bootboot-lite: dual boot without bootboot!

bootboot-lite.rb

# Require this from your Gemfile
#   - Adds support for programmatically setting the cache path

Bundler::Settings.class_eval do
  attr_writer :app_cache_path
end

Gemfile

source "https://rubygems.org"

ruby "~> " + File.read("#{Bundler.root}/.ruby-version").strip

require_relative 'dual_boot/bootboot-lite'

if ENV['DEPENDENCIES_NEXT']
  Bundler.settings.app_cache_path =  "vendor/cache-rails_5.0"

  # Add any gem you want here, they will be loaded only when running
  # bundler command prefixed with `DEPENDENCIES_NEXT=1`.
  eval_gemfile File.dirname(__FILE__) + "/gemfiles/rails5.0.gemfile"
else
  eval_gemfile File.dirname(__FILE__) + "/gemfiles/rails4.2.gemfile"
end

The earlier revisions of this did not work consistently with bundler 1.17.x. In some cases the bundler would take a code path that let the patches take affect before it tried to read the lock file, e.g. DEPENDENCIES_NEXT=1 bundle install. However, it failed when running commands through bundle exec ... because it tried to read the lockfile before processing the Gemfile or any extensions loaded in it.

The latest revision (revision 4 at the time of this writing) essentially removes all of the bootboot patches and only leaves in the attr_writer for app_cache_path.

If you symlink Gemfile_next to Gemfile and use BUNDLE_GEMFILE=Gemfile_next then everything will work across bundler invocations.

It's possible to remove the app_cache_path writer and just rely on BUNDLE_CACHE_PATH but it's left in place as a way to not require that env var to be set and to rely on DEPENDENCIES_NEXT to always set the cache path appropriately.