Engwar

Pissing people off

From time to time, if you feel the need to piss someone off for their own good, let me assure you that it’s a perfectly natural thing. If you see someone living their lives without getting pissed off even for few minutes, do them a favour by pissing them off, at least once. Just like eustress people need eupiss for a long healthy life. Besides, it shakes things a little bit which helps people to bring some variety to their monotonous existence. So, if the conventional wisdom has shown you that pissing people off is bad, well, it’s because it’s conventional. Make a note on your todo lists, or your GTD system and make it an items that you could do in less than 2 minutes. Then do it. After all it’s for their own good.

Now, let me show you a very easy method to piss someone off. This works extremely well if the pissee is a tech guy. More specifically a programmer. It’s a well tested and trustworthy method which people used in mailing lists from time to time to start holy wars. This method is most successful if the pissee uses a big stupid language and actually LIKE it. All you have to do is utter “my language is better than you one” phrase in a slightly offensive tone and throw an example on their face. It’s that simple. Lemme give you one example.

Artificial examples on blogs sucks. So let me take some code which I was playing with today and explain.

I was writing some Ruby code this afternoon with WSF/Ruby. In order to talk to an SSL endpoint you need to supply the CA cert to the client as an option. Rampart expects you give the cert with the certificate delimiters removed (first and the last line of a certificate). WSF/Perl will read the certificate for you and do the right thing when you pass the filename. But not WSF/Ruby (note to self: make this part of the lib).

client = WSClient.new( 
  { "to" => "/your/end/point",
    "ca_cert" => "cert content as a string" }, logfile )

The beauty of the language you’re using spring up in situations like this. Without further ado, here’s the one liner to get the cert content as a string with certificate delimiters removed,

File.open('server.cert').readlines[1..-2].join.gsub(/\n/, '')

You got to appreciate the beauty of the above line. Although I’m quite fond of it I prefer the Python version over it. Here’s one way to do it,

"".join([line.strip() for line in open("server.cert")][1:-2])

Yes, the list comprehension beats it all. It’s simple and elegant. Most importantly beautiful and pleasing to look at. My first exposure to list comprehension has been via Erlang. Purely due to the fact that I started reading about it before Python.

Anyhoo, all you have to do now is find someone who uses a big stupid language and throw an example like above to their face and tell them to beat it. See if they can write it more elegantly using their language. The beauty of the trick is that there’s no way in this world that’s gonna happen. So, you’ve already won. One word of caution though. When taking an example take something slightly more complex than a single function call. Like opening a file. In both Ruby and Python it’s one function call. DON’T take this type of examples. Why? Because there’s a very high probability that the other person, yes, the one who’s using a big stupid language, will get so angry and beat you up. You don’t want this. So, avoid at all costs.

There you go. Do some good to the world by pissing off few people with this technique. It’s good for your karma too. Happy pissing!

Update: Ok, I admit that I got a bit carried away with that example which made it artificial, hence sucked. When you write an SSL client using WSF/Ruby you only have to give the filename of the certificate, none of the certificate delimiter removal plus newline removal is necessary. But, I’ll keep the examples to keep the central theme in alignment. BTW, here’s the correct SSL client example with WSF/Ruby,

client = WSClient.new(
  { "to" =>  "/your/end/point",
    "ca_cert" => "server.cert" }, logfile )

Update 2: Before it gets a bit out of hand, let me reiterate if it was not evident from the first reading. This whole thing was meant to be a joke. Don’t take it too seriously, have a laugh and return to you editors, grow some sense of humour for chrissake. Oh, one more thing, I’m no Erlang wiz. I’m just started to grok the Erlang landscape.

Comments (20)

Perl embedding woes

While developing server side components that enables providing Web services using Perl bumped into a brick wall. First the requirement is when a client request comes, need to extract the payload and pass it to WSF/C which acts as the underlying Web services engine. So WSF/C knows how to process the incoming XML payload and do what it does and return another XML payload which is then passed back to the client who send the request. And the business logic is written using Perl. Simple, I know.

