'Compare the checksum of the tarball_path with the .checksum file
in the dest_dir and return False if it matches. The untar
of the package happens only if the checksums do not match.'

def untar_required(self, tarball_path, dest_dir):

    checksum_path = os.path.join(dest_dir, '.checksum')
    try:
        existing_checksum = self._run_command(('cat    ' + checksum_path)).stdout
    except (error.CmdError, error.AutoservRunError):
        return True
    new_checksum = self.compute_checksum(tarball_path)
    return (new_checksum.strip() != existing_checksum.strip())
