Function overwriteFileIfChanged

internal/shellgen/generate.go:106–142

overwriteFileIfChanged checks that the contents of f == data, and overwrites f if they differ. It also ensures that f's permissions are set to perm.

(path string, data []byte, perm os.FileMode)

// overwriteFileIfChanged checks that the contents of f == data, and overwrites
// f if they differ. It also ensures that f's permissions are set to perm.
func overwriteFileIfChanged(path string, data []byte, perm os.FileMode) (changed bool, err error) {
	flag := os.O_RDWR | os.O_CREATE
	file, err := os.OpenFile(path, flag, perm)
	if errors.Is(err, os.ErrNotExist) {
		if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
			return false, err
		}

		// Definitely a new file if we had to make the directory.
		return true, os.WriteFile(path, data, perm)
	}
	if err != nil {
		return false, err
	}
	defer file.Close()

	fi, err := file.Stat()
	if err != nil || fi.Mode().Perm() != perm {
		if err := file.Chmod(perm); err != nil {
			return false, err
		}
	}

	// Fast path - check if the lengths differ.
	if err == nil && fi.Size() != int64(len(data)) {
		return true, overwriteFile(file, data, 0)
	}

	r := bufio.NewReader(file)
	for offset := range data {
		b, err := r.ReadByte()
		if err != nil || b != data[offset] {
			return true, overwriteFile(file, data, offset)
		}
	}
	return false, nil
}

// overwriteFile truncates f to len(data) and writes data[offset:] beginning at
// the same offset in f.