Add the "provider" API to the V1 plugin.

lib/vagrant/plugin/v1/plugin.rb

@@ -181,6 +181,19 @@ module Vagrant
           data[:hosts]
         end
 
+        # Registers additional providers to be available.
+        #
+        # @param [Symbol] name Name of the provider.
+        def self.provider(name=UNSET_VALUE, &block)
+          data[:providers] ||= Registry.new
+
+          # Register a new provider class only if a name was given
+          data[:providers].register(name.to_sym, &block) if name != UNSET_VALUE
+
+          # Return the registry
+          data[:providers]
+        end
+
         # Registers additional provisioners to be available.
         #
         # @param [String] name Name of the provisioner.

plugins/providers/virtualbox/plugin.rb

@@ -0,0 +1,18 @@
+require "vagrant"
+
+module VagrantPlugins
+  module ProviderVirtualBox
+    class Plugin < Vagrant.plugin("1")
+      name "virtualbox provider"
+      description <<-EOF
+      The VirtualBox provider allows Vagrant to manage and control
+      VirtualBox-based virtual machines.
+      EOF
+
+      provider("virtualbox") do
+        require File.expand_path("../provider", __FILE__)
+        Provider
+      end
+    end
+  end
+end

plugins/providers/virtualbox/provider.rb

@@ -0,0 +1,6 @@
+module VagrantPlugins
+  module ProviderVirtualBox
+    class Provider < Vagrant.plugin("1", :provider)
+    end
+  end
+end

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

@@ -195,6 +195,34 @@ describe Vagrant::Plugin::V1::Plugin do
     end
   end
 
+  describe "providers" do
+    it "should register provider classes" do
+      plugin = Class.new(described_class) do
+        provider("foo") { "bar" }
+      end
+
+      plugin.provider[:foo].should == "bar"
+    end
+
+    it "should lazily register provider classes" do
+      # Below would raise an error if the value of the config 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
+          provider("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.provider[:foo]
+      }.to raise_error(StandardError)
+    end
+  end
+
   describe "provisioners" do
     it "should register provisioner classes" do
       plugin = Class.new(described_class) do