Writing plugins

Plugins use Alien::Build::Plugin, which sets the appropriate base class, and provides you with the "has" property builder. "has" takes two arguments, the name of the property and the default value. (As with Moose and Moo, you should use a code reference to specify default values for non-string defaults). No not set this as your plugin's base class directly:

use parent qw( Alien::Build::Plugin );  # wrong
use Alien::Build::Plugin;               # right

The only method that you need to implement is "init". From this method you can add hooks to change the behavior of the alienfile recipe. This is a very simple example of a probe hook, with the actual probe logic removed:

sub init
{
  my($self, $meta) = @_;
  $meta->register_hook(
    probe => sub {
      my($build) = @_;
      if( ... )
      {
        return 'system';
      }
      else
      {
        return 'share';
      }
    },
  );
}

Hooks get the Alien::Build instance as their first argument, and depending on the hook may get additional arguments.

Modifying hooks

You can also modify hooks using "before_hook", "around_hook" and "after_hook", similar to Moose modifiers:

sub init
{
  my($self, $meta) = @_;

  $meta->before_hook(
    build => sub {
      my($build) = @_;
      $build->log('this runs before the build');
    },
  );

  $meta->after_hook(
    build => sub {
      my($build) = @_;
      $build->log('this runs after the build');
    },
  );

  $meta->around_hook(
    build => sub {
      my $orig = shift;

      # around hooks are useful for setting environment variables
      local $ENV{CPPFLAGS} = '-I/foo/include';

      $orig->(@_);
    },
  );
}

Testing plugins

You can and should write tests for your plugin. The best way to do this is using Test::Alien::Build, which allows you to write an inline alienfile in your test. Here is an example:

use Test::V0;
use Test::Alien::Build;

my $build = alienfile_ok q{
  use alienfile;
  plugin 'Build::MyPlugin' => (
    arg1 => 'override for arg1',
    arg2 => [ 'something', 'else' ],
  );
  ...
};

# you can interrogate $build, it is an instance of L<Alien::Build>.

my $alien = alien_build_ok;

# you can interrogate $alien, it is an instance of L<Alien::Base>.

Negotiator plugins

A Negotiator plugin doesn't itself typically implement anything on its own, but picks the best plugin to achieve a particular goal.

The "best" plugin can in some cases vary depending on the platform or tools that are available. For example The download negotiator might choose to use the fetch plugin that relies on the command line "curl", or it might choose the fetch plugin that relies on the Perl module HTTP::Tiny depending on the platform and what is already installed. (For either to be useful they have to support SSL).

The Negotiator plugin is by convention named something like "Alien::Build::Plugin::*::Negotiate", but is typically invoked without the "::Negotiate" suffix. For example:

plugin 'Download'; # is short for Alien::Build::Plugin::Download::Negotiator

Here is a simple example of a negotiator which picks "curl" if already installed and HTTP::Tiny otherwise. (The actual download plugin is a lot smarter and complicated than this, but this is a good simplified example).

package Alien::Build::Plugin::Download::Negotiate;

use strict;
use warnings;
use Alien::Build::Plugin;
use File::Which qw( which );

sub init
{
  my($self, $meta) = @_;

  if(which('curl')) {
    $meta->apply_plugin('Fetch::Curl');
  } else {
    $meta->apply_plugin('Fetch::HTTPTiny');
  }
}

Hooks

The remainder of this document is a reference for the hooks that you can register. Generally speaking you can register any hook that you like, but some care must be taken as some hooks have default behavior that will be overridden when you register a hook. The hooks are presented in alphabetical order. The execution order is shown in the flowchart above (if you are browsing the HTML version of this document), or the Perlish pseudo code in the synopsis section.

build hook

$meta->register_hook( build => sub {
  my($build) = @_;
  ...
});

This does the main build of the alienized project and installs it into the staging area. The current directory is the build root. You need to run whatever tools are necessary for the project, and install them into "$build-"install_prop->{prefix}> ("%{.install.prefix}").

build_ffi hook

$meta->register_hook( build_ffi => sub {
  my($build) = @_;
  ...
});

This is the same as build, except it fires only on a FFI build.

decode hook

$meta->register_hook( decode => sub {
  my($build, $res) = @_;
  ...
}

This hook takes a response hash reference from the "fetch" hook above with a type of "html" or "dir_listing" and converts it into a response hash reference of type "list". In short it takes an HTML or FTP file listing response from a fetch hook and converts it into a list of filenames and links that can be used by the prefer hook to choose the correct file to download. See the fetch hook for the specification of the input and response hash references.

