Allow the definition of communicators in plugins

lib/vagrant/machine.rb

@@ -109,6 +109,7 @@ module Vagrant
     def communicate
       # For now, we always return SSH. In the future, we'll abstract
       # this and allow plugins to define new methods of communication.
+
     end
 
     # This sets the unique ID associated with this machine. This will

lib/vagrant/plugin/v1/plugin.rb

@@ -103,6 +103,24 @@ module Vagrant
           data[:command]
         end
 
+        # Defines additional communicators to be available. Communicators
+        # should be returned by a block passed to this method. This is done
+        # to ensure that the class is lazy loaded, so if your class inherits
+        # from or uses any Vagrant internals specific to Vagrant 1.0, then
+        # the plugin can still be defined without breaking anything in future
+        # versions of Vagrant.
+        #
+        # @param [String] name Communicator name.
+        def self.communicator(name=UNSET_VALUE, &block)
+          data[:communicator] ||= Registry.new
+
+          # Register a new communicator class only if a name was given.
+          data[:communicator].register(name.to_sym, &block) if name != UNSET_VALUE
+
+          # Return the registry
+          data[:communicator]
+        end
+
         # Defines additional configuration keys to be available in the
         # Vagrantfile. The configuration class should be returned by a
         # block passed to this method. This is done to ensure that the class

test/unit/vagrant/plugin/v1/plugin_test.rb

@@ -100,6 +100,34 @@ describe Vagrant::Plugin::V1::Plugin do
     end
   end
 
+  describe "communicators" do
+    it "should register communicator classes" do
+      plugin = Class.new(described_class) do
+        communicator("foo") { "bar" }
+      end
+
+      plugin.communicator[:foo].should == "bar"
+    end
+
+    it "should lazily register communicator classes" do
+      # Below would raise an error if the value of the class was
+      # evaluated immediately. By asserting that this does not raise an
+      # error, we verify that the value is actually lazily loaded
+      plugin = nil
+      expect {
+        plugin = Class.new(described_class) do
+        communicator("foo") { raise StandardError, "FAIL!" }
+        end
+      }.to_not raise_error
+
+      # Now verify when we actually get the configuration key that
+      # a proper error is raised.
+      expect {
+        plugin.communicator[:foo]
+      }.to raise_error(StandardError)
+    end
+  end
+
   describe "configuration" do
     it "should register configuration classes" do
       plugin = Class.new(described_class) do