src/cmd/go/internal/renameio/renameio.go

// 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 (
	"bytes"
	"io"
	"io/ioutil"
	"os"
	"path/filepath"
)

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 {
			f.Close()
			os.Remove(f.Name())
		}
	}()

	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 https://golang.org/issue/22397#issuecomment-380831736.
	if err := f.Sync(); err != nil {
		return err
	}
	if err := f.Close(); err != nil {
		return err
	}
	return os.Rename(f.Name(), filename)
}