By Graham Ollis on 4 April 2017

Last week I introduced the alienfile recipe system and we wrote a simple alienfile that provides the tool xz and the library liblzma. I also showed how to test it using App::af. Today we are going to take that alienfile and integrate it into a fully functional Alien distribution.

The main motiviation for alienfile + Alien::Build was to separate the alien detection and installer code from the perl installer code. (In fact your alienfile is fully usable without any Perl installer at all; you can use your alienfile from a Perl script or Perl module using Alien::Build directly).

For our Alien, we will be creating Alien-xz, and we will use Alien::Build::MM to provide the thin layer of functionality needed between ExtUtils::MakeMaker (EUMM) and Alien::Build. This is what our Makefile.PL should look like:

use strict;
use warnings;
use ExtUtils::MakeMaker;
use Alien::Build::MM;

my %WriteMakefileArgs = (
  "ABSTRACT" => "Find or build xz",
  "AUTHOR" => "Graham Ollis <plicease\@cpan.org>",
  "VERSION_FROM" => "lib/Alien/xz.pm",
  "CONFIGURE_REQUIRES" => {
    "ExtUtils::MakeMaker" => "6.52",
  },
  "PREREQ_PM" => {
    "Alien::Base" => "0.038",
  },
  "DISTNAME" => "Alien-xz",
  "LICENSE" => "perl",
  "NAME" => "Alien::xz",
  "PREREQ_PM" => {
    "Alien::Base" => "0.038"
  },
);

my $abmm = Alien::Build::MM->new;
%WriteMakefileArgs = $abmm->mm_args(%WriteMakefileArgs);

WriteMakefile(%WriteMakefileArgs);

sub MY::postamble {
  $abmm->mm_postamble;
}

Most of this will be pretty recognizable to anyone who has hand crafted a Makefile.PL before. After constructing the basic arguments that will be passed to WriteMakefile, we create an instance of Alien::Build::MM and pass the arguments into the mm_args method, which returns a modified version of those arguments. This method will decide if it can find xz and liblzma from what is provided by the operating system, or if it should build the tool and library from source. To make this decision it consults the alienfile. There are dynamic prerequisite that depend on the outcome of this decision. Typcially a build from source may pull in additional prerequisite to download or unpack an archive in an unusual format, or perhaps the build system requires an extra module or two. A system install may also require additional modules, although that is less common. At the bottom of the Makefile.PL we add a postamble to the Makefile. This will add the make targets needed to build xz.

Note that we declare Alien::Base as a prerequisite because our Alien class will be a subclass of that. We also declare EUMM as a configure time prerequisite which is a good practice for any EUMM dist. We do NOT need to declare Alien::Build::MM as a configure time prerequisite even though it is used in the Makefile.PL because that is one of the prerequisite that are addedin by mm_arg.

As mentioned earlier, alienfile is not tied to any one Perl installer, so if you prefer you can use Module::Build via Alien::Build::MB. I personally do not recommend this. It just adds extra prerequisite to your Alien. The sole reason I wrote Alien::Build::MB was to act as a proof of concept that: yes alienfile + Alien::Build can be used with any installer that meets its requirements. Including hopefully future installers that are more capable than EUMM. (Module::Build::Tiny will likely never be supported, as it is useful for a different subset of things).

If (like me) you do not like mucking about in Makefile.PL or Build.PL files you can instead use the Dist::Zilla plugin Dist::Zilla::Plugin::AlienBuild.

[MakeMaker]
[AlienBuild]
; a little shorter than the Makefile.PL above eh?

The next thing that we need is the actual Perl module! We will call that lib/Alien/xz.pm, and will look like this:

package Alien::xz;

use strict;
use warnings;
use base qw( Alien::Base );

our $VERSION = '1.00';

1;

Not a lot is there? For most Aliens you will find that the base class does everything that you need. The only thing missing here really, (and I do not reproduce it here for the sake of brevity) is documentation. You should provide your users with enough information in the form of POD to be able to use this module! (See Alien::Build::Manual::AlienUser for clues as to what should be included). If you are lazy like me, you will want to use the Dist::Zilla plugin Dist::Zilla::Plugin::AlienBase::Doc to generate synopsis and description.

You should of course also provide other common distribution files, such as a MANIFEST and a Changes file, but all of that is beyond the scope of this document. (Always wanted to say that).

