disable observers without cross-thread side effects

lib/rails/observers/active_model/disabled_observers_registry.rb

@@ -0,0 +1,13 @@
+module ActiveModel
+  class DisabledObserversRegistry
+    extend ActiveSupport::PerThreadRegistry
+
+    attr_accessor :disabled_observers_per_class
+    attr_accessor :disabled_observers_stacks_per_class
+
+    def initialize
+      @disabled_observers_per_class         = {}
+      @disabled_observers_stacks_per_class  = {}
+    end
+  end
+end

lib/rails/observers/active_model/observer_array.rb

@@ -73,7 +73,11 @@ def enable(*observers, &block)
     protected
 
       def disabled_observers #:nodoc:
-        @disabled_observers ||= Set.new
+        DisabledObserversRegistry.disabled_observers_per_class[model_class] ||= Set.new
+      end
+
+      def disabled_observers= observers #:nodoc:
+        DisabledObserversRegistry.disabled_observers_per_class[model_class] = observers
       end
 
       def observer_class_for(observer) #:nodoc:
@@ -95,11 +99,11 @@ def start_transaction #:nodoc:
       end
 
       def disabled_observer_stack #:nodoc:
-        @disabled_observer_stack ||= []
+        DisabledObserversRegistry.disabled_observers_stacks_per_class[model_class] ||= []
       end
 
       def end_transaction #:nodoc:
-        @disabled_observers = disabled_observer_stack.pop
+        self.disabled_observers = disabled_observer_stack.pop
         each_subclass_array do |array|
           array.end_transaction
         end

lib/rails/observers/active_model/observing.rb

@@ -1,4 +1,5 @@
 require 'singleton'
+require 'rails/observers/active_model/disabled_observers_registry'
 require 'rails/observers/active_model/observer_array'
 require 'active_support/core_ext/module/aliasing'
 require 'active_support/core_ext/module/remove_method'

test/observer_array_test.rb

@@ -109,7 +109,6 @@ def assert_observer_not_notified(model_class, observer_class)
 
   test "can disable observers on individual models without affecting those observers on other models" do
     Widget.observers.disable :all
-
     assert_observer_not_notified Widget, WidgetObserver
     assert_observer_notified     Budget, BudgetObserver
     assert_observer_not_notified Widget, AuditTrail
@@ -162,6 +161,13 @@ def assert_observer_not_notified(model_class, observer_class)
     assert_observer_notified     Budget, AuditTrail
   end
 
+  test "can enable observer without side effects on other threads" do
+    Thread.new do
+      Widget.observers.disable :audit_trail
+    end.join()
+    assert_observer_notified Widget, AuditTrail
+  end
+
   test "raises an appropriate error when a developer accidentally enables or disables the wrong class (i.e. Widget instead of WidgetObserver)" do
     assert_raise ArgumentError do
       ORM.observers.enable :widget