blob: 8f59e1a57732579bfc547e49b227955f300cdba5 [file] [log] [blame]
// Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package renameio writes files atomically by renaming temporary files.
package renameio
import (
const patternSuffix = "*.tmp"
// Pattern returns a glob pattern that matches the unrenamed temporary files
// created when writing to filename.
func Pattern(filename string) string {
return filepath.Join(filepath.Dir(filename), filepath.Base(filename)+patternSuffix)
// WriteFile is like ioutil.WriteFile, but first writes data to an arbitrary
// file in the same directory as filename, then renames it atomically to the
// final name.
// That ensures that the final location, if it exists, is always a complete file.
func WriteFile(filename string, data []byte) (err error) {
return WriteToFile(filename, bytes.NewReader(data))
// WriteToFile is a variant of WriteFile that accepts the data as an io.Reader
// instead of a slice.
func WriteToFile(filename string, data io.Reader) (err error) {
f, err := ioutil.TempFile(filepath.Dir(filename), filepath.Base(filename)+patternSuffix)
if err != nil {
return err
defer func() {
// Only call os.Remove on f.Name() if we failed to rename it: otherwise,
// some other process may have created a new file with the same name after
// that.
if err != nil {
if _, err := io.Copy(f, data); err != nil {
return err
// Sync the file before renaming it: otherwise, after a crash the reader may
// observe a 0-length file instead of the actual contents.
// See
if err := f.Sync(); err != nil {
return err
if err := f.Close(); err != nil {
return err
return os.Rename(f.Name(), filename)