selfpost/internal/atomicfile/atomicfile.go

package atomicfile

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

// Write writes data to path via a temp file in the same directory followed by
// a rename, so readers only ever see the complete old or new file.
func Write(path string, data []byte, perm os.FileMode) error {
	dir := filepath.Dir(path)
	tmp, err := os.CreateTemp(dir, ".tmp-*")
	if err != nil {
		return fmt.Errorf("create temp in %s: %w", dir, err)
	}
	tmpName := tmp.Name()
	cleanup := true
	defer func() {
		if cleanup {
			_ = os.Remove(tmpName)
		}
	}()

	if _, err := tmp.Write(data); err != nil {
		tmp.Close()
		return fmt.Errorf("write temp %s: %w", tmpName, err)
	}
	if err := tmp.Chmod(perm); err != nil {
		tmp.Close()
		return fmt.Errorf("chmod temp %s: %w", tmpName, err)
	}
	if err := tmp.Sync(); err != nil {
		tmp.Close()
		return fmt.Errorf("sync temp %s: %w", tmpName, err)
	}
	if err := tmp.Close(); err != nil {
		return fmt.Errorf("close temp %s: %w", tmpName, err)
	}
	if err := os.Rename(tmpName, path); err != nil {
		return fmt.Errorf("rename %s -> %s: %w", tmpName, path, err)
	}
	cleanup = false
	return nil
}