getting bitten by universal methods

When I work on code, I nearly always replace UNIVERSAL::isa and UNIVERSAL::can with block-eval of a method call instead. This lets objects that overload isa or can work properly, and still avoids program death when the invocant is an invalid invocant.

A few weeks ago, while working on memory issues, though, we found a horrible problem with this. Some of our code, which could accept either an Email::Simple or a string, was doing something like this:

if (eval { $message->isa('Email::Simple') }) { ... }

The code functioned as expected, and worked. Sometimes, though, the process would grow to ridiculous sizes. The reason is that this code was running under perl-5.6, which has some issues.

See, in perl-5.6, when you try to call a method on something that is not yet defined, perl creates the package for you. This creates a hash entry in the stash for that package’s variables to go, even though it has none. Here’s an example:

japh@perl-tester:~$ /opt/perl/perl-5.6.1/bin/perl -l test-mem "xy" 1000000
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME
japh      3779  0.0  0.4   3956  1132 pts/0    S+   03:55   0:00

Then we run: eval { (xy x 1000000)->can('bloat') }; 

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME
japh      3779  0.0  3.4  11780  8976 pts/0    R+   03:55   0:00

So, a two meg message called as a method can bloat you up quite a lot! What email message is going to be “xy” repeated a million times, though? Let’s try something more realistic, just a bit:

japh@perl-tester:~$ /opt/perl/perl-5.6.1/bin/perl -l test-mem "Subject: hi mom\n\nHow's the world's best mom?" 45454
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME
japh      3782  0.0  0.4   3956  1128 pts/0    S+   03:58   0:00

Killed

It ran out of memory, even though it’s very nearly the same number of characters. What happened? It’s the apostrophes. They’re equivalent to ::, and they cause much deeper structures to be created in the stash. Here’s some output that didn’t kill the process:

japh@perl-tester:~$ /opt/perl/perl-5.6.1/bin/perl -l test-mem "Subject: hi mom\n\nHow's the world's best mom?" 2500
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME
japh      3801  0.0  0.4   3956  1128 pts/0    S+   04:01   0:00

Then we run: eval { (Subject: hi mom\n\nHow's the world's best mom? x 2500)->can('bloat') }; 

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME
japh      3801  6.9 74.9 286708 192868 pts/0   S+   04:01   0:02

That’s only a 110k message. For a lot of fun, dump \%:: after doing that. This is just another reason to use perl-5.8:

japh@perl-tester:~$ /opt/perl/perl-5.8.0/bin/perl -l test-mem "Subject: hi
mom\n\nHow's the world's best mom?" 2500
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME
japh      3807  0.0  0.4   4260  1148 pts/0    R+   04:03   0:00

Then we run: eval { (Subject: hi mom\n\nHow's the world's best mom? x
2500)->can('bloat') }; 

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME 
japh      3807  0.0  0.5   4428  1400 pts/0    R+   04:03   0:00

Of course, if you can’t upgrade, there’s always my old friend _INVOCANT, which is now found in Params::Util. If you say something like this:

eval { _INVOCANT($x) && $x->can('do_awesome_stuff') }

Everything is much closer to normal:

japh@perl-tester:~$ /opt/perl/perl-5.6.1/bin/perl -l test-mem-invocant "Subject: hi mom\n\nHow's the world's best mom?" 2500
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME
japh      3905  0.0  0.4   3948  1184 pts/0    R+   04:11   0:00

Then we run our code.

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME
japh      3905  0.0  0.5   4112  1476 pts/0    R+   04:11   0:00

Despite this, you still won’t see me using UNIVERSAL:: methods as functions. I think maybe Params::Util just needs a _CAN.

Written on December 15, 2006
perl   programming