Performance optimizations to handle cases of instrumentors that are not listened to. Also, fix a possible concurrency issue.

activesupport/lib/active_support/notifications.rb

@@ -41,10 +41,30 @@ module ActiveSupport
     autoload :Event, 'active_support/notifications/instrumenter'
     autoload :Fanout, 'active_support/notifications/fanout'
 
+    @instrumenters = Hash.new { |h,k| h[k] = notifier.listening?(k) }
+
     class << self
       attr_writer :notifier
-      delegate :publish, :subscribe, :unsubscribe, :to => :notifier
-      delegate :instrument, :to => :instrumenter
+      delegate :publish, :unsubscribe, :to => :notifier
+
+      def instrument(name, payload = {})
+        if @instrumenters[name]
+          instrumenter.instrument(name, payload) { yield payload if block_given? }
+        else
+          yield payload if block_given?
+        end
+      end
+
+      def subscribe(*args, &block)
+        notifier.subscribe(*args, &block).tap do
+          @instrumenters.clear
+        end
+      end
+
+      def unsubscribe(*args)
+        notifier.unsubscribe(*args)
+        @instrumenters.clear
+      end
 
       def notifier
         @notifier ||= Fanout.new

activesupport/lib/active_support/notifications/fanout.rb

@@ -9,15 +9,16 @@ module ActiveSupport
       end
 
       def subscribe(pattern = nil, block = Proc.new)
-        @listeners_for.clear
-        Subscriber.new(pattern, block).tap do |s|
+        subscriber = Subscriber.new(pattern, block).tap do |s|
           @subscribers << s
         end
+        @listeners_for.clear
+        subscriber
       end
 
       def unsubscribe(subscriber)
-        @listeners_for.clear
         @subscribers.reject! {|s| s.matches?(subscriber)}
+        @listeners_for.clear
       end
 
       def publish(name, *args)
@@ -28,6 +29,10 @@ module ActiveSupport
         @listeners_for[name] ||= @subscribers.select { |s| s.subscribed_to?(name) }
       end
 
+      def listening?(name)
+        listeners_for(name).any?
+      end
+
       # This is a sync queue, so there is not waiting.
       def wait
       end

activesupport/lib/active_support/notifications/instrumenter.rb

@@ -19,7 +19,7 @@ module ActiveSupport
       def instrument(name, payload={})
         begin
           @started = Time.now
-          yield(payload) if block_given?
+          yield
         rescue Exception => e
           payload[:exception] = [e.class.name, e.message]
           raise e