sub _waitpid_blocking {
    my $self = shift;
    # pseudo-blocking  
    if( my $sleep_period = $self->{waitpid_blocking_sleep} ) {
        while() {
            my $pid = $self->_waitpid_non_blocking;
            return $pid if $pid;
            # sleep $sleep_period;
            select( '','','', $sleep_period );
        }
    }

    return waitpid -1, 0;
}