We used SWIG to generate some wrapper functions (plus some custom written C code) which belongs to WSF/C and wrote a Perl module encapsulating the lower level stuff. So when you have to write a Web service in Perl all you have to do is use WSO2::WSF::Service; in your script and call a couple of functions. As the deployment scenario we choose Apache with ModPerl. That combination seemd logical and some of the heavy trafficked sites are using that.

It gets interesting when you test the whole thing. Apache is configured to run ModPerl on a folder named /perl so requests matching /perl/something will go through ModPerl. ResponseHandler is ModPerl::Registry and also GlobalRequest has been enabled to get the global request via Apache::RequestUtil->request().

Say, we have a service script named echo_service.pl which simply return back whatever it gets. Here’s the code,

#!/usr/bin/perl
 
use strict;
use WSO2::WSF;
use WSO2::WSF::WSService;
use Apache2::RequestUtil ();
 
my $payload =<<E;
<ns1:echoString xmlns:ns1="http://perl.axis2.org/samples">
  <text>plurk u can haz</text>
</ns1:echoString>
E
 
sub echoFunction {
    my $arg = $_[0];
    my $message = new WSO2::WSF::WSMessage ({'payload' => $arg});
    return $message;
}
 
my %operations = ( 'echoString' => 'echoFunction');
 
if (defined $ENV{MOD_PERL}) {
  my $r = Apache2::RequestUtil->request();
 
  my $service = new WSO2::WSF::WSService(
    {'operations' => \%operations, 'action' => 'testAction'}
  );
 
  $service->reply( $r );
}

When it goes to the reply method it’ll call up some C routines. Inside one of those C functions, after determining the function name to call by looking at the request payload, we need to call it. From a C function. How can you do that? By embedding the Perl interpreter. AFAIK, you cannot call a Perl function from C without embedding the interpreter (please correct me if I’m wrong). In WSF/Ruby server side there was no such issue because the entire Ruby module (WSService class and methods) is written in C.

Let me show you the code that embeds the interpreter and calls the function after looking at the XML payload in the request,

static axiom_node_t *
wsf_xml_msg_recv_invoke_other(axis2_msg_recv_t* msg_recv,
        const axutil_env_t* env,
        wsf_svc_info_t* svc_info,
        axis2_msg_ctx_t* in_msg_ctx,
        axis2_msg_ctx_t* out_msg_ctx,
        axis2_char_t* function_name,
        axis2_char_t* class_name)
{
    AXIS2_PARAM_CHECK(env->error, svc_info, NULL);
    AXIS2_PARAM_CHECK(env->error, in_msg_ctx, NULL);
    AXIS2_PARAM_CHECK(env->error, out_msg_ctx, NULL);
 
    axiom_node_t *node = NULL;
    axiom_node_t *om_node = NULL;
    axiom_soap_envelope_t *envelope = NULL;
    axiom_soap_body_t *body = NULL;
    axis2_char_t *retstr = NULL;
    axiom_node_t *soap_body_node = NULL;
 
    /* extracting payload from the soap message */
    envelope = axis2_msg_ctx_get_soap_envelope(in_msg_ctx, env);
    if (!envelope)
    {
        AXIS2_LOG_ERROR(env->log, AXIS2_LOG_SI, "[wsf-perl-service] soap envelope not found");
        return NULL;
    }
 
    body = axiom_soap_envelope_get_body(envelope, env);
    if (!body)
    {
        AXIS2_LOG_ERROR(env->log, AXIS2_LOG_SI, "[wsf-perl-service] soap body not found");
        return NULL;
    }
 
    soap_body_node = axiom_soap_body_get_base_node(body, env);
    if (!soap_body_node)
    {
        AXIS2_LOG_ERROR(env->log, AXIS2_LOG_SI, "[wsf-perl-service] soap body base node not found");
        return NULL;
    }
 
