woo.go

package main

import (
	"fmt"
	"math/rand"
	"os"
	"path/filepath"
	"strconv"
	"time"

	"github.com/pkg/errors"

	"golang.org/x/sys/unix"
)

func main() {
	rand.Seed(time.Now().UnixNano())

	data := fmt.Sprintf("%d", rand.Int63())
	err := atomicWriteOnce("file", []byte(data), 0644)
	if err != nil {
		panic(err)
	}
}

func atomicWriteOnce(path string, data []byte, mode os.FileMode) error {
	dir := filepath.Dir(path)
	file, err := os.OpenFile(dir, unix.O_TMPFILE|os.O_RDWR, mode) // nolint: gosec
	if err != nil {
		return err
	}
	// warning: error return value not checked
	defer file.Close()
	_, err = file.Write(data)
	if err != nil {
		return err
	}

	err = file.Sync()
	if err != nil {
		return err
	}

	oldpath := filepath.Join("/proc", "self", "fd", strconv.Itoa(int(file.Fd())))
	err = unix.Linkat(unix.AT_FDCWD, oldpath, unix.AT_FDCWD, path, unix.AT_SYMLINK_FOLLOW)
	if err == nil {
		return nil
	} else if err != unix.EEXIST {
		return err
	}
	// Woo, we gotta do the renameat exchange dance
	tmpFileName := filepath.Join(filepath.Dir(path), fmt.Sprintf("%d", rand.Int63()))
	defer unix.Unlink(tmpFileName)
	err = unix.Linkat(unix.AT_FDCWD, oldpath, unix.AT_FDCWD, tmpFileName, unix.AT_SYMLINK_FOLLOW)
	if err != nil {
		return errors.Wrap(err, "Temp file name already existed!")
	}
	return unix.Renameat2(unix.AT_FDCWD, tmpFileName, unix.AT_FDCWD, path, unix.RENAME_EXCHANGE)
}