just use an attr_accessor so we do not pay ||= on every notification call

activesupport/lib/active_support/notifications.rb

@@ -44,7 +44,7 @@ module ActiveSupport
     @instrumenters = Hash.new { |h,k| h[k] = notifier.listening?(k) }
 
     class << self
-      attr_writer :notifier
+      attr_accessor :notifier
 
       def publish(name, *args)
         notifier.publish(name, *args)
@@ -69,13 +69,11 @@ module ActiveSupport
         @instrumenters.clear
       end
 
-      def notifier
-        @notifier ||= Fanout.new
-      end
-
       def instrumenter
         Thread.current[:"instrumentation_#{notifier.object_id}"] ||= Instrumenter.new(notifier)
       end
     end
+
+    self.notifier = Fanout.new
   end
 end

activesupport/test/notifications_test.rb

@@ -3,14 +3,19 @@ require 'abstract_unit'
 module Notifications
   class TestCase < ActiveSupport::TestCase
     def setup
-      ActiveSupport::Notifications.notifier = nil
-      @notifier = ActiveSupport::Notifications.notifier
+      @old_notifier = ActiveSupport::Notifications.notifier
+      @notifier = ActiveSupport::Notifications::Fanout.new
+      ActiveSupport::Notifications.notifier = @notifier
       @events = []
       @named_events = []
       @subscription = @notifier.subscribe { |*args| @events << event(*args) }
       @named_subscription = @notifier.subscribe("named.subscription") { |*args| @named_events << event(*args) }
     end
 
+    def teardown
+      ActiveSupport::Notifications.notifier = @old_notifier
+    end
+
   private
 
     def event(*args)