[Bio-packaging] Ruby gems/bundler support

[Pjotr Prins]

We are starting on supporting Ruby gems in GNU Guix. So far I have written up
(a recent copy at
https://github.com/pjotrp/guix-notes/blob/master/RUBYGEMS-Nokogiri.org).

David, currently in guix/build/ruby-build-system.scm you create a gem from the
tgz file and then run gem install. Would it be logical to download the gem file
directly?

Below you can see that I propose to give the Ruby gem command a full build
environment so it can do native builds. We may even hack gem itself (a little)
to provide the correct PATHs by default.


* Introduction

Ruby developers have multiple needs when running a system when it
comes to Ruby versions and modules (called gems). GNU Guix has great
support for versioning and control of the dependency graph. Here we
discuss gem support in GNU Guix (this is a work in progress).

Basically we are aware of the following use cases after installing
a version of Ruby (say 2.1.6):

1. A user/sysadmin wants to install a gem in the GNU Guix store 
2. A user wants to install a gem using rubygems outside GNU Guix and
   local to $HOME
3. A developer wants to use bundler which installs gems in the source
   tree for development and testing

In all cases, when Ruby code needs to include a file, resolving the
location of the gem should start in 3, next 2, then 1. As an example
of supporting a complex gem we use the example of Nokogiri which has
over 1,000 installation related questions on Stack Overflow.

* Current state

** GNU Guix installed gem

GNU Guix now build gems from a tar-ball using a Ruby build
environment. Installing ruby-hoe

: guix package -i ruby-hoe

creates a symlink to the executable sow in

: /home/wrk/.guix-profile/bin/sow -> /gnu/store/ccx2ri5l8y6qbrzkw4p0gq8l647kxn0g-ruby-hoe-3.13.1/bin/sow

and the libraries are available through the symlink in

: /home/wrk/.guix-profile/lib/ruby/gems/2.2.0/gems/hoe-3.13.1/ -> /gnu/store/ccx2ri5l8y6qbrzkw4p0gq8l647kxn0g-ruby-hoe-3.13.1/lib/ruby/gems/2.2.0/gems/hoe-3.13.1

Running sow means you'll need to add the GNU Guix gem locations to the
GEM_PATH, e.g.,

: env GEM_PATH=~/.guix-profile/lib/ruby/gems/2.2.0 sow

*** TODO Make the GNU Guix gem path automatically available

At this point the GEM_PATH is not set properly.

*** TODO Provide a build system that uses simple gem descriptions

We could extract Gem information from Rubygems - something Nix was
capable of doing - and create expressions from those (even on the
fly).

** Gem in $HOME

The gem tool comes with the GNU Guix Ruby installation. This means you can run
gem after tweaking the local PATHs. An example can be found [[https://github.com/pjotrp/guix-notes/blob/master/scripts/ruby-guix-env][here]]. Run it as

: . ruby-guix-env

Note: it also sets the path for GNU Guix global gems.

*** TODO Supporting native gems

Native gems include some C or C++ code.

Currently the GNU Guix gem tool does not build native gems. The reason
is that the build environment is not complete for the gem tool that
comes with the installed Ruby. It is possible to force native builds
by tweaking C_INCLUDE_PATH, LD_LIBRARY_PATH and friends, but that is
not encouraged.

To move forward I think we should create a 'rubygems' package that
includes the most important build tools and a 'rubygems-extra' for
some of the rarer tools.

** Bundler

Bundler works in GNU Guix, both installed though Guix and through
gems, but at this point there are no native builds for the same reason.

*** TODO Support native gems for bundler

* Nokogiri