Vagrantfile [except].rb

# Handles auto-installing required vagrant plugins:
class PluginAutoInstaller < Vagrant.plugin("2")
  name "Plugin Auto-Installer"

  description <<-DESC
  This (pseudo-plugin) code fragment is intended to be
  pasted into the top of your Vagrantfile to ensure your
  required manifest of plugins are automatically installed
  (or uninstalled) as your Vagrantfile expects.
  DESC

  def to_s
    name
  end

  class PluginChecker
    def initialize(app, env)
      @app = app
    end

    def self.manifest=(new_manifest)
      @@required_plugin_manifest = new_manifest.tap do |map|
        map.each do |plugin, expectation|
          raise "Invalid plugin expectation #{expectation} for plugin #{plugin}" unless [ :required, :prohibited ].include?(expectation)
        end
      end
    end

    @@required_plugin_manifest = {}

    def call(env)
      env[:ui].output("Checking installed plugins for event #{env[:action_name]}")
      plugins_to_alter = @@required_plugin_manifest.select do |plugin, expectation|
        (Vagrant.has_plugin? plugin) != (expectation == :required)
      end
      if not plugins_to_alter.empty?
        plugins_by_state = Hash.new { |h, k| h[(k == :required) ? :install : :uninstall ] = [] }
        plugins_to_alter.each { |k, v| plugins_by_state[v] << k }
        plugins_by_state.each do |action, plugin_alter_list|
          env[:ui].output("#{(action == :install) ? 'Installing' : 'Removing'} plugins: #{plugin_alter_list.join(' ')}")
          # (un)install the plugin(s) in a separate vagrant process
          if system "vagrant plugin #{action} #{plugin_alter_list.join(' ')}"
          else
            raise "#{(action == :install) ? 'Installation' : 'Removal'} of one or more plugins has failed. Aborting."
          end
        end
        # scrap current vagrant process, and start over:
        env[:ui].output("Restarting vagrant...")
        exec ‘vagrant’, *ARGV
        raise "The Ruby interperter should have already exited!"
      end
      @app.call(env)
    end
  end

  # Only respond to actions that will create / boot / resume a box (not a halt or destroy,
  # for instance)
  [ :machine_action_boot,      :machine_action_provision, :machine_action_reload,
    :machine_action_resume,    :machine_action_start,     :machine_action_up,
    :provisioner_run
  ]. each do |event_name|
    action_hook("auto_install_plugins", event_name) do |hook|
        hook.prepend(PluginChecker)
    end
  end
end

PluginAutoInstaller::PluginChecker.manifest = {
  "vagrant-vbguest" => :required,
  "vagrant-reload"  => :required,
  "vagrant-cachier" => :prohibited }