str)
	case reflect.Int8:
		err = setValueWithIntX(value, str, Int8Size)
	case reflect.Int16:
		err = setValueWithIntX(value, str, Int16Size)
	case reflect.Int, reflect.Int32:
		err = setValueWithIntX(value, str, Int32Size)
	case reflect.Int64:
		err = setValueWithIntX(value, str, Int64Size)
	case reflect.Uint8:
		err = setValueWithUintX(value, str, Uint8Size)
	case reflect.Uint16:
		err = setValueWithUintX(value, str, Uint16Size)
	case reflect.Uint, reflect.Uint32:
		err = setValueWithUintX(value, str, Uint32Size)
	case reflect.Uint64:
		err = setValueWithUintX(value, str, Uint64Size)
	case reflect.Float32:
		err = setValueWithFloatX(value, str, Float32Size)
	case reflect.Float64:
		err = setValueWithFloatX(value, str, Float64Size)

	case reflect.Slice:
		sp, ok := field.Tag.Lookup("separator")
		if !ok {
			sp = ":"
		}

		err = setValueWithSlice(value, str, sp)

	default:
		return fmt.Errorf("%w: %s", errUnsupportedType, kind.String())
	}

	if err != nil {
		err = fmt.Errorf("%s: %w", field.Name, err)
	}

	return err
}

func usageFieldValueEnv(out io.Writer, field reflect.StructField, prefix string) error {
	envName := field.Tag.Get("env")
	if envName == "" {
		return nil
	}

	envName = prefix + envName
	msg := "   " + envName + "\t" + field.Tag.Get("usage")

	if def := field.Tag.Get("default"); def != "" {
		msg += " (default: " + def + ")"
	}

	if _, err := out.Write([]byte(msg + "\n")); err != nil {
		return fmt.Errorf("write usage: %w", err)
	}

	return nil
}