check_digest hook

# implement the well known FOO-92 digest
$meta->register_hook( check_digest => sub {
  my($build, $file, $algorithm, $digest) = @_;
  if($algorithm ne 'FOO92') {
    return 0;
  }
  my $actual = foo92_hex_digest($file);
  if($actual eq $digest) {
    return 1;
  } else {
    die "Digest FOO92 does not match: got $actual, expected $digest";
  }
});

This hook should check the given $file (the format is the same as used by the fetch hook) matches the given $digest using the given $algorithm. If the plugin does not support the given algorithm, then it should return a false value. If the digest does not match, it should throw an exception. If the digest matches, it should return a true value.

clean_install

$meta->register_hook( clean_install => sub {
  my($build) = @_;
});

This hook allows you to remove files from the final install location before the files are installed by the installer layer (examples: Alien::Build::MM, Alien::Build::MB or App::af). This hook is not called by default, and must be enabled via the interface to the installer layer (example: "clean_install" in Alien::Build::MM).

This hook SHOULD NOT remove the "_alien" directory or its content from the install location.

The default implementation removes all the files EXCEPT the "_alien" directory and its content.

download hook

$meta->register_hook( download => sub {
  my($build) = @_;
  ...
});

This hook is used to download from the internet the source. Either as an archive (like tar, zip, etc), or as a directory of files ("git clone", etc). When the hook is called, the current working directory will be a new empty directory, so you can save the download to the current directory. If you store a single file in the directory, Alien::Build will assume that it is an archive, which will be processed by the extract hook. If you store multiple files, Alien::Build will assume the current directory is the source root. If no files are stored at all, an exception with an appropriate diagnostic will be thrown.

Note: If you register this hook, then the fetch, decode and prefer hooks will NOT be called, unless you call them yourself from this hook.

extract hook

$meta->register_hook( extract => sub {
  my($build, $archive) = @_;
  ...
});

This hook is used to extract an archive that has already been downloaded. Alien::Build already has plugins for the most common archive formats, so you will likely only need this to add support for new or novel archive formats. When this hook is called, the current working directory will be a new empty directory, so you can save the content of the archive to the current directory. If a single directory is written to the current directory, Alien::Build will assume that is the root directory of the package. If multiple files and/or directories are present, that will indicate that the current working directory is the root of the package. The logic typically handles correctly the default behavior for tar (where packages are typically extracted to a subdirectory) and for zip (where packages are typically extracted to the current directory).

fetch hook

package Alien::Build::Plugin::MyPlugin;

use strict;
use warnings;
use Alien::Build::Plugin;
use Carp ();

has '+url' => sub { Carp::croak "url is required property" };

sub init
{
  my($self, $meta) = @_;

  $meta->register_hook( fetch => sub {
    my($build, $url, %options) = @_;
    ...
  }
}

1;

Used to fetch a resource. The first time it will be called without an argument (or with $url set to "undef", so the configuration used to find the resource should be specified by the plugin's properties. On subsequent calls the first argument will be a URL.

The %options hash may contain these options:

http_headers

HTTP request headers, if an appropriate protocol is being used. The headers are provided as an array reference of key/value pairs, which allows for duplicate header keys with multiple values.

If a non-HTTP protocol is used, or if the plugin cannot otherwise send HTTP request headers, the plugin SHOULD issue a warning using the "$build->log" method, but because this option wasn't part of the original spec, the plugin MAY no issue that warning while ignoring it.

Note that versions of Alien::Build prior to 2.39 did not pass the options hash into the fetch plugin.

Normally the first fetch will be to either a file or a directory listing. If it is a file then the content should be returned as a hash reference with the following keys:

# content of file stored in Perl
return {
  type     => 'file',
  filename => $filename,
  content  => $content,
  version  => $version,  # optional, if known
  protocol => $protocol, # AB 2.60 optional, but recommended
};

# content of file stored in the filesystem
return {
  type     => 'file',
  filename => $filename,
  path     => $path,     # full file system path to file
  version  => $version,  # optional, if known
  tmp      => $tmp,      # optional
  protocol => $protocol, # AB 2.60 optional, but recommended
};

$tmp if set will indicate if the file is temporary or not, and can be used by Alien::Build to save a copy in some cases. The default is true, so Alien::Build assumes the file or directory is temporary if you don't tell it otherwise. Probably the most common situation when you would set "tmp" to false, is when the file is bundled inside the Alien distribution. See Alien::Build::Plugin::Fetch::Local for example.

