Reduce memory used by ActiveSupport::Callbacks

Previously, callbacks which were shared with subclasses would not share
between them the procs generated in Filters::Before and Filters::After.
This was also lazily generated so would cause memory growth after an
application is booted (and not sharable between workers via CoW).

This was because we would rebuild the objects used to invoke the
callbacks via CallbackChain#compile, so any difference in the callback
chain would result in all of the callback procs being rebuilt.

This commit changes before and after callbacks (but not around!) to be
shared between all subclasses of where it was defined. This is done by
changing Filters::Before and Filters::After to plain classes which
respond to call instead of generating procs (which wasn't strictly
necessary but was easier to implement, and also results in simpler
objects which use less memory). These objects avoid referencing and tied
to a specific callback sequence and so can be memoized and reused.

This has the most impact on applications with many Controllers, and many
callbacks in the ApplicationController (or similar).

I also took this opportunity to merge together all the different forms
of procs generated (halting, halting_and_conditional, conditional,
simple) into one form with if statements. There isn't any significant
performance benefit from the specialization previously being done.

Author: jhawthorn

activesupport/lib/active_support/callbacks.rb

@@ -150,7 +150,7 @@ def run_callbacks(kind, type = nil)
       def halted_callback_hook(filter, name)
       end
 
-      module Conditionals # :nodoc:
+      module Conditionals # :nodoc: all
         class Value
           def initialize(&block)
             @block = block
@@ -159,128 +159,76 @@ def call(target, value); @block.call(value); end
         end
       end
 
-      module Filters
+      module Filters # :nodoc: all
         Environment = Struct.new(:target, :halted, :value)
 
         class Before
-          def self.build(callback_sequence, user_callback, user_conditions, chain_config, filter, name)
+          def initialize(user_callback, user_conditions, chain_config, filter, name)
             halted_lambda = chain_config[:terminator]
-
-            if user_conditions.any?
-              halting_and_conditional(callback_sequence, user_callback, user_conditions, halted_lambda, filter, name)
-            else
-              halting(callback_sequence, user_callback, halted_lambda, filter, name)
-            end
+            @user_callback, @user_conditions, @halted_lambda, @filter, @name = user_callback, user_conditions, halted_lambda, filter, name
+            freeze
           end
+          attr_reader :user_callback, :user_conditions, :halted_lambda, :filter, :name
 
-          def self.halting_and_conditional(callback_sequence, user_callback, user_conditions, halted_lambda, filter, name)
-            callback_sequence.before do |env|
-              target = env.target
-              value  = env.value
-              halted = env.halted
+          def call(env)
+            target = env.target
+            value  = env.value
+            halted = env.halted
 
-              if !halted && user_conditions.all? { |c| c.call(target, value) }
-                result_lambda = -> { user_callback.call target, value }
-                env.halted = halted_lambda.call(target, result_lambda)
-                if env.halted
-                  target.send :halted_callback_hook, filter, name
-                end
+            if !halted && user_conditions.all? { |c| c.call(target, value) }
+              result_lambda = -> { user_callback.call target, value }
+              env.halted = halted_lambda.call(target, result_lambda)
+              if env.halted
+                target.send :halted_callback_hook, filter, name
               end
-
-              env
             end
-          end
-          private_class_method :halting_and_conditional
-
-          def self.halting(callback_sequence, user_callback, halted_lambda, filter, name)
-            callback_sequence.before do |env|
-              target = env.target
-              value  = env.value
-              halted = env.halted
 
-              unless halted
-                result_lambda = -> { user_callback.call target, value }
-                env.halted = halted_lambda.call(target, result_lambda)
-                if env.halted
-                  target.send :halted_callback_hook, filter, name
-                end
-              end
+            env
+          end
 
