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.