Add support for "upgrade safe" config classes to plugins.

These are classes that use NO core classes of Vagrant, and are therefore
safe to load for upgrades. i.e. a V2 core can load a V1 config class
that is deemed upgrade safe without crashing Vagrant.

lib/vagrant/plugin/v1/manager.rb

@@ -42,6 +42,25 @@ module Vagrant
           result
         end
 
+        # This returns all the registered configuration classes that were
+        # marked as "upgrade safe."
+        #
+        # @return [Hash]
+        def config_upgrade_safe
+          result = {}
+
+          @registered.each do |plugin|
+            configs = plugin.data[:config_upgrade_safe]
+            if configs
+              configs.each do |key|
+                result[key] = plugin.config.get(key)
+              end
+            end
+          end
+
+          result
+        end
+
         # This returns all the registered guests.
         #
         # @return [Hash]

lib/vagrant/plugin/v1/plugin.rb

@@ -126,7 +126,16 @@ module Vagrant
           data[:config] ||= Registry.new
 
           # Register a new config class only if a name was given.
-          data[:config].register(name.to_sym, &block) if name != UNSET_VALUE
+          if name != UNSET_VALUE
+            data[:config].register(name.to_sym, &block)
+
+            # If we were told this is an upgrade safe configuration class
+            # then we add it to the set.
+            if upgrade_safe
+              data[:config_upgrade_safe] ||= Set.new
+              data[:config_upgrade_safe].add(name.to_sym)
+            end
+          end
 
           # Return the registry
           data[:config]
@@ -218,18 +227,19 @@ module Vagrant
           data[:provisioners]
         end
 
-        protected
-
-        # Sentinel value denoting that a value has not been set.
-        UNSET_VALUE = Object.new
-
-        # Returns the internal data associated with this plugin.
+        # Returns the internal data associated with this plugin. This
+        # should NOT be called by the general public.
         #
         # @return [Hash]
         def self.data
           @data ||= {}
         end
 
+        protected
+
+        # Sentinel value denoting that a value has not been set.
+        UNSET_VALUE = Object.new
+
         # Helper method that will set a value if a value is given, or otherwise
         # return the already set value.
         #

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

@@ -45,6 +45,22 @@ describe Vagrant::Plugin::V1::Manager do
     instance.config[:bar].should == "baz"
   end
 
+  it "should enumerate registered upgrade safe config classes" do
+    pA = plugin do |p|
+      p.config("foo", true) { "bar" }
+    end
+
+    pB = plugin do |p|
+      p.config("bar") { "baz" }
+    end
+
+    instance.register(pA)
+    instance.register(pB)
+
+    instance.config_upgrade_safe.length.should == 1
+    instance.config_upgrade_safe[:foo].should == "bar"
+  end
+
   it "should enumerate registered guest classes" do
     pA = plugin do |p|
       p.guest("foo") { "bar" }