while( 1 ) {
    my $node = $queue->dequeue; 
    if( !defined $node and $queue->pending ){
        $queue->enqueue( undef );
        next;
    }
    else {
        last;
    }
    if( $node->{canBeSet} ) {
        ## deal with this node
    }
    else {
        $node->{canBeSet} = 1;
        $Q->enqueue( $node ); ## push back for next time.
    }
}