Merge pull request #8559 from mshytikov/runner_method_is_slow_with_thread_safe

Attempt to fix slow runner name method

activesupport/lib/active_support/callbacks.rb

+require 'thread_safe'
 require 'active_support/concern'
 require 'active_support/descendants_tracker'
 require 'active_support/core_ext/class/attribute'
@@ -351,10 +352,18 @@ def __reset_runner(symbol)
         undef_method(name) if method_defined?(name)
       end
 
-      def __callback_runner_name(kind)
+      def __callback_runner_name_cache
+        @__callback_runner_name_cache ||= ThreadSafe::Cache.new {|cache, kind| cache[kind] = __generate_callback_runner_name(kind) }
+      end
+
+      def __generate_callback_runner_name(kind)
         "_run__#{self.name.hash.abs}__#{kind}__callbacks"
       end
 
+      def __callback_runner_name(kind)
+        __callback_runner_name_cache[kind]
+      end
+
       # This is used internally to append, prepend and skip callbacks to the
       # CallbackChain.
       def __update_callbacks(name, filters = [], block = nil) #:nodoc: