Safer tests by not mutating current process

[schneems]

If you don't know if code will mutate a process information, such as environment variables then run it in a fork to ensure it does not affect the parent process.

Currently when you run any LanguagePack::<Klass>.use? such as LanguagePack::Rails6.use? it will mutate the environment by loading in bundler and modifying the LOAD_PATH.

This happens because the use? method relies on bundler:

  def self.use?
    instrument "rails6.use" do
      rails_version = bundler.gem_version('railties')
      return false unless rails_version
      is_rails = rails_version >= Gem::Version.new('6.x') &&
        rails_version < Gem::Version.new('7.0.0')
      return is_rails
    end
  end

Which loads and initializes this wrapper:

  def self.bundler
    @@bundler ||= LanguagePack::Helpers::BundlerWrapper.new.install
  end

And the wrapper install method mutates globals:

  def install
    ENV['BUNDLE_GEMFILE'] = @gemfile_path.to_s

    fetch_bundler
    $LOAD_PATH << @path
    require "bundler"
    self
  end

This is an issue because other tests such as the rake_runner_spec tests expect a specific version of Rake to be available however if the load path is mutated and bundler is required then only one specific version of rake will be available on the system.

While I believe that this fixes some flappy tests, I'm not totally sure why it only fails sometimes. You would think that it would either always pass or always fail for a given order. This test order is taken from a failing test run:

$ cat << EOL > log/test_order_fails_sometimes.log
./spec/helpers/rake_runner_spec.rb[1:2]
./spec/helpers/shell_spec.rb[1:2:1]
./spec/helpers/jvm_installer_spec.rb[1:4]
./spec/installers/yarn_installer_spec.rb[1:1:1:1]
./spec/installers/heroku_ruby_installer_spec.rb[1:1:1]
./spec/helpers/rake_runner_spec.rb[1:4]
./spec/hatchet/stack_spec.rb[1:1]
./spec/hatchet/rails6_spec.rb[1:1]
./spec/hatchet/jvm_spec.rb[1:1]
./spec/helpers/rake_runner_spec.rb[1:1]
./spec/helpers/jvm_installer_spec.rb[1:6]
./spec/helpers/jvm_installer_spec.rb[1:1]
./spec/hatchet/bugs_spec.rb[1:3:1]
./spec/hatchet/bundler_spec.rb[1:1]
./spec/installers/heroku_ruby_installer_spec.rb[1:1:2:1]
./spec/hatchet/ruby_spec.rb[1:4:2:1]
./spec/hatchet/rubies_spec.rb[1:3]
./spec/hatchet/node_spec.rb[1:2]
./spec/hatchet/getting_started_spec.rb[1:1]
EOL
$ while bundle exec rspec-queue --queue log/test_order_fails_sometimes.log --build 1 --worker 2; do :; done

Test output link https://dashboard.heroku.com/pipelines/ac057663-170b-4bdd-99d0-87560eb3a570/tests/519

This PR possible to the feature added in heroku/hatchet#65

[schneems]

Waiting on compliance before I rebase and re-run tests since I have a lot of commits waiting to be merged and every time I merge one I have to update the branch and re-run tests on all of them. This PR needs to be prioritized since it affects the stability of tests