May 2008


Yesterday at work someone was trying to pass traditional Apache SSI directives through an XSL transformation on a Google search appliance. Long story short, they vanished: HTML comments don’t make it out of that device.

Anyway…I had a simple solution. Since we were pumping the Google results through a Perl CGI anyway, there was no reason we couldn’t just output a fake HTML tag which the CGI would then turn into an SSI comment for Apache. This was born <ssi virtual="/foo/bar.html" />.

Then a simple s/<ssi (virtual=\"[^\"]+\")><\/ssi>/<!--#include $1 -->/; in Perl will give something Apache can understand.

That solved the immediate problem, but got me thinking about emulating the full Apache mod_include set of SSI directives using the <ssi> tag. I’m thinking of something like this:

<ssi element="include" virtual="/foo/bar.html" />

<ssi element="set" var="FOO" value="BAR" />

<ssi element="if">
  <ssi_if expr="test_condition">YES!</ssi_if>
  <ssi_elif expr="test_condition">MAYBE!</ssi_elif>
  <ssi_else>NO!</ssi_else>
</ssi>

And with that format, the original version still works if you assume a missing “element” attribute implies element="include". The <!--#if --> block isn’t quite satisfying here — any text nodes inside the <ssi> block but outside the <ssi_(if|elif|else)> blocks would be ignored, but that’s no different than odd content in, say, a <table> that doesn’t actually fall into a cell.

I don’t actually have the Perl that would do the transformation, but it wouldn’t be hard. I’ll wait until someone actually needs it.

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mod_perl 2 has an annoying…feature. Because the system environ struct is not thread safe, mod_perl’s perl-script handler unties the %ENV hash from the actual environment. That means, anything that uses the C getenv/setenv/unsetenv functions to read the environment will not see changes that were made to %ENV.

An obvious example is Perl’s localtime function. It actually calls the system localtime function, which uses the C getenv to check the current value of the timezone environment variable TZ. If you try to change the timezone in a mod_perl2 program by assigning to $ENV{TZ}, localtime won’t know it.

The solution is to use the Env::C module and it’s getenv/setenv/unsetenv wrappers. It works fine, but it’s a bit cumbersome. But a simple module, loaded at server-startup time, can wrap the system localtime in a function that takes care of the environment.

package Apache2::Localtime;

use Env::C;
use Exporter;
use strict;

our @ISA = qw(Exporter);
our @EXPORT = qw(localtime);

sub import {
  my $class = shift;
  $class->export('CORE::GLOBAL', 'localtime');
}

sub localtime {
  my $time = shift || time;
  return localtime($time) unless $ENV{TZ};

  my $orig_tz = Env::C::getenv('TZ');
  Env::C::setenv('TZ', $ENV{TZ}, 1);
  my(@ret, $ret);
  if(wantarray) {
    @ret = CORE::localtime($time);
  } else {
    $ret = CORE::localtime($time);
  }
  if(defined $orig_tz) {
    Env::C::setenv('TZ', $orig_tz, 1);
  } else {
    Env::C::unsetenv('TZ');
  }
  return wantarray ? @ret : $ret;
}

1;

Put that in your @INC path at Apache2/Localtime.pm and then add use Apache2::Localtime to a PerlRequire .../initialize.pl script or something similar. The new function should override the built-in localtime and keep your timeonzes in sync.

The code was mostly taken from here.