-              env
-            end
+          def apply(callback_sequence)
+            callback_sequence.before(self)
           end
-          private_class_method :halting
         end
 
         class After
-          def self.build(callback_sequence, user_callback, user_conditions, chain_config)
-            if chain_config[:skip_after_callbacks_if_terminated]
-              if user_conditions.any?
-                halting_and_conditional(callback_sequence, user_callback, user_conditions)
-              else
-                halting(callback_sequence, user_callback)
-              end
-            else
-              if user_conditions.any?
-                conditional callback_sequence, user_callback, user_conditions
-              else
-                simple callback_sequence, user_callback
-              end
-            end
+          attr_reader :user_callback, :user_conditions, :halting
+          def initialize(user_callback, user_conditions, chain_config)
+            halting = chain_config[:skip_after_callbacks_if_terminated]
+            @user_callback, @user_conditions, @halting = user_callback, user_conditions, halting
+            freeze
           end
 
-          def self.halting_and_conditional(callback_sequence, user_callback, user_conditions)
-            callback_sequence.after do |env|
-              target = env.target
-              value  = env.value
-              halted = env.halted
-
-              if !halted && user_conditions.all? { |c| c.call(target, value) }
-                user_callback.call target, value
-              end
+          def call(env)
+            target = env.target
+            value  = env.value
+            halted = env.halted
 
-              env
+            if (!halted || !@halting) && user_conditions.all? { |c| c.call(target, value) }
+              user_callback.call target, value
             end
-          end
-          private_class_method :halting_and_conditional
-
-          def self.halting(callback_sequence, user_callback)
-            callback_sequence.after do |env|
-              unless env.halted
-                user_callback.call env.target, env.value
-              end
 
-              env
-            end
+            env
           end
-          private_class_method :halting
-
-          def self.conditional(callback_sequence, user_callback, user_conditions)
-            callback_sequence.after do |env|
-              target = env.target
-              value  = env.value
 
-              if user_conditions.all? { |c| c.call(target, value) }
-                user_callback.call target, value
-              end
-
-              env
-            end
+          def apply(callback_sequence)
+            callback_sequence.after(self)
           end
-          private_class_method :conditional
+        end
 
-          def self.simple(callback_sequence, user_callback)
-            callback_sequence.after do |env|
-              user_callback.call env.target, env.value
+        class Around
+          def initialize(user_callback, user_conditions)
+            @user_callback, @user_conditions = user_callback, user_conditions
+            freeze
+          end
 
-              env
-            end
+          def apply(callback_sequence)
+            callback_sequence.around(@user_callback, @user_conditions)
           end
-          private_class_method :simple
         end
       end
 
-      class Callback # :nodoc:#
+      class Callback # :nodoc:
         def self.build(chain, filter, kind, options)
           if filter.is_a?(String)
             raise ArgumentError, <<-MSG.squish
@@ -329,19 +277,26 @@ def duplicates?(other)
           end
         end
 
+        def compiled
+          @compiled ||=
+            begin
+              user_conditions = conditions_lambdas
+              user_callback = CallTemplate.build(@filter, self)
+
+              case kind
+              when :before
+                Filters::Before.new(user_callback.make_lambda, user_conditions, chain_config, @filter, name)
+              when :after
+                Filters::After.new(user_callback.make_lambda, user_conditions, chain_config)
+              when :around
+                Filters::Around.new(user_callback, user_conditions)
+              end
+            end
+        end
+
         # Wraps code with filter
         def apply(callback_sequence)
-          user_conditions = conditions_lambdas
-          user_callback = CallTemplate.build(@filter, self)
-
-          case kind
-          when :before
-            Filters::Before.build(callback_sequence, user_callback.make_lambda, user_conditions, chain_config, @filter, name)
-          when :after
-            Filters::After.build(callback_sequence, user_callback.make_lambda, user_conditions, chain_config)
-          when :around
-            callback_sequence.around(user_callback, user_conditions)
-          end
+          compiled.apply(callback_sequence)
         end
 
         def current_scopes
@@ -375,7 +330,7 @@ def conditions_lambdas
 
       # A future invocation of user-supplied code (either as a callback,
       # or a condition filter).
-      module CallTemplate # :nodoc:
+      module CallTemplate # :nodoc: all
         class MethodCall
           def initialize(method)
             @method_name = method
@@ -566,12 +521,12 @@ def initialize(nested = nil, call_template = nil, user_conditions = nil)
           @after = []
         end
 
-        def before(&before)
+        def before(before)
           @before.unshift(before)
           self
         end
 
-        def after(&after)
+        def after(after)
           @after.push(after)
           self
         end