    om_node = axiom_node_get_first_child(soap_body_node, env);
    if (!om_node)
    {
        return NULL;
    }
 
    axis2_char_t *embedding[] = {"-M'WSO2::WSF::C; WSO2::WSF::Service;'", ""};
    if (!svc_info->script_filename)
    {
        AXIS2_LOG_ERROR(env->log, AXIS2_LOG_SI,
                "perl function invocation failed, script_file name not found for "
                "service %s", svc_info->svc_name);
        return NULL;
    }
    /* passing script real path into perl interpreter. */
    embedding[1] = svc_info->script_filename;
 
    my_perl = perl_alloc();
    PL_perl_destruct_level = 0;
/*    PL_use_safe_putenv = 1; */
    PERL_SET_CONTEXT(my_perl);
    perl_construct(my_perl);
 
    PL_origalen = 1;
    PERL_SET_CONTEXT(my_perl);
    if (perl_parse(my_perl, xs_init, 2, embedding, NULL))
    {
        AXIS2_LOG_ERROR(env->log, AXIS2_LOG_SI, "perl_parse method failed");
        return NULL;
    }
    PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
    /* perl_run(my_perl); */
 
    if (SvTRUE(ERRSV))
    {
        AXIS2_LOG_DEBUG(env->log, AXIS2_LOG_SI, "invoke perl function failed");
    }
 
    retstr = invoke_perl_function(env, om_node, function_name, NULL);
 
    if (retstr)
    {
        node = wsf_util_deserialize_buffer(env, retstr);
    }
 
    PERL_SET_CONTEXT(my_perl);
    perl_destruct(my_perl);
    PERL_SET_CONTEXT(my_perl);
    perl_free(my_perl);
    PERL_SYS_TERM();
    AXIS2_LOG_DEBUG(env->log, AXIS2_LOG_SI, axiom_node_to_string(node, env));
    return node;
}
 
static axis2_char_t *
invoke_perl_function(const axutil_env_t *env, axiom_node_t *om_node,
        axis2_char_t *operation, axis2_char_t *class_name)
{
    int count = 0;
    axis2_char_t *inmsg = NULL;
    SV **wsmsg_str = NULL; /* hold the value for the key 'payload' in WSMessage */
    SV *wsmsg_ref = NULL; /* reference to a WSMessage object */
    HV *wsmsg = NULL; /* WSMessage object */
    char *tmp_str = NULL;
    axis2_char_t *res_payload_str = NULL;
 
    if (!operation)
    {
        AXIS2_LOG_DEBUG(env->log, AXIS2_LOG_SI,
                "invoking perl function failed, operation not available");
        return NULL;
    }
 
    if (om_node)
    {
        inmsg = axiom_node_to_string(om_node, env);
    }
 
    /* declare and init a local copy of the Perl stack pointer */
    dSP;
 
    /* mortal SVs for the stack */
    ENTER;
    SAVETMPS;
 
    /* "record" the current stack pointer */
    PUSHMARK(SP);
 
    /* push parameters to the stack */
    XPUSHs(sv_2mortal(newSVpv(inmsg, 0)));
 
    /* make the global copy of the stack pointer same as the local copy */
    PUTBACK;
 
    /* call the Perl function, expecting a scalar to be returned */
    count = call_pv(operation, G_SCALAR);
 
    /* refreshing the local copy of the stack pointer, call_pv might have reallocated it */
    SPAGAIN;
 
    if (count != 1)
    {
        croak("perl function invocation failed") ;
        AXIS2_LOG_ERROR(env->log, AXIS2_LOG_SI, "perl function %s invocation failed", operation);
    }
 
    /* get the scalar reference from the stack */
    wsmsg_ref = (SV *) POPs;
 
    /* getting the object from the reference */
    wsmsg = SvRV(wsmsg_ref);
 
    /* fetching the value of the member variable 'payload' from WSMessage */
    wsmsg_str = hv_fetch(wsmsg, "payload", 7, FALSE);
 
