Fix default scope thread safety. Thanks @thedarkone for reporting.

activerecord/lib/active_record/base.rb

@@ -1206,6 +1206,14 @@ def current_scope=(scope) #:nodoc:
           Thread.current["#{self}_current_scope"] = scope
         end
 
+        def ignore_default_scope? #:nodoc:
+          Thread.current["#{self}_ignore_default_scope"]
+        end
+
+        def ignore_default_scope=(ignore) #:nodoc:
+          Thread.current["#{self}_ignore_default_scope"] = ignore
+        end
+
         # Use this macro in your model to set a default scope for all operations on
         # the model.
         #
@@ -1259,24 +1267,27 @@ def default_scope(scope = {})
         # The @ignore_default_scope flag is used to prevent an infinite recursion situation where
         # a default scope references a scope which has a default scope which references a scope...
         def build_default_scope #:nodoc:
-          return if defined?(@ignore_default_scope) && @ignore_default_scope
-          @ignore_default_scope = true
-
-          if method(:default_scope).owner != Base.singleton_class
-            default_scope
-          elsif default_scopes.any?
-            default_scopes.inject(relation) do |default_scope, scope|
-              if scope.is_a?(Hash)
-                default_scope.apply_finder_options(scope)
-              elsif !scope.is_a?(Relation) && scope.respond_to?(:call)
-                default_scope.merge(scope.call)
-              else
-                default_scope.merge(scope)
+          return if ignore_default_scope?
+
+          begin
+            self.ignore_default_scope = true
+
+            if method(:default_scope).owner != Base.singleton_class
+              default_scope
+            elsif default_scopes.any?
+              default_scopes.inject(relation) do |default_scope, scope|
+                if scope.is_a?(Hash)
+                  default_scope.apply_finder_options(scope)
+                elsif !scope.is_a?(Relation) && scope.respond_to?(:call)
+                  default_scope.merge(scope.call)
+                else
+                  default_scope.merge(scope)
+                end
               end
             end
+          ensure
+            self.ignore_default_scope = false
           end
-        ensure
-          @ignore_default_scope = false
         end
 
         # Returns the class type of the record using the current module as a prefix. So descendants of

activerecord/test/cases/relation_scoping_test.rb

@@ -524,4 +524,22 @@ def test_default_scope_include_with_count
 
     assert_equal 1, DeveloperWithIncludes.where(:audit_logs => { :message => 'foo' }).count
   end
+
+  def test_default_scope_is_threadsafe
+    if in_memory_db?
+      skip "in memory db can't share a db between threads"
+    end
+
+    threads = []
+    assert_not_equal 1, ThreadsafeDeveloper.unscoped.count
+
+    threads << Thread.new do
+      Thread.current[:long_default_scope] = true
+      assert_equal 1, ThreadsafeDeveloper.all.count
+    end
+    threads << Thread.new do
+      assert_equal 1, ThreadsafeDeveloper.all.count
+    end
+    threads.each(&:join)
+  end
 end

activerecord/test/models/developer.rb

@@ -227,3 +227,12 @@ class EagerDeveloperWithCallableDefaultScope < ActiveRecord::Base
 
   default_scope OpenStruct.new(:call => includes(:projects))
 end
+
+class ThreadsafeDeveloper < ActiveRecord::Base
+  self.table_name = 'developers'
+
+  def self.default_scope
+    sleep 0.05 if Thread.current[:long_default_scope]
+    limit(1)
+  end
+end