Alien::Base past, present and future (upcoming change in behavior)

Alien::Base has made a great deal of progress since last September when Joel Berger turned over day to day development for the project to the then newly created Alien::Base team. We’ve closed most of the major issues and pull requests. One remaining important issue will hopefully be solved soon (I will get to that later; if you are an Alien::Base user you may want to skip to The Future below). Stability and reliability has improved to the point where it is good enough to be used by real projects. My own FFI::Platypus depends on Alien::Base tech to provide libffi, as an example. I’d like to see some other projects take advantage of Alien::Base as well.

What have we fixed? A number of things:

  • Builds on Microsoft Windows are now doable, thanks to integration with Alien::MSYS that provides the necessary tools to build autotools based packages
  • Relocation of library directories is now supported, including support for DESTDIR and AFS
  • Support for 64 bit Perls in hybrid 32/64 bit environments like Solaris
  • Integration with FFI and Inline based distributions

Why? A lot of XS developers when they use an external non-CPAN dependency will take one of two approaches:

  1. Bundle the dependency with the module
  2. Use Devel::CheckLib to find the library as provided by the operating system

These approaches are somewhat inflexible. If you bundle the dependency then you are increasing the bloat of your distribution. The bundled version may become out of date, allowing bugs and security issues that have been fixed upstream to persist in your version. It also may be problematic for integrators. One distribution developer confided in me that his module was unlikely to be integrated into his distribution because the bundled C code needed to be reviewed and documented for legal issues that would not be a problem if the code wasn’t bundled. The distribution already provided the library as a separate package.

If you depend on the operating system to provide the library then you have a moral obligation to instruct your user as to how to install that dependency, which may be different on different platforms. If the dependency isn’t a common one then you will likely not get any useful feedback from cpan testers. Downstream developers may be reluctant to use your module as a dependency since it may not install reliably.

Alien defines a non-Perl or non-CPAN dependency so that it can be used by your CPAN distribution. Alien::Base provides a base class that you can use to easily construct these dependencies. When properly configured, an Alien::Base based distribution will use the system provided library, and if it can’t be found, attempt to download from the Internet and install it into a share directory so that it will not interfere with any system packages.

As an example, FFI::Platypus depends on Alien::FFI which will use the system libffi if it is usually available, and if not it downloads and installs it for you.

The Future:

The one remaining important issue that I mentioned above has to do with the way packages are installed into the share directory. Historically, Alien::Base has installed packages into their final destination when the cpan client (or user) runs the ./Build install command. The problem with this is that many cpan testers do not necessarily install the modules, preferring instead to add the blib paths to the PERL5LIB. The work around for this was for Alien::Base to attempt to detect such an environment and install the package into the blib in just that circumstances. This has improved the reliability of cpan testers, but it does mean that cpan testers installs for Alien::Base based distributions work in differently in a subtle way.

With recent advances in relocation of packages included in Alien::Base we can now install into the blib and allow the installer to install these files into the final location, just like normal Perl distributions. The current version just released today has this capability, but is off by default to maintain backward compatibility. In the next version 0.017 we plan to make this the default. So if you are the owner of an Alien::Base based distribution, now would be a great time to test this change in behavior. You can do this by installing the development version of Alien::Base 0.016_01, or by explicitly setting alien_stage_install to 1 in your Build.PL (or dist.ini the latest version of the Alien Dist::Zilla plugin also supports this option). Here is an example of how Alien::LibYAML was modified to explicitly set this value:

You should also make Alien::Base 0.016 a prerequisite for your distribution.

If this new behavior is going to break your distribution in a way that is not otherwise fixable, you can explicitly set alien_stage_install to 0, and the old behavior will be maintained, even when Alien::Base 0.017 is released. If you decide to do this, please contact the Alien::Base team, as at some point we would like to deprecate and perhaps even remove the old behavior. Contact us on either GitHub:

or the Alien::Base mailing list:!forum/perl5-alien


Two quick clarifications, its "Berger" and I'm still involved, though Graham certainly is doing the lion's share of the work.

Leave a comment

About Graham Ollis

user-pic My interests in Perl lie in Alien (Alien::Base etc) and FFI (FFI::Platypus).