Fix another race condition.

From 2c667f6.

Thanks @pwnall for the heads-up.

Conflicts:

	activerecord/lib/active_record/core.rb

activerecord/CHANGELOG.md

@@ -1,3 +1,8 @@
+## Rails 3.2.1 (unreleased) ##
+
+*   Fix possible race condition when two threads try to define attribute
+    methods for the same class.
+
 ## Rails 3.2.0 (January 20, 2012) ##
 
 *   Added a `with_lock` method to ActiveRecord objects, which starts

activerecord/lib/active_record/attribute_methods.rb

@@ -1,6 +1,5 @@
 require 'active_support/core_ext/enumerable'
 require 'active_support/deprecation'
-require 'thread'
 
 module ActiveRecord
   # = Active Record Attribute Methods
@@ -39,8 +38,6 @@ module ClassMethods
       def define_attribute_methods
         # Use a mutex; we don't want two thread simaltaneously trying to define
         # attribute methods.
-        @attribute_methods_mutex ||= Mutex.new
-
         @attribute_methods_mutex.synchronize do
           return if attribute_methods_generated?
           superclass.define_attribute_methods unless self == base_class

activerecord/lib/active_record/base.rb

@@ -5,6 +5,7 @@
 
 require 'yaml'
 require 'set'
+require 'thread'
 require 'active_support/benchmarkable'
 require 'active_support/dependencies'
 require 'active_support/descendants_tracker'
@@ -390,12 +391,18 @@ class Base
 
     class << self # Class methods
       def inherited(child_class) #:nodoc:
-        # force attribute methods to be higher in inheritance hierarchy than other generated methods
-        child_class.generated_attribute_methods
-        child_class.generated_feature_methods
+        child_class.initialize_generated_modules
         super
       end
 
+      def initialize_generated_modules #:nodoc:
+        @attribute_methods_mutex = Mutex.new
+
+        # force attribute methods to be higher in inheritance hierarchy than other generated methods
+        generated_attribute_methods
+        generated_feature_methods
+      end
+
       def generated_feature_methods
         @generated_feature_methods ||= begin
           mod = const_set(:GeneratedFeatureMethods, Module.new)

activerecord/test/cases/attribute_methods/read_test.rb

@@ -1,5 +1,6 @@
 require "cases/helper"
 require 'active_support/core_ext/object/inclusion'
+require 'thread'
 
 module ActiveRecord
   module AttributeMethods
@@ -19,6 +20,13 @@ def self.base_class; self; end
 
           include ActiveRecord::AttributeMethods
 
+          def self.define_attribute_methods
+            # Created in the inherited/included hook for "proper" ARs
+            @attribute_methods_mutex ||= Mutex.new
+
+            super
+          end
+
           def self.column_names
             %w{ one two three }
           end