sub fetch_item {
    my( $item, $on_success )= @_;
    my $fetch; $fetch= sub {
        http_get( $item, 
            success => sub {
                undef $fetch; # we are done
                goto &$on_success;
            };
            error => sub {
                if( can_retry( $item ) ) {
                    goto &$fetch;
                } else {
                    undef $fetch; # we are done retrying
                };
            },
    };
};