UPDATE 2015-06-26: Change gem.files and gem.test_files in gemspecs

Currently nmatrix relies on the ATLAS library, which can be a big pain to install. We want people to be able to build and use nmatrix without having to install ATLAS, but at the same time allow those who do have ATLAS to use it as before. This means separating nmatrix into two gems: the core one nmatrix and the plugin nmatrix-atlas. Eventually there may be other nmatrix plugins nmatrix-xxx (see ruby naming conventions). We would like to develop both gems in the same repository so that development of the two gems will stay in sync.

I found a quite useful blog post on the same subject, which is what this post is based on, however the nmatrix case is a bit more complicated since both the core gem and the plugin rely on C code.

The design is maybe not totally finalized yet, I will update if changes happen.

Repository directory structure

ext/nmatrix/          nmatrix C extension
ext/nmatrix_atlas/    nmatrix-atlas C extension
lib/nmatrix.rb        main file for nmatrix
lib/nmatrix/          auxillary ruby files
lib/nmatrix/atlas.rb  main file for nmatrix-atlas, so the extension can be
                        loaded with `require 'nmatrix/atlas'` as required
                        by the naming convention
spec/                 shared tests, tests for nmatrix, auxillary files for tests
spec/plugins/atlas/   tests for nmatrix-atlas

Common header files which are needed by both C extensions are located in ext/nmatrix. How should nmatrix_atlas get access to these common header files? I thought of three ways to do this:

  1. Install the nmatrix gem and use the installed header files to build nmatrix-atlas. This is a bad solution because it makes the build process clunky for developers. You would have to gem install nmatrix before you could even build nmatrix-atlas.

  2. Just add ext/nmatrix to the include path for the nmatrix-atlas build process, and then package all the necessary headers with the nmatrix-atlas gem. This has the downside that you will end up packaging identical header files twice in two (or more) different gems.

  3. Deal with the issue differently depending on whether we are building in a development environment or building during a gem installation. In the first case, use the headers from ext/nmatrix in the tree, in the second case use the headers from the installed nmatrix gem. This would mean that the headers would only need to be packaged in one place, but it would add complexity to the build process.

I chose to go with option 2.

Plugin gemspecs

First we need to make a gemspec for each gem: nmatrix.gemspec, nmatrix-atlas.gemspec. What to put in these gemspecs?

For nmatrix-atlas.gemspec, it looks like a normal gemspec except for a few interesting things:

lib = File.expand_path('../lib/', __FILE__)
$:.unshift lib unless $:.include?(lib)

require 'nmatrix/version'

Gem::Specification.new do |gem|
  gem.name = "nmatrix-atlas"
  gem.version = NMatrix::VERSION::STRING #use the same version as the main gem

  # [...] some boring stuff goes here

  gem.files         = ["lib/nmatrix/atlas.rb"]
  gem.files         += `git ls-files -- ext/nmatrix_atlas`.split("\n")
  gem.files         += `git ls-files -- ext/nmatrix | grep ".h$"`.split("\n") #need nmatrix header files to compile
  gem.test_files    = `git ls-files -- spec`.split("\n")
  gem.test_files    -= `git ls-files -- spec/plugins`.split("\n")
  gem.test_files    += `git ls-files -- spec/plugins/atlas`.split("\n")
  gem.extensions = ['ext/nmatrix_atlas/extconf.rb']
  gem.require_paths = ["lib"]

  gem.required_ruby_version = '>= 1.9'

  gem.add_dependency 'nmatrix', NMatrix::VERSION::STRING

The important thing is making sure we add all the needed files to gem.files: all the needed ruby files (here we have only one), all of the files from ext/nmatrix_atlas, and the common header files that we will need to build the extension when the user runs gem install. Setting gem.test_files doesn't actually do anything from the perspective of RubyGems, but we will make use of it when setting up the spec task in our Rakefile.

Then of course we need to make our plugin gem dependent on the core nmatrix gem.

