// Package safeio implements convenient I/O routines that provide additional
// levels of safety in the presence of unexpected failures.
package safeio

import (
	"io/ioutil"
	"os"
	"path"
)

// WriteFile writes data to a file named by filename, atomically.
// It's a wrapper to ioutil.WriteFile, but provides atomicity (and increased
// safety) by writing to a temporary file and renaming it at the end.
//
// Note this relies on same-directory Rename being atomic, which holds in most
// reasonably modern filesystems.
func WriteFile(filename string, data []byte, perm os.FileMode) error {
	// Note we create the temporary file in the same directory, otherwise we
	// would have no expectation of Rename being atomic.
	tmpf, err := ioutil.TempFile(path.Dir(filename), path.Base(filename))
	if err != nil {
		return err
	}

	if err = os.Chmod(tmpf.Name(), perm); err != nil {
		tmpf.Close()
		os.Remove(tmpf.Name())
		return err
	}

	if _, err = tmpf.Write(data); err != nil {
		tmpf.Close()
		os.Remove(tmpf.Name())
		return err
	}

	if err = tmpf.Close(); err != nil {
		os.Remove(tmpf.Name())
		return err
	}

	return os.Rename(tmpf.Name(), filename)
}