[CHEF-3432] use shared variable to track resource subclasses

Use a class (@@var) instead of class instance variable to track
subclasses of Chef::Resource. This ensures that subclasses of subclasses
will be added to the list.

lib/chef/resource.rb

@@ -132,7 +132,7 @@ def fix_notifier_reference(resource_collection)
     # Track all subclasses of Resource. This is used so names can be looked up
     # when attempting to deserialize from JSON. (See: json_compat)
     def self.resource_classes
-      @resource_classes ||= []
+      @@resource_classes ||= []
     end
 
     # Callback when subclass is defined. Adds subclass to list of subclasses.

spec/unit/resource_spec.rb

@@ -35,6 +35,21 @@ class ResourceTestHarness < Chef::Resource
     @resource = Chef::Resource.new("funk", @run_context)
   end
 
+  describe "when inherited", :focus do
+
+    it "adds an entry to a list of subclasses" do
+      subclass = Class.new(Chef::Resource)
+      Chef::Resource.resource_classes.should include(subclass)
+    end
+
+    it "keeps track of subclasses of subclasses" do
+      subclass = Class.new(Chef::Resource)
+      subclass_of_subclass = Class.new(subclass)
+      Chef::Resource.resource_classes.should include(subclass_of_subclass)
+    end
+
+  end
+
   describe "when declaring the identity attribute" do
     it "has no identity attribute by default" do
       Chef::Resource.identity_attr.should be_nil