From 0668c22a41e4c88ccb7600fc5b4bfb1a5b76e45d Mon Sep 17 00:00:00 2001
From: Kir Shatrov <shatrov@me.com>
Date: Sat, 24 Jun 2017 10:57:36 -0400
Subject: [PATCH] Eager load controller and mailer actions

On the first request, ActionController::Base#action_methods computes
and memoized the list of available actions [1]. With this PR we move
this expensive operation into eager load step to reduce response time
of the first request served in production.

This also reduces the memory footprint when running on forking server
like Unicorn.

[1] https://github.com/rails/rails/blob/a3813dce9a0c950a4af7909111fa730a2622b1db/actionpack/lib/abstract_controller/base.rb#L66-L77
---
 actionmailer/lib/action_mailer/railtie.rb     |  6 ++
 actionpack/lib/action_controller/railtie.rb   |  6 ++
 .../test/application/configuration_test.rb    | 60 +++++++++++++++++++
 3 files changed, 72 insertions(+)

diff --git a/actionmailer/lib/action_mailer/railtie.rb b/actionmailer/lib/action_mailer/railtie.rb
index 36c2e5866d..69578471b0 100644
--- a/actionmailer/lib/action_mailer/railtie.rb
+++ b/actionmailer/lib/action_mailer/railtie.rb
@@ -58,6 +58,12 @@ module ActionMailer
       end
     end
 
+    initializer "action_mailer.eager_load_actions" do
+      ActiveSupport.on_load(:after_initialize) do
+        ActionMailer::Base.descendants.each(&:action_methods) if config.eager_load
+      end
+    end
+
     config.after_initialize do |app|
       options = app.config.action_mailer
 
diff --git a/actionpack/lib/action_controller/railtie.rb b/actionpack/lib/action_controller/railtie.rb
index 1c1cd58732..0d7a230b17 100644
--- a/actionpack/lib/action_controller/railtie.rb
+++ b/actionpack/lib/action_controller/railtie.rb
@@ -77,5 +77,11 @@ module ActionController
         end
       end
     end
+
+    initializer "action_controller.eager_load_actions" do
+      ActiveSupport.on_load(:after_initialize) do
+        ActionController::Metal.descendants.each(&:action_methods) if config.eager_load
+      end
+    end
   end
 end
diff --git a/railties/test/application/configuration_test.rb b/railties/test/application/configuration_test.rb
index 9f62ca8eb8..aa5a1b09fd 100644
--- a/railties/test/application/configuration_test.rb
+++ b/railties/test/application/configuration_test.rb
@@ -238,6 +238,66 @@ module ApplicationTests
       assert_instance_of Pathname, Rails.public_path
     end
 
+    test "does not eager load controller actions in development" do
+      app_file "app/controllers/posts_controller.rb", <<-RUBY
+        class PostsController < ActionController::Base
+          def index;end
+          def show;end
+        end
+      RUBY
+
+      app "development"
+
+      assert_nil PostsController.instance_variable_get(:@action_methods)
+    end
+
+    test "eager loads controller actions in production" do
+      app_file "app/controllers/posts_controller.rb", <<-RUBY
+        class PostsController < ActionController::Base
+          def index;end
+          def show;end
+        end
+      RUBY
+
+      add_to_config <<-RUBY
+        config.eager_load = true
+        config.cache_classes = true
+      RUBY
+
+      app "production"
+
+      assert_equal %w(index show).to_set, PostsController.instance_variable_get(:@action_methods)
+    end
+
+    test "does not eager load mailer actions in development" do
+      app_file "app/mailers/posts_mailer.rb", <<-RUBY
+        class PostsMailer < ActionMailer::Base
+          def noop_email;end
+        end
+      RUBY
+
+      app "development"
+
+      assert_nil PostsMailer.instance_variable_get(:@action_methods)
+    end
+
+    test "eager loads mailer actions in production" do
+      app_file "app/mailers/posts_mailer.rb", <<-RUBY
+        class PostsMailer < ActionMailer::Base
+          def noop_email;end
+        end
+      RUBY
+
+      add_to_config <<-RUBY
+        config.eager_load = true
+        config.cache_classes = true
+      RUBY
+
+      app "production"
+
+      assert_equal %w(noop_email).to_set, PostsMailer.instance_variable_get(:@action_methods)
+    end
+
     test "initialize an eager loaded, cache classes app" do
       add_to_config <<-RUBY
         config.eager_load = true