Today I Learned: fatpack and PERL5LIB


Today I Learned that fatpack relies on absolute paths in PERL5LIB.

While attempting to fatpack the main program for skfold:

$ local/bin/fatpack pack bin/skf > skf

I got out a file without the embedded Perl modules. How come?!?

Due preliminary note

As you may have noticed, I installed fatpack inside local, along with the supporting modules for skfold. To make sure that the modules there are found, I set this:

$ export PERL5LIB='local/lib/perl5'

The relative path does not really bother me - I’m not planning to call skfold from any other directory than the project’s root while hacking on it.

So let’s troubleshoot this

The command I called was the all-in-one, so I thought it best to break it down into the different components.

It turns out that there are four stages:

  • trace: some magic figures out which modules are needed by our program and produces a list of their associated module files;
  • packlists-for: from a list of module files, a list of the files included in the package each was installed for is produced (these files are installed as packlist, this is why the command is called packlists-for);
  • tree: from a list of .packlist files, produce a sub-directory fatlib with all the associated files (including, as it seems, POD files);
  • file: from a file, a lib and a fatlib, produce the final fat-packed artifact.

Let’s start!


While the other sub-commands seem to work fine with the standard file descriptors (standard input and standard output), the initial tracing command defaults to producing a yadda-yadda-yadda.trace file, that we will eventually need to cat anyway. Luckily, there’s an option to send it to the standard output right away:

$ local/bin/fatpack trace --to=- bin/skf

The list seems fine to me, and the target modules that are not part of CORE are there, so let’s move on.

Packlist files

The next step is producing the list of .packlist files:

$ local/bin/fatpack packlists-for \
    $(local/bin/fatpack trace --to=- bin/skf)
bin/skf syntax OK

Again, it seems encouraging because it detected exactly the right modules that we need to include in our fat-packed version. Let’s move on!


Let’s call the tree sub-command now:

$ local/bin/fatpack tree $(local/bin/fatpack packlists-for \
    $(local/bin/fatpack trace --to=- bin/skf))
bin/skf syntax OK

$ find fatlib

Ouch! Files are not copied to the fatlib sub-directory, here’s why they don’t appear in the overall file.

A quick look at the code is helpful (from here):

 1 sub packlists_to_tree {
 2   my ($self, $where, $packlists) = @_;
 3   rmtree $where;
 4   mkpath $where;
 5   foreach my $pl (@$packlists) {
 6     my ($vol, $dirs, $file) = splitpath $pl;
 7     my @dir_parts = splitdir $dirs;
 8     my $pack_base;
 9     PART: foreach my $p (0 .. $#dir_parts) {
10       if ($dir_parts[$p] eq 'auto') {
11         # $p-2 normally since it's <wanted path>/$Config{archname}/auto but
12         # if the last bit is a number it's $Config{archname}/$version/auto
13         # so use $p-3 in that case
14         my $version_lib = 0+!!($dir_parts[$p-1] =~ /^[0-9.]+$/);
15         $pack_base = catpath $vol, catdir @dir_parts[0..$p-(2+$version_lib)];
16         last PART;
17       }
18     }
19     die "Couldn't figure out base path of packlist ${pl}" unless $pack_base;
20     foreach my $source (lines_of $pl) {
21       # there is presumably a better way to do "is this under this base?"
22       # but if so, it's not obvious to me in File::Spec
23       next unless substr($source,0,length $pack_base) eq $pack_base;
24       my $target = rel2abs( abs2rel($source, $pack_base), $where );
25       my $target_dir = catpath((splitpath $target)[0,1]);
26       mkpath $target_dir;
27       copy $source => $target;
28     }
29   }
30 }

The culprit is in line 23, excused by the comment in lines 21 and 22.

Setting PERL5LIB to a relative path makes variable $pack_base (declared in line 8, populated in line 15) hold a relative path too, like (in our case) local/lib/perl5.

On the other hand, paths inside the .packlist files are listed with absolute paths:

$ cat local/lib/perl5/x86_64-linux-gnu-thread-multi/auto/Path/Tiny/.packlist 

For this reason, the comparison in line 23 is going to put an absolute portion of a path on the left hand side (i.e. the output of the substr call) and a relative path on the right hand side.

Setting PERL5LIB to an absolute value solves the problem:

$ export PERL5LIB="$PWD/local/lib/perl5"
$ local/bin/fatpack tree $(local/bin/fatpack packlists-for \
    $(local/bin/fatpack trace --to=- bin/skf))
bin/skf syntax OK

$ find fatlib/


Needless to say… I’ll put items in PERL5LIB as absolute paths from now on!

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