perltutorial
imp
When working with Apache::Registry it is very easy to create accidental closures.<br>
This is due to the way Apache::Registry makes a fake package for your script, as I'll demonstrate in this tutorial.
<p>
The only indication that something is amiss (other than the unpredictable behaviour) will be the following line in your error log: <i>Variable "$foo" will not stay shared at ...</i>
<p>
The following code demonstrates the problem with accidental closures when using Apache::Registry.
<code>
use strict;
use warnings;
my $foo = 5;
print "Content-type: text/plain\n";
print "Content-disposition: inline; filename=foo.txt\n\n";
printf "Package: %s\n", __PACKAGE__;
printf "[%s] Before: %s\n", $$, $foo;
badness(5);
printf "[%s] After: %s\n", $$, $foo;
sub badness {
my $val = shift;
printf "[%s] badness: %s\n", $$, $foo;
$foo += $val;
}
</code>
<p>
Apache::Registry will take the above code and create a new package for it based on the ServerName and the name of the script,
and then wrap the code in a sub handler {} block.
<p>
If your script is running on "foo.com" and is named "test.pl", then this is what the above code will look like after Apache::Registry is done:
<p>
<code>
package Apache::ROOTfoo_2ecom::test_2epl;
use Apache qw(exit);
sub handler {
#line 1 /www/foo.com/test.pl
use strict;
use warnings;
my $foo = 5;
print "Content-type: text/plain\n";
print "Content-disposition: inline; filename=foo.txt\n\n";
printf "Package: %s\n", __PACKAGE__;
printf "[%s] Before: %s\n", $$, $foo;
badness(5);
printf "[%s] After: %s\n", $$, $foo;
sub badness {
my $val = shift;
printf "[%s] badness: %s\n", $$, $foo;
$foo += $val;
}
}
</code>
<u>First run:</u>
<code>
Package: Apache::ROOTfoo_2ecom::test_2epl
[13520] Before: 5
[13520] badness: 5
[13520] After: 10
</code>
<u>Second:</u>
<code>
Package: Apache::ROOTfoo_2ecom::test_2epl
[19331] Before: 5
[19331] badness: 5
[19331] After: 10
</code>
<u>Third:</u>
<code>
Package: Apache::ROOTfoo_2ecom::test_2epl
[19331] Before: 5
[19331] badness: 10
[19331] After: 5
</code>
<u>Fourth:</u>
<code>
Package: Apache::ROOTfoo_2ecom::test_2epl
[19331] Before: 5
[19331] badness: 15
[19331] After: 5
</code>
Notice how the number within the badness sub is increasing for each process, but the $foo that is seen by the instance script is never modified after 'badness' after the first execution for that process.
<p>
This is because the badness function is actually an inner function now,
and it keeps a reference to the instance of $foo that was created for the
first run.
<p>
<b>Edit - example of how to avoid this issue added, per rhesa's suggestion</b><br>
<p>
Thankfully it is easy to avoid these problems once you know why they occur.<br>
Tips:
<ul>
<li>Keep your toplevel script minimal</li>
<li>Subroutines should only use the variables that were passed</li>
<li>Encapsulate behaviour in supporting objects</li>
</ul>
Example of a working alternative:
<code>
use strict;
use warnings;
my $foo = 5;
print "Content-type: text/plain\n";
print "Content-disposition: inline; filename=foo.txt\n\n";
printf "Package: %s\n", __PACKAGE__;
printf "[%s] Before: %s\n", $$, $foo;
badness(\$foo, 5);
badness(\$foo, 5);
printf "[%s] After: %s\n", $$, $foo;
sub badness {
my ($foo,$val) = @_;
printf "[%s] badness: %s\n", $$, $$foo;
$$foo += $val;
}
</code>