Core gemspec

The original post had a neat trick for adding all files to the main gem, except for those that were added to plugin gems, but it's a little more complicated for us since we have shared header files that we want to be installed with all gems:

lib = File.expand_path('../lib/', __FILE__)
$:.unshift lib unless $:.include?(lib)

require 'nmatrix/version'

#get files that are used by plugins rather than the main nmatrix gem
plugin_files = []
Dir["nmatrix-*.gemspec"].each do |gemspec_file|
  gemspec = eval(File.read(gemspec_file))
  plugin_files += gemspec.files
plugin_lib_files = plugin_files.select { |file| file.match(/^lib\//) }

Gem::Specification.new do |gem|
  gem.name = "nmatrix"
  gem.version = NMatrix::VERSION::STRING

  # [...] boring stuff goes here

  gem.files         = `git ls-files -- ext/nmatrix`.split("\n")
  gem.files         += `git ls-files -- lib`.split("\n")
  gem.files         -= plugin_lib_files
  gem.test_files    = `git ls-files -- spec`.split("\n")
  gem.test_files    -= `git ls-files -- spec/plugins`.split("\n")
  gem.extensions = ['ext/nmatrix/extconf.rb']
  gem.require_paths = ["lib"]

  gem.required_ruby_version = '>= 1.9'

  # add ordinary dependencies here


This is pretty much the same as in the original post:

source 'https://rubygems.org'

#main gemspec
gemspec :name => 'nmatrix'

#plugin gemspecs
Dir['nmatrix-*.gemspec'].each do |gemspec_file|
  plugin_name = gemspec_file.match(/(nmatrix-.*)\.gemspec/)[1]
  gemspec(:name => plugin_name, :development_group => :plugin)


The Rakefile is where most of the magic goes down.

I think in the original post the author was assuming that a developer would always want to build all the plugins. But for us since nmatrix-atlas has a nasty dependency, we don't want to force everyone to build it. So we pass an argument to rake to tell it what plugins we want. You can pass arguments to rake by calling the command rake task arg1=val1. This sets an environment variable arg1 that you can access from within rake. I use the nmatrix_plugins env variable to specify which plugins rake should build/package/test/whatever:

#Specify plugins to build on the command line like:
#rake whatever nmatrix_plugins=atlas,lapack
#rake whatever nmatrix_plugins=all
#If you want to build *only* plugins and not the core nmatrix gem:
#rake whatever nmatrix_plugins=all nmatrix_core=false
if ENV["nmatrix_plugins"] == "all"
  gemspecs = Dir["*.gemspec"]
  plugins = []
  plugins = ENV["nmatrix_plugins"].split(",") if ENV["nmatrix_plugins"]
  gemspecs = ["nmatrix.gemspec"] #always include the main nmatrix gem
  plugins.each do |plugin|
    gemspecs << "nmatrix-#{plugin}.gemspec"
if ENV["nmatrix_core"] == "false"
  gemspecs -= ["nmatrix.gemspec"]
gemspecs.map! { |gemspec| eval(IO.read(gemspec)) }

So now gemspecs is an array of all the gemspecs that we want rake to deal with.

So we create a Rake::ExtensionTask for each C extension that we have, which makes a single compile task that compiles all of the extensions:

require 'rake'
require "rake/extensiontask"

gemspecs.each do |gemspec|
  next unless gemspec.extensions
  gemspec.extensions.each do |extconf|
    ext_name = extconf.match(/ext\/(.*)\/extconf\.rb/)[1]
    Rake::ExtensionTask.new do |ext|
      ext.name = ext_name
      ext.ext_dir = "ext/#{ext_name}"
      ext.lib_dir = 'lib/'
      ext.source_pattern = "**/*.{c,cpp,h}"

and a Gem::PackageTask for each gemspec, which gives us package and repackage tasks:

gemspecs.each do |gemspec|

and a custom install task:

desc "Build and install into system gems."
task :install => :package do
  gemspecs.each do |gemspec|
    gem_file = "pkg/#{gemspec.name}-#{gemspec.version}.gem"
    system "gem install '#{gem_file}'"

Note that Bundler::GemHelper.install_tasks does not work with multiple gems in the same directory.

The spec task for using RSpec was a bit more complicated so I split that off into a different blog post.

I still haven't tried this with travis-cl, maybe that will require a little bit more work.