    /* perl scalar to c string */
    tmp_str = SvPVutf8_nolen(*wsmsg_str);
    res_payload_str = (axis2_char_t *) savepvn(tmp_str, strlen(tmp_str));
 
    PUTBACK;
 
    /* cleaning up mortal SVs */
    FREETMPS;
    LEAVE;
 
    return res_payload_str;
}

Full source is here (note that the actual source might be different from what you see here since it’s being modified at the time of this writing). Also my_perl is declared as a static variable. The problem is when my_perl gets freed it looks like ModPerl is also affected. A valgrind report shows that it tries to read from a pointer after perl_free() gets called. As the following valgrind exerpt shows,

==16101== Invalid read of size 4
==16101==    at 0x62AD118: _wrap_wsf_worker_process_request (wsf_wrap.c:4167)
==16101==    by 0x5476D68: Perl_pp_entersub (pp_hot.c:2847)
==16101==    by 0x5438092: Perl_runops_debug (dump.c:1931)
==16101==    by 0x546FF97: Perl_call_sv (perl.c:2646)
==16101==    by 0x4DC2AF3: modperl_callback (modperl_callback.c:101)
==16101==    by 0x4DC388B: modperl_callback_run_handlers (modperl_callback.c:262)
==16101==    by 0x4DC4289: modperl_callback_per_dir (modperl_callback.c:369)
==16101==    by 0x4DBB53E: modperl_response_handler_run (mod_perl.c:1004)
==16101==    by 0x4DBB704: modperl_response_handler_cgi (mod_perl.c:1099)
==16101==    by 0x25A6C: ap_run_handler (config.c:158)
==16101==    by 0x295BE: ap_invoke_handler (config.c:372)
==16101==    by 0x35D80: ap_process_request (http_request.c:258)
==16101==  Address 0x649051c is 12 bytes inside a block of size 1,692 free'd
==16101==    at 0x480590A: free (vg_replace_malloc.c:323)
==16101==    by 0x546AED4: perl_free (perl.c:1394)
==16101==    by 0x62BD599: wsf_xml_msg_recv_invoke_business_logic_sync (wsf_xml_msg_recv.c:452)
==16101==    by 0x613D503: axis2_msg_recv_invoke_business_logic (msg_recv.c:392)
==16101==    by 0x613DB34: axis2_msg_recv_receive_impl (msg_recv.c:319)
==16101==    by 0x613D583: axis2_msg_recv_receive (msg_recv.c:431)
==16101==    by 0x61327D4: axis2_engine_receive (engine.c:315)
==16101==    by 0x6167177: axis2_http_transport_utils_process_http_post_request (http_transport_utils.c:595)
==16101==    by 0x62BE9B3: wsf_worker_process_request (wsf_worker.c:301)
==16101==    by 0x62AD0F7: _wrap_wsf_worker_process_request (wsf_wrap.c:4166)
==16101==    by 0x5476D68: Perl_pp_entersub (pp_hot.c:2847)
==16101==    by 0x5438092: Perl_runops_debug (dump.c:1931)

Still haven’t been able to pin down what exactly happens when perl_free() gets called

Comments (2)

WSF/Perl 1.1

Just uploaded WSF/Perl 1.1 to CPAN. Doesn’t show up there yet but will be available within a few hours. This has many things added since the last release (which was ages ago). You could now do WS-ReliableMessaging, WS-Security, WS-SecurityPolicy. The release became ridiculously late due to couple of bugs which took me REALLY LONG to debug and fix. Ouch. Download and give it a shot. It’ll take couple of seconds to compile. If you’re in a generous mood I recommend contributing to a good cause in those few seconds.

This release again has only support only on the client side, if you’re looking forward to write/host services with WSF/Perl you still have to wait a bit more. Dinesh and Danushka has done some excellent progress on that front and will be finishing it soon. If you wanna know how it’s taking shape, then by all means.

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