conf/config.go

// Package conf defines utils to simplify configuration
// management.
package conf

import (
	"encoding/json"
	"errors"
	"fmt"
	"io/ioutil"
	"os"
	"path/filepath"
)

// LoadFile reads the file at path, parses its json content and fills the struct
// with the content of the file.
func LoadFile(path string, data interface{}) error {
	return read(path, data)
}

// LoadFiles tries to load all the given paths in the given order.
//
// If a path does not exist, it is ignored.
//
// It returns an error only if the content of a file is invalid, i.e. it
// cannot be unmarshaled by json.
func LoadFiles(data interface{}, paths ...string) error {
	for _, p := range paths {
		err := read(p, data)
		if err != nil && !errors.Is(err, os.ErrNotExist) {
			return fmt.Errorf("cannot load %q: %w", p, err)
		}
	}

	return nil
}

// SaveFile writes the given data serialized in JSON in the given path.
func SaveFile(path string, data interface{}) error {
	return write(path, data)
}

// LoadAndUpdateFile reads the config fileat path and
// updates it, meaning that it adds new options, removes
// old ones, and update it by calling the Update method of
// data if it implements the interface Updater.
//
// If no file is found at path, it is created and
// initialized with the default values.
//
// An error is returned only if the config file cannot be
// written.
func LoadAndUpdateFile(path string, data interface{}) error {
	if _, err := os.Stat(path); !os.IsNotExist(err) {
		err2 := read(path, data)
		if err2 != nil {
			return err2
		}

		if data, ok := data.(Updater); ok {
			data.Update()
		}
	}

	return write(path, data)
}

// Updater is the interface that can be implemented by
// config structs. If it is implemented, Update() is
// called by LoadAndUpdateFile(). It allows one to modify
// the data and persist those changes, for example to
// change default values.
type Updater interface {
	Update()
}

func read(path string, data interface{}) error {
	content, err := ioutil.ReadFile(filepath.Clean(path))
	if err != nil {
		return fmt.Errorf("cannot read config file: %w", err)
	}

	err = json.Unmarshal(content, &data)
	if err != nil {
		return fmt.Errorf("cannot parse config file: %w", err)
	}

	return nil
}

func write(path string, data interface{}) error {
	content, err := json.MarshalIndent(data, "", "  ")
	if err != nil {
		return fmt.Errorf("cannot generate config content: %w", err)
	}

	err = ioutil.WriteFile(path, content, 0o600)
	if err != nil {
		return fmt.Errorf("cannot write config file '%s': %w", path, err)
	}

	return nil
}