Now we can install our Alien like any other distribution. Create the make file:

% perl Makefile.PL

You can download the xz tarball using the alien_download target:

% make alien_download
"/Users/ollisg/opt/perl/5.24.0/bin/perl" -MAlien::Build::MM=cmd -e prefix site /Users/ollisg/opt/perl/5.24.0/my/dev/lib/perl5/darwin-2level /Users/ollisg/opt/perl/5.24.0/my/dev/lib/perl5/darwin-2level /Users/ollisg/opt/perl/5.24.0/my/dev/lib/perl5/darwin-2level
main> prefix /Users/ollisg/opt/perl/5.24.0/my/dev/lib/perl5/darwin-2level/auto/share/dist/Alien-xz
"/Users/ollisg/opt/perl/5.24.0/bin/perl" -MAlien::Build::MM=cmd -e download
Alien::Build::Plugin::Core::Download> decoding html
Alien::Build::Plugin::Core::Download> candidate *http://tukaani.org/xz/xz-5.2.3.tar.gz
Alien::Build::Plugin::Core::Download> setting version based on archive to 5.2.3
Alien::Build::Plugin::Core::Download> downloaded xz-5.2.3.tar.gz

(hint, if you are testing this and the system xz and liblzma are being detected, the download step will be a noisy NOOP. You can set ALIEN_INSTALL_TYPE to share to override this and force a source code build.)

You can then build xz and liblzma using the alien_build target:

% make alien_build
"/Users/ollisg/opt/perl/5.24.0/bin/perl" -MAlien::Build::MM=cmd -e build
Alien::Build::CommandSequence> + ./configure --prefix=/Users/ollisg/opt/perl/5.24.0/my/dev/lib/perl5/darwin-2level/auto/share/dist/Alien-xz --with-pic --disable-shared

XZ Utils 5.2.3

System type:
checking build system type... x86_64-apple-darwin15.6.0
checking host system type... x86_64-apple-darwin15.6.0
...

(copious output not included).

You can also just do a regular make all and it will build the alien_download and alien_build targets, along with the necessary Perl specific targets.

Now we are ready to run the tests:

% make test
PERL_DL_NONLAZY=1 "/Users/ollisg/opt/perl/5.24.0/bin/perl" "-MExtUtils::Command::MM" "-MTest::Harness" "-e" "undef *Test::Harness::Switches; test_harness(0, 'blib/lib', 'blib/arch')" t/*.t
t/run.t ...... ok
t/xs.t ....... ok
All tests successful.
Files=2, Tests=7,  1 wallclock secs ( 0.03 usr  0.01 sys +  0.70 cusr  0.56 csys =  1.30 CPU)
Result: PASS

I waited until now to remind you that you need tests! It is important to know that the Alien will work with your XS module. You don't want to find out it doesn't work when you are installing that. The best way to do this is to use Test::Alien, which tests your Alien with that same tools that your Alien will actually be used with. Here is a very basic Test::Alien test that ensures the alienized liblzma works correctly with XS:

use Test2::Bundle::Extended;
use Test::Alien;
use Alien::xz;

alien_ok 'Alien::xz';

xs_ok do { local $/; <DATA> }, with_subtest {
  my $version = lzma::lzma_version_string();
  ok $version;
  note "version = $version";
};

done_testing;

__DATA__

#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
#include <lzma.h>

MODULE = lzma PACKAGE = lzma

const char *
lzma_version_string()

...and here is the tool test that ensures the alienized command line xz works correctly:

use Test2::Bundle::Extended;
use Test::Alien;
use Alien::xz;

alien_ok 'Alien::xz';

run_ok(['xz', '--version'])
  ->success
  ->out_like(qr{XZ Utils})
  ->note;

done_testing;

You are all done writing your Alien. Although it may seem like you went through a lot, this is a lot less work than if you tried to roll your own Alien. Now we can finally install our Alien, and just eyeball test that it works from the command line.

% make install
...
% perl -MAlien::xz -E 'say Alien::xz->version'
5.2.3
% perl -MAlien::xz -E 'say Alien::xz->cflags'
-I/Users/ollisg/opt/perl/5.24.0/my/dev/lib/perl5/darwin-2level/auto/share/dist/Alien-xz/include
% perl -MAlien::xz -E 'say Alien::xz->libs'
-L/Users/ollisg/opt/perl/5.24.0/my/dev/lib/perl5/darwin-2level/auto/share/dist/Alien-xz/lib -llzma

Next time we will use the Alien that we have crafted here from an XS or FFI module, which is ultimately the reason for all of this prep work.

Correction: a previous version of this blogity blog incorrectly referred to Alien::Build::MM as a dynamic prerequisite. It is always added so it is strictly speaking a static prerequisite.

This article was originally posted to blogs.perl.org: here