Tender Lovemaking

Right now people who publish native gems targeting the windows platform have a problem. Our problem is supporting ruby 1.8 and 1.9 at the same time. Right now, we can’t build one gem targeting 1.8 and one gem targeting 1.9, and have rubygems differentiate the two. I have a solution: fat binary gems. We can build a gem that contains dynamic libraries that target ruby 1.8 and ruby 1.9 on windows, with no changes to rubygems whatsoever. I’ve put together a proof of concept that I want to share. I will walk through the steps for building a fat binary gem with the tools we have today. The steps I am going to present are not necessarily the best steps, they are just the steps I took to get this idea working.

The tools I will use are MinGW for cross compiling, hoe and rake-compiler for their packaging and compiling tasks, multiruby for cross compiling 1.8 and 1.9, and use nokogiri as the target gem to be built.

Here is the basic strategy for making dreams happen:

1. Gem entry point must be written in Ruby

When someone does “require ‘whatever’” on your library, that ‘whatever.rb’ file must be written in ruby and work with both 1.8 and 1.9. The reason is because we will:

2. Dynamically determine the correct SO file to load

We can determine at runtime the current ruby version, then load the appropriate SO file at runtime.

Let’s get down to business and see it in action.

Getting our hands dirty

The first thing we need to do is make sure that the so file from the ruby 1.8 build and the ruby 1.9 build are in a different place. The way I accomplished this was by customizing my Rake::Extension task (from rake-compiler), and adding a prerequisite to the cross task:

RET = Rake::ExtensionTask.new("nokogiri", HOE.spec) do |ext|
  ext.lib_dir = "ext/nokogiri"
end

task :muck_with_lib_dir do
  RET.lib_dir += "/#{RUBY_VERSION.sub(/\.\d$/, '')}"
  FileUtils.mkdir_p(RET.lib_dir)
end
if Rake::Task.task_defined?(:cross)
  Rake::Task[:cross].prerequisites << "muck_with_lib_dir"
end

This code will make sure the so file goes to “ext/nokogiri/1.8″ when compiling with ruby 1.8, and “ext/nokogiri/1.9″ when compiling with 1.9. Then, all you have to do is compile your extension twice:


$ ~/.multiruby/install/1.8.6-p114/bin/rake cross compile
$ rm -rf tmp
$ $ ~/.multiruby/install/1.9.1-rc2/bin/rake cross compile
[/sourcecode]
WARNING! Watch out for that "rm -rf". That is removing the tmp directory that rake-compiler made. rake-compiler doesn't seem to know that I switched ruby versions. In order to get the two different compilations working, I had to manually remove the already compiled objects.

if RUBY_PLATFORM =~/(mswin|mingw)/i
  # Fat binary gems, you make the Rockin' world go round
  require "nokogiri/#{RUBY_VERSION.sub(/\.\d+$/, '')}/nokogiri"
else
  require 'nokogiri/nokogiri'
end

task :add_dll_to_manifest do
  HOE.spec.files += Dir['ext/nokogiri/**.{dll,so}']
  HOE.spec.files += Dir['ext/nokogiri/{1.8,1.9}/**.{dll,so}']
end

if Rake::Task.task_defined?(:cross)
  Rake::Task[:cross].prerequisites << :add_dll_to_manifest
end