Validate max_staleness.

def _validate_max_staleness(max_staleness):
    """Validate max_staleness."""
    if max_staleness == -1:
        return -1

    if not isinstance(max_staleness, integer_types):
        raise TypeError(_invalid_max_staleness_msg(max_staleness))

    if max_staleness <= 0:
        raise ValueError(_invalid_max_staleness_msg(max_staleness))

    return max_staleness