Memory Leak in Perl

I always find the topic Memory Management very fascinating. Perl being the Perl, there is nothing you need to worry about. All memory management is done for us for FREE.

Still, there was something, I always wanted to find out about memory management in Perl.

All along, I knew that there is an easy way I can force my script to leak memory.

How?

Just create a data structure with circular reference.

Easy?

But the question is how do I demonstrate the memory leak?

Let me take you back in the memory, in the year 2015, Gabor Szabo handed over the maintenance of PDF::Create. At that time I had no clue about the internals of the module. I was hoping to keep it alive by fixing/patching bugs as and when time permits. During my peak, I had the opportunity to look into some of the core functions of the module. At that time I noticed the use of weaken for the first time. After little research, I found out more about it.

So now, I had two very important informations: circular reference can create memory leak and weaken can fix the leak.

Even, then I am back to my original question, how can I easily demonstrate the behaviour?

So, I decided to ask for help on my Twitter handle.

To my request, I got this post by Gabor Szabo. I tried to replicate the behaviour on my box but without any luck.

So what next?

I didn’t give up this time, I wanted to get to the bottom of it, no matter what.

After couple of evenings doing research, I found this very handy CPAN module Devel::Leak::Object by Karen Etheridge, famously known as ETHER.

At this point, I felt little better.

So time to demonstrate the memory leak?

Just for fun, I am using Perl v5.36.

 1 #!/usr/bin/perl
 2
 3 package L {
 4     use Devel::Leak::Object qw{ GLOBAL_bless };
 5     $Devel::Leak::Object::TRACKSOURCELINES = 1;
 6     sub new { bless { self_ref => undef }, $_[0] }
 7 }
 8
 9 package main;
10
11 use v5.36;
12
13 my $l = L->new;
14
15 # This would leak memory.
16 $l->{self_ref} = $l;

Time to see the memory leak

Thanks to the module Devel::Leak::Object, I can clearly see the source of leak i.e. line 13.

Well, it is good to know that we have memory leak.

How to fix the memory leak?

Before Perl v5.36, you would take the help of CPAN module Scalar::Util which gives us the handy function weaken. Since I am playing with the latest toy, I would take the luxury and use the experimental builtin by the same name.

 1 #!/usr/bin/perl
 2
 3 package L {
 4     use Devel::Leak::Object qw{ GLOBAL_bless };
 5     $Devel::Leak::Object::TRACKSOURCELINES = 1;
 6     sub new { bless { self_ref => undef }, $_[0] }
 7 }
 8
 9 package main;
10
11 use v5.36;
12 use experimental qw(builtin);
13 use builtin qw(weaken);
14
15 my $l = L->new;
16
17 # Fix the memory leak.
18 $l->{self_ref} = weaken $l;

Let us check, if the script still leak memory?

Wow, no more memory leak.

Finally I have script that demonstrates the memory leak and then fix to the leak as well.

I am very happy now.

As you know, most of my CPAN distributions use Moo for OO purpose. Can I use Moo object to demo the memory leak and fix as well?

Why not? I sound more confident now. :-)

 1 #!/usr/bin/perl
 2
 3 package M {
 4     use Moo;
 5     use Devel::Leak::Object qw{ GLOBAL_bless };
 6     $Devel::Leak::Object::TRACKSOURCELINES = 1;
 7     has self_ref => (is => 'rw');
 8 }
 9
10 package main;
11
12 use v5.36;
13
14 my $m = M->new;
15
16 # This would leak memory.
17 $m->self_ref($m);

Lets run the script and see the memory leak.

So we have seen the memory leak, time to fix the leak. To fix the leak, we can use weak_ref attribute supported by Moo.

 1 #!/usr/bin/perl
 2
 3 package M {
 4     use Moo;
 5     use Devel::Leak::Object qw{ GLOBAL_bless };
 6     $Devel::Leak::Object::TRACKSOURCELINES = 1;
 7     has self_ref => (is => 'rw', weak_ref => 1);
 8 }
 9
10 package main;
11
12 use v5.36;
13
14 my $m = M->new;
15
16 # Fix the memory leak.
17 $m->self_ref($m);