core: Vagrant.has_plugin? tries to match gem name first

Search primary from the list of gem names which have been loaded by
`Vagrant.require_plugin`. Fall back to matching registered plugin names.

lib/vagrant.rb

@@ -140,7 +140,10 @@ module Vagrant
   # be used from the Vagrantfile to easily branch based on plugin
   # availability.
   def self.has_plugin?(name)
-    plugin("2").manager.registered.any? { |plugin| plugin.name == name }
+    manager = plugin("2").manager
+
+    manager.required.any? { |gem_name| gem_name == name } ||
+      manager.registered.any? { |plugin| plugin.name == name }
   end
 
   # Returns a superclass to use when creating a plugin for Vagrant.
@@ -198,6 +201,7 @@ module Vagrant
     # Attempt the normal require
     begin
       require name
+      plugin("2").manager.plugin_required(name)
     rescue Exception => e
       # Since this is a rare case, we create a one-time logger here
       # in order to output the error

lib/vagrant/plugin/v2/manager.rb

@@ -8,10 +8,12 @@ module Vagrant
       # those plugins as a single unit.
       class Manager
         attr_reader :registered
+        attr_reader :required
 
         def initialize
           @logger = Log4r::Logger.new("vagrant::plugin::v2::manager")
           @registered = []
+          @required = []
         end
 
         # This returns all the action hooks.
@@ -164,10 +166,20 @@ module Vagrant
           end
         end
 
+        # This registers a required plugin. This should _NEVER_ be called by
+        # the public and should only be called from within Vagrant.
+        def plugin_required(gem_name)
+          if !@required.include?(gem_name)
+            @logger.info("Registered required plugin: #{gem_name}")
+            @required << gem_name
+          end
+        end
+
         # This clears out all the registered plugins. This is only used by
         # unit tests and should not be called directly.
         def reset!
           @registered.clear
+          @required.clear
         end
 
         # This unregisters a plugin so that its components will no longer

test/unit/vagrant/plugin/v2/manager_test.rb

@@ -188,4 +188,16 @@ describe Vagrant::Plugin::V2::Manager do
     instance.synced_folders[:foo].should == ["bar", 10]
     instance.synced_folders[:bar].should == ["baz", 50]
   end
+
+  it "should list the required plugins" do
+    instance.plugin_required("foo")
+    instance.plugin_required("bar")
+    expect(instance.required).to eq(["foo", "bar"])
+  end
+
+  it "should list the required plugins only once" do
+    instance.plugin_required("foo")
+    instance.plugin_required("foo")
+    expect(instance.required).to eq(["foo"])
+  end
 end

test/unit/vagrant_test.rb

@@ -54,6 +54,12 @@ describe Vagrant do
         to_not raise_error
     end
 
+    it "should add the gem name to plugin manager" do
+      expect(described_class.plugin("2").manager).
+        to receive(:plugin_required).with("set")
+      described_class.require_plugin "set"
+    end
+
     it "should raise an error if the file doesn't exist" do
       expect { described_class.require_plugin("i_dont_exist") }.
         to raise_error(Vagrant::Errors::PluginLoadError)
@@ -72,20 +78,27 @@ describe Vagrant do
   end
 
   describe "has_plugin?" do
-    after(:each) do
-      described_class.plugin('2').manager.reset!
-    end
-
-    it "should return true if the plugin is installed" do
-      plugin = Class.new(described_class.plugin('2')) do
+    before(:each) do
+      Class.new(described_class.plugin("2")) do
         name "i_am_installed"
       end
+      manager.plugin_required("plugin_gem")
+    end
+    after(:each) do
+      manager.reset!
+    end
+    let(:manager) { described_class.plugin("2").manager }
 
-      described_class.has_plugin?("i_am_installed").should be_true
+    it "should find the installed plugin by the gem name" do
+      expect(described_class.has_plugin?("plugin_gem")).to be_true
+    end
+
+    it "should find the installed plugin by the registered name" do
+      expect(described_class.has_plugin?("i_am_installed")).to be_true
     end
 
     it "should return false if the plugin is not installed" do
-      described_class.has_plugin?("i_dont_exist").should be_false
+      expect(described_class.has_plugin?("i_dont_exist")).to be_false
     end
   end