Atomic write testing in Golang (why this fails on Windows?)

atomic-test.go

package main

import (
	"bytes"
	"fmt"
	"io"
	"io/ioutil"
	"os"
	"path/filepath"

	"golang.org/x/sync/errgroup"
)

func main() {
	if err := run(); err != nil {
		fmt.Println(err)
		os.Exit(1)
	}
	fmt.Printf("OK\n")
}

func run() error {
	f, err := ioutil.TempFile("", "atomic-test")
	if err != nil {
		return err
	}
	f.Close()
	os.Remove(f.Name())
	defer os.Remove(f.Name())

	contents := bytes.Repeat([]byte("abcde"), 100)
	var eg errgroup.Group
	for i := 0; i < 10; i++ {
		eg.Go(func() error {
			return writeFile(f.Name(), bytes.NewReader(contents))
		})
	}
	if err := eg.Wait(); err != nil {
		return err
	}

	written, err := ioutil.ReadFile(f.Name())
	if err != nil {
		return err
	}
	if string(written) != string(contents) {
		return fmt.Errorf("file contents should be %q but got %q", string(contents), string(written))
	}
	return nil
}

func writeFile(name string, r io.Reader) error {
	dir, base := filepath.Split(name)
	f, err := ioutil.TempFile(dir, base)
	if err != nil {
		return err
	}
	defer os.Remove(f.Name())
	if _, err := io.Copy(f, r); err != nil {
		return err
	}
	f.Close()
	return os.Rename(f.Name(), name)
}

Note that atomic write ensures it does not create a halfway file but not guarantees that all the multiple write request to be succeeded. Rename can fail, that's all.