If the URL points to a directory listing you should return it as either a hash reference containing a list of files:

return {
  type => 'list',
  list => [
    # filename: each filename should be just the
    #   filename portion, no path or url.
    # url: each url should be the complete url
    #   needed to fetch the file.
    # version: OPTIONAL, may be provided by some fetch or prefer
    { filename => $filename1, url => $url1, version => $version1 },
    { filename => $filename2, url => $url2, version => $version2 },
  ],
  protocol => $protocol, # AB 2.60 optional, but recommended
};

or if the listing is in HTML format as a hash reference containing the HTML information:

return {
  type => 'html',
  charset  => $charset, # optional
  base     => $base,    # the base URL: used for computing relative URLs
  content  => $content, # the HTML content
  protocol => $protocol, # optional, but recommended
};

or a directory listing (usually produced by an FTP servers) as a hash reference:

return {
  type     => 'dir_listing',
  base     => $base,
  content  => $content,
  protocol => $protocol, # AB 2.60 optional, but recommended
};

[version 2.60]

For all of these responses $protocol is optional, since it was not part of the original spec, however it is strongly recommended that you include this field, because future versions of Alien::Build will use this to determine if a file was downloaded securely (that is via a secure protocol such as SSL).

Some plugins (like decode plugins ) trans late a file hash from one type to another, they should maintain the $protocol from the old to the new representation of the file.

gather_ffi hook

$meta->register_hook( gather_ffi => sub {
  my($build) = @_;
  $build->runtime_prop->{cflags}  = ...;
  $build->runtime_prop->{libs}    = ...;
  $build->runtime_prop->{version} = ...;
});

This hook is called for a FFI build to determine the properties necessary for using the library or tool. These properties should be stored in the runtime_prop hash as shown above. Typical properties that are needed for libraries are cflags and libs. If at all possible you should also try to determine the version of the library or tool.

gather_share hook

$meta->register_hook( gather_share => sub {
  my($build) = @_;
  $build->runtime_prop->{cflags}  = ...;
  $build->runtime_prop->{libs}    = ...;
  $build->runtime_prop->{version} = ...;
});

This hook is called for a share install to determine the properties necessary for using the library or tool. These properties should be stored in the runtime_prop hash as shown above. Typical properties that are needed for libraries are cflags and libs. If at all possible you should also try to determine the version of the library or tool.

gather_system hook

$meta->register_hook( gather_system => sub {
  my($build) = @_;
  $build->runtime_prop->{cflags}  = ...;
  $build->runtime_prop->{libs}    = ...;
  $build->runtime_prop->{version} = ...;
});

This hook is called for a system install to determine the properties necessary for using the library or tool. These properties should be stored in the runtime_prop hash as shown above. Typical properties that are needed for libraries are cflags and libs. If at all possible you should also try to determine the version of the library or tool.

override hook

$meta->register_hook( override => sub {
  my($build) = @_;
  return $ENV{ALIEN_INSTALL_TYPE} || '';
});

This allows you to alter the override logic. It should return one of "share", "system", "default" or ''. The default implementation is shown above. Alien::Build::Plugin::Probe::Override and Alien::Build::Plugin::Probe::OverrideCI are examples of how you can use this hook.

patch hook

$meta->register_hook( patch => sub {
  my($build) = @_;
  ...
});

This hook is completely optional. If registered, it will be triggered after extraction and before build. It allows you to apply any patches or make any modifications to the source if they are necessary.

patch_ffi hook

$meta->register_hook( patch_ffi => sub {
  my($build) = @_;
  ...
});

This hook is exactly like the patch hook, except it fires only on an FFI build.

prefer hook

$meta->register_hook( prefer => sub {
  my($build, $res) = @_;
  return {
    type => 'list',
    list => [sort @{ $res->{list} }],
  };
}

This hook sorts candidates from a listing generated from either the "fetch" or "decode" hooks. It should return a new list hash reference with the candidates sorted from best to worst. It may also remove candidates that are totally unacceptable.

probe hook

$meta->register_hook( probe => sub {
  my($build) = @_;
  return 'system' if ...; # system install
  return 'share';         # otherwise
});

$meta->register_hook( probe => [ $command ] );

This hook should return the string "system" if the operating system provides the library or tool. It should return "share" otherwise.

You can also use a command that returns true when the tool or library is available. For example for use with "pkg-config":

$meta->register_hook( probe =>
  [ '%{pkgconf} --exists libfoo' ] );

Or if you needed a minimum version:

$meta->register_hook( probe =>
  [ '%{pkgconf} --atleast-version=1.00 libfoo' ] );

Note that this hook SHOULD NOT gather system properties, such as cflags, libs, versions, etc, because the probe hook will be skipped in the event the environment variable "ALIEN_INSTALL_TYPE" is set. The detection of these properties should instead be done by the gather_system hook.

Multiple probe hooks can be given. These will be used in sequence, stopping at the first that detects a system installation.