Function SaveReaderSafe

cmn/cos/io.go:527–537

same as above, plus rename

(tmpfqn, fqn string, reader io.Reader, buf []byte, cksumType string,
	size int64, dirMustExist string)

// same as above, plus rename
func SaveReaderSafe(tmpfqn, fqn string, reader io.Reader, buf []byte, cksumType string,
	size int64, dirMustExist string) (cksum *CksumHash, err error) {
	if cksum, err = SaveReader(tmpfqn, reader, buf, cksumType, size, dirMustExist); err != nil {
		return nil, err
	}
	if err := Rename(tmpfqn, fqn); err != nil {
		os.Remove(tmpfqn)
		return nil, err
	}
	return cksum, nil
}

// Read only the first line of a file.
// Do not use for big files: it reads all the content and then extracts the first