Config merging

lib/vagrant/config/v1.rb

@@ -10,6 +10,60 @@ module Vagrant
       # @param [Proc] config_proc
       # @return [Object]
       def self.load(config_proc)
+        # Create a root configuration object
+        root = new_root_object
+
+        # Call the proc with the root
+        config_proc.call(root)
+
+        # Return the root object, which doubles as the configuration object
+        # we actually use for accessing as well.
+        root
+      end
+
+      # Merges two configuration objects.
+      #
+      # @param [V1::Root] old The older root config.
+      # @param [V1::Root] new The newer root config.
+      # @return [V1::Root]
+      def self.merge(old, new)
+        # Grab the internal states, we use these heavily throughout the process
+        old_state = old.__internal_state
+        new_state = new.__internal_state
+
+        # The config map for the new object is the old one merged with the
+        # new one.
+        config_map = old_state["config_map"].merge(new_state["config_map"])
+
+        # Merge the keys.
+        old_keys = old_state["keys"]
+        new_keys = new_state["keys"]
+        keys     = {}
+        old_keys.each do |key, old|
+          if new_keys.has_key?(key)
+            # We need to do a merge, which we expect to be available
+            # on the config class itself.
+            keys[key] = old.merge(new_keys[key])
+          else
+            # We just take the old value, but dup it so that we can modify.
+            keys[key] = old.dup
+          end
+        end
+
+        new_keys.each do |key, new|
+          # Add in the keys that the new class has that we haven't merged.
+          if !keys.has_key?(key)
+            keys[key] = new.dup
+          end
+        end
+
+        # Return the final root object
+        V1::Root.new(config_map, keys)
+      end
+
+      protected
+
+      def self.new_root_object
         # Get all the registered plugins
         config_map = {}
         Vagrant.plugin("1").registered.each do |plugin|
@@ -19,14 +73,7 @@ module Vagrant
         end
 
         # Create the configuration root object
-        root = V1::Root.new(config_map)
+        V1::Root.new(config_map)
-
-        # Call the proc with the root
-        config_proc.call(root)
-
-        # Return the root object, which doubles as the configuration object
-        # we actually use for accessing as well.
-        root
       end
     end
   end

lib/vagrant/config/v1/root.rb

@@ -8,8 +8,8 @@ module Vagrant
         # configuration classes.
         #
         # @param [Hash] config_map Map of key to config class.
-        def initialize(config_map)
+        def initialize(config_map, keys=nil)
-          @keys       = {}
+          @keys       = keys || {}
           @config_map = config_map
         end
 
@@ -29,6 +29,17 @@ module Vagrant
             super
           end
         end
+
+        # Returns the internal state of the root object. This is used
+        # by outside classes when merging, and shouldn't be called directly.
+        # Note the strange method name is to attempt to avoid any name
+        # clashes with potential configuration keys.
+        def __internal_state
+          {
+            "config_map" => @config_map,
+            "keys"       => @keys
+          }
+        end
       end
     end
   end

test/unit/vagrant/config/v1/root_test.rb

@@ -17,4 +17,18 @@ describe Vagrant::Config::V1::Root do
     instance = described_class.new({})
     expect { instance.foo }.to raise_error(NoMethodError)
   end
+
+  it "can be created with initial state" do
+    instance = described_class.new({}, { :foo => "bar" })
+    instance.foo.should == "bar"
+  end
+
+  it "should return internal state" do
+    map      = { "foo" => Object, "bar" => Object }
+    instance = described_class.new(map)
+    instance.__internal_state.should == {
+      "config_map" => map,
+      "keys"       => {}
+    }
+  end
 end

test/unit/vagrant/config/v1_test.rb

@@ -31,4 +31,51 @@ describe Vagrant::Config::V1 do
       end
     end
   end
+
+  describe "merging" do
+    it "should merge available configuration keys" do
+      old = Vagrant::Config::V1::Root.new({ :foo => Object })
+      new = Vagrant::Config::V1::Root.new({ :bar => Object })
+      result = described_class.merge(old, new)
+      result.foo.should be_kind_of(Object)
+      result.bar.should be_kind_of(Object)
+    end
+
+    it "should merge instantiated objects" do
+      config_class = Class.new do
+        attr_accessor :value
+      end
+
+      old = Vagrant::Config::V1::Root.new({ :foo => config_class })
+      old.foo.value = "old"
+
+      new = Vagrant::Config::V1::Root.new({ :bar => config_class })
+      new.bar.value = "new"
+
+      result = described_class.merge(old, new)
+      result.foo.value.should == "old"
+      result.bar.value.should == "new"
+    end
+
+    it "should merge conflicting classes by calling `merge`" do
+      config_class = Class.new do
+        attr_accessor :value
+
+        def merge(new)
+          result       = self.class.new
+          result.value = @value + new.value
+          result
+        end
+      end
+
+      old = Vagrant::Config::V1::Root.new({ :foo => config_class })
+      old.foo.value = 10
+
+      new = Vagrant::Config::V1::Root.new({ :foo => config_class })
+      new.foo.value = 15
+
+      result = described_class.merge(old, new)
+      result.foo.value.should == 25
+    end
+  end
 end