List actually compares state with gems

plugins/commands/plugin/action/list_plugins.rb

@@ -1,18 +1,36 @@
 require "rubygems"
+require "set"
 
 module VagrantPlugins
   module CommandPlugin
     module Action
+      # This middleware lists all the installed plugins.
+      #
+      # This is a bit more complicated than simply listing installed
+      # gems or what is in the state file as installed. Instead, this
+      # actually compares installed gems with what the state file claims
+      # is installed, and outputs the appropriate truly installed
+      # plugins.
       class ListPlugins
         def initialize(app, env)
           @app = app
         end
 
         def call(env)
-          env[:gem_helper].with_environment do
+          # Get the list of installed plugins according to the state file
-            specs = Gem::Specification.find_all
+          installed = Set.new(env[:plugin_state_file].installed_plugins)
 
-            specs.each do |spec|
+          # Get the actual specifications of installed gems
+          specs = env[:gem_helper].with_environment do
+            Gem::Specification.find_all
+          end
+
+          # Go through each spec and if it is an installed plugin, then
+          # output it. This means that both the installed state and
+          # gem match up.
+          specs.each do |spec|
+            if installed.include?(spec.name)
+              # TODO: Formatting
               env[:ui].info spec.name
             end
           end

plugins/commands/plugin/gem_helper.rb

@@ -55,7 +55,7 @@ module VagrantPlugins
         # Clear paths so that it reads the new GEM_HOME setting
         Gem.clear_paths
 
-        yield
+        return yield
       ensure
         # Restore the old GEM_HOME
         ENV["GEM_HOME"] = old_gem_home

plugins/commands/plugin/state_file.rb

@@ -21,6 +21,16 @@ module VagrantPlugins
         save!
       end
 
+      # This returns a list of installed plugins according to the state
+      # file. Note that this may _not_ directly match over to actually
+      # installed gems.
+      #
+      # @return [Array<String>]
+      def installed_plugins
+        @data["installed"] ||= []
+        @data["installed"]
+      end
+
       # This saves the state back into the state file.
       def save!
         @path.open("w+") do |f|