Don't merge config keys that start with __.

This allows config classes to store internal state somehow.

lib/vagrant/config/base.rb

@@ -33,7 +33,12 @@ module Vagrant
       def merge(other)
         result = self.class.new
         instance_variables_hash.merge(other.instance_variables_hash).each do |key, value|
-          result.instance_variable_set("@#{key}".to_sym, value)
+          # Ignore keys that start with a double underscore. This allows
+          # configuration classes to still hold around internal state
+          # that isn't propagated.
+          if !key.start_with?("__")
+            result.instance_variable_set("@#{key}".to_sym, value)
+          end
         end
 
         result

test/unit/vagrant/config/base_test.rb

@@ -22,4 +22,26 @@ describe Vagrant::Config::Base do
     result.one.should == 2
     result.two.should == 5
   end
+
+  it "doesn't merge values that start with a double underscore" do
+    bar_class = Class.new(foo_class) do
+      @@counter = 0
+      def initialize
+        @__test = @@counter
+        @@counter += 1
+      end
+    end
+
+    one = bar_class.new
+    one.one = 2
+    one.two = 1
+
+    two = bar_class.new
+    two.two = 5
+
+    # Verify the counters
+    one.instance_variable_get(:@__test).should == 0
+    two.instance_variable_get(:@__test).should == 1
+    one.merge(two).instance_variable_get(:@__test).should == 2
+  end
 end