the hazards of coding for webservers vs. shell scripts

[kareila]

When I was doing some code cleanup a few months ago, I saw some lines that looked like this:

my $foo;
$foo = $bar unless $baz;

I decided that looked redundant and changed it to:

my $foo = $bar unless $baz;

Take my advice: don't combine "my" statements and postfix conditionals. Ever.

I've updated the programming guidelines to warn people away from this as well.

The manifestation of this particular bug was to cause the $foo variable to be referenced in global scope if the conditional was false. In that situation, the assigned value would persist across execution threads, such that if two people in a row had a true $baz, the second user would see the first user's value of $foo. Even with use warnings and use strict in effect, Perl apparently thinks this is perfectly fine.

ETA: exor674 says this command will find it (but not in BML files):

perlcritic --single-policy ProhibitConditionalDeclarations

Comments

[jadelennox]

Huh. Nice catch.

Conway's Perl Best Practices' general reaction to postfix conditionals in any circumstance is STAB STAB STABBITY STAB, which is why perlcritic can be set to warn against them, but I know this is direct opposition to our coding guidelines.

[kareila]

I've always liked postfix conditionals myself, although I understand and respect the argument against them. This is the first time I can remember one biting me in the ass like this, though.

[vlion]

I've usually found postfix conditions to wind up being more of a pain than otherwise; they tend to get factored into prefix conditionals.

[sophie]

I must admit, I'm mostly the same; I also don't tend to use unless at all, as my brain prefers consistency; I can deal with negated ifs.

But of course I stick to the coding guidelines for DW.