Atomic file writes with temp and rename

package main

import (
	"fmt"
	"os"
	"path/filepath"
)

// WriteFileAtomic replaces path with data via a temp file and rename, so
// readers never observe a partially written file.
func WriteFileAtomic(path string, data []byte) error {
	tmp, err := os.CreateTemp(filepath.Dir(path), "."+filepath.Base(path)+".*")
	if err != nil {
		return fmt.Errorf("create temp file: %w", err)
	}
	name := tmp.Name()
	defer os.Remove(name) // clean up if the rename fails
	if _, err := tmp.Write(data); err != nil {
		tmp.Close()
		return fmt.Errorf("write temp file: %w", err)
	}
	if err := tmp.Close(); err != nil {
		return fmt.Errorf("close temp file: %w", err)
	}
	if err := os.Chmod(name, 0o644); err != nil {
		return fmt.Errorf("chmod temp file: %w", err)
	}
	if err := os.Rename(name, path); err != nil {
		return fmt.Errorf("rename temp file: %w", err)
	}
	return nil
}

func main() {
	_ = WriteFileAtomic("settings.json", []byte(`{"theme":"dark"}`))
	fmt.Println("saved settings.json")
}