Engwar

I’ve been rewriting the Perl StockTrader client using Catalyst for the past week or so. The first version of the client was done using (gasp!) Perl/CGI. So it was about time to do it right. Thanks to the excellent documentation Catalyst provide, it was a straightforward task.

The main annoyance I faced was the lack of a proper error reporting mechanism. When you have an error on your controller, the script that starts the server simply refuse to start you can see the following VERY helpful error message in the console.

Unknown error
Compilation failed in require at /usr/share/perl5/Catalyst/Utils.pm line 270.
Compilation failed in require at ./script/stocktrader_server.pl line 55.

You cannot find out where the error occured. Only conclusion that you can make out of this is that there is an error in one of your controllers. Errors in models are shown correctly as they should. Template errors can easily be fixed. If you haven’t been doing incremental development, meaning you add a little bit of code then start the server and see whether everything has worked ok up to now, you’ll be in serious trouble. If you have the habit of writing large amounts of code without testing and keep testing for the last minute, Catalyst will torture you and put you through hellish debugging sessions for hours. I spent an inordinate amount of time trying to figure out the error and finally spotted that I’ve forgotten to make a single variable lexically scoped with my.

I also configured Log4perl to log all the errors to a log file and enabled the option to log warnings. Sadly I could not see any more log messages than I see in the console.

That aside, Catalyst is a fantastic framework to write web applications if you know Perl. As a big fan of frameworks it’s so exciting to see high quality open source MVC frameworks being developed that drastically reduce the effort of building web applications. Whatever the language you’re comfortable in programming, you can easily find a decent MVC framework for it these days. Well .. almost.

Last time I looked at Catalyst for a project, the time was really tight and had to do it by other means. This time however, I had slightly more time available so I started from Catalyst::Manual::Tutorial and boy, the docs are amazingly well written. Kennedy Clark has done a wonderful job with the documentation.

Catalyst is a MVC framework written in Perl. If you’ve written any applications with Ruby on Rails or Django, the first thing you’ll notice is, like anything written in Perl, speed. Also, another difference that I see with Catalyst is the unprecedented flexibility it gives to the hands of the application developer. For a framework providing this level of flexibility, Catalyst is surprisingly easy to program with. Will write more about my Catalyst chronicles in another post.

Since there was no progress from my last experiment, writing a rudimentary program without any WSF/C code seemed like a logical next step. For those of you who don’t know what I tried to do, here’s a brief explanation. I want to let other people provide Web services in Perl. My objective is to enable people to just copy their Perl scripts to, say, a folder and it’ll automatically become a Web service. Let me explain that with an example. Say you have the following Perl script,

#!/usr/bin/env perl
 
sub getRandomNumber {
    return 4;
}

I’m going to assuming you have configured your web server to execute Perl scripts ending with .pl via ModPerl for a directory called perl. You copy the file and it’s accessible from http://localhost/perl/random.pl. And it’s a web service now. So you can send an XML payload like,

<getRandomNumber />

And get back something like,

<number>4</number>

Since I’m using WSF/C as the underlying Web service engine and it provides various other facilities like WS-Security, WS-ReliableMessaging etc… at blazing speed, I wanna use it.

Here’s a simplified graphical version of the story,

1. Request comes to rand.pl via ModPerl and it gets executed.
2. rand.pl then calls some methods in Service.pm module which in turn calls to some C functions.
3. From one of those C functions we need to call a Perl function residing in the script the request came into.

If you’re wondering how the graphic relates to the story of copying your Perl script and making it a service (‘cos there’s a Service.pm in the picture), please note that I’ve not yet figured out every detail in the system

Perl is segfaulting at all the bizzare places when this is executed. When things get this bad it’s time to write a very simple application that does the same thing without any WSF/C code. Here’s how I did it.

I started by creating a new Perl extension in C.

$ h2xs -A -n ICanHaz

This’ll create the skeleton and all I have to do is edit the .xs file and put my code there. Here’s my ICanHaz.xs file,

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include <string.h>
 
#include "ppport.h"
 
static	PerlInterpreter *my_perl;
 
void cheeseburger( void )
{
    char *embedding[] = {"/usr/local/apache2/perl/exec-this.pl"};
 
    my_perl = perl_alloc();
    perl_construct(my_perl);
    PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
    perl_parse(my_perl, NULL, 1, embedding, NULL);
    perl_run(my_perl);
    perl_destruct(my_perl);
    perl_free(my_perl);
    PERL_SYS_TERM();
}
 
MODULE = ICanHaz		PACKAGE = ICanHaz		
 
void
cheeseburger()

Now you have to edit Makefile.PL and give the output of, perl -MExtUtils::Embed -e ccopts -e ldopts to the compiler flags. After that you can compile and install the module. Here’s what’s inside exec-this.pl,

sub keele_teh_urth {
    print "oh noez. i iz gud kitteh";
}

Just a simple function. Then I wrote another Perl script to invoke the method in my C extension. Here’s the program,

#!/usr/local/bin/perl
 
use ICanHaz;
 
ICanHaz::cheeseburger();
 
print "Content-type: text/html\n\n";
print "dis segfaults, srsly.\n";

Executing this file via ModPerl results in a segmentation fault. As you can see even the simplest case of embedding will not execute through ModPerl. Switched to CGI and got the same results. Possible alternatives to try is 1) move all the embedding stuff to it’s own executable, when you need to call the Perl function from C do a fork followed by exec, pass the payload as a parameter, store the result in a shared memory and once done read from it (this .. um .. a bit ugly to say the least). Second alternative is to use PPI. I haven’t looked into PPI yet so don’t know for sure whether it’s possible.