Defer route drawing to the first request, or when url_helpers called.

Executes the first routes reload in middleware, or when the route set
url_helpers is called. Previously, this was executed unconditionally on
boot, which can slow down boot time unnecessarily for larger apps with
lots of routes.

actionpack/lib/action_dispatch.rb

@@ -71,6 +71,7 @@ class MissingController < NameError
     autoload :DebugView
     autoload :ExceptionWrapper
     autoload :Executor
+    autoload :Once
     autoload :Flash
     autoload :PublicExceptions
     autoload :Reloader

actionpack/lib/action_dispatch/middleware/once.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+# :markup: markdown
+
+module ActionDispatch
+  class Once # :nodoc:
+    def initialize(app, block)
+      @app, @block, @called = app, block, false
+    end
+
+    def call(env)
+      @called ||= begin
+        @block.call
+        true
+      end
+      @app.call(env)
+    end
+  end
+end

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -345,9 +345,7 @@ def self.default_resources_path_names
         { new: "new", edit: "edit" }
       end
 
-      def self.new_with_config(config)
-        route_set_config = DEFAULT_CONFIG
-
+      def self.new_with_config(config, route_set_config = DEFAULT_CONFIG)
         # engines apparently don't have this set
         if config.respond_to? :relative_url_root
           route_set_config.relative_url_root = config.relative_url_root

actionpack/test/dispatch/once_test.rb

@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require "abstract_unit"
+
+module ActionDispatch
+  class OncesTest < ActionDispatch::IntegrationTest
+    setup do
+      @call_count = 0
+      @call_increment = proc { @call_count += 1 }
+      build_app
+    end
+
+    def test_runs_block_only_once
+      5.times { get "/test" }
+
+      assert_equal(@call_count, 1)
+    end
+
+    private
+      def build_app
+        @app = self.class.build_app do |middleware|
+          middleware.use Once, @call_increment
+        end
+      end
+  end
+end

actionview/lib/action_view/rendering.rb

@@ -63,6 +63,7 @@ def build_view_context_class(klass, supports_path, routes, helpers)
 
         Class.new(klass) do
           if routes
+            routes.application.routes_reloader.execute_unless_loaded if routes.respond_to?(:application)
             include routes.url_helpers(supports_path)
             include routes.mounted_helpers
           end

railties/lib/rails/application/default_middleware_stack.rb

@@ -63,6 +63,7 @@ def build_stack
           end
 
           if config.reloading_enabled?
+            middleware.use ::Rails::Rack::LoadRoutes, app.routes_reloader unless app.config.eager_load
             middleware.use ::ActionDispatch::Reloader, app.reloader
           end
 

railties/lib/rails/application/finisher.rb

@@ -159,7 +159,6 @@ def self.complete(_state)
       initializer :set_routes_reloader_hook do |app|
         reloader = routes_reloader
         reloader.eager_load = app.config.eager_load
-        reloader.execute
         reloaders << reloader
 
         app.reloader.to_run do
@@ -177,7 +176,12 @@ def self.complete(_state)
           ActiveSupport.run_load_hooks(:after_routes_loaded, self)
         end
 
-        ActiveSupport.run_load_hooks(:after_routes_loaded, self)
+        if reloader.eager_load
+          reloader.execute
+          ActiveSupport.run_load_hooks(:after_routes_loaded, self)
+        elsif reloader.loaded
+          ActiveSupport.run_load_hooks(:after_routes_loaded, self)
+        end
       end
 
       # Set clearing dependencies after the finisher hook to ensure paths

railties/lib/rails/application/routes_reloader.rb

@@ -7,7 +7,7 @@ class Application
     class RoutesReloader
       include ActiveSupport::Callbacks
 
-      attr_reader :route_sets, :paths, :external_routes
+      attr_reader :route_sets, :paths, :external_routes, :loaded
       attr_accessor :eager_load
       attr_writer :run_after_load_paths # :nodoc:
       delegate :execute_if_updated, :execute, :updated?, to: :updater
@@ -17,9 +17,11 @@ def initialize
         @route_sets = []
         @external_routes = []
         @eager_load = false
+        @loaded = false
       end
 
       def reload!
+        @loaded = true
         clear!
         load_paths
         finalize!
@@ -28,6 +30,13 @@ def reload!
         revert
       end
 
+      def execute_unless_loaded
+        unless @loaded
+          execute
+          true
+        end
+      end
+
     private
       def updater
         @updater ||= begin

railties/lib/rails/commands/routes/routes_command.rb

@@ -23,6 +23,7 @@ def invoke_command(*)
       desc "routes", "List all the defined routes"
       def perform(*)
         boot_application!
+        Rails.application.routes_reloader.execute_unless_loaded
         require "action_dispatch/routing/inspector"
 
         say inspector.format(formatter, routes_filter)

railties/lib/rails/engine.rb

@@ -349,6 +349,7 @@ module Rails
   #   config.railties_order = [Blog::Engine, :main_app, :all]
   class Engine < Railtie
     autoload :Configuration, "rails/engine/configuration"
+    autoload :RouteSet,      "rails/engine/route_set"
 
     class << self
       attr_accessor :called_from, :isolated
@@ -544,7 +545,7 @@ def env_config
     # Defines the routes for this engine. If a block is given to
     # routes, it is appended to the engine.
     def routes(&block)
-      @routes ||= ActionDispatch::Routing::RouteSet.new_with_config(config)
+      @routes ||= RouteSet.new_with_config(config)
       @routes.append(&block) if block_given?
       @routes
     end
@@ -599,6 +600,7 @@ def load_seed
         app.routes_reloader.paths.unshift(*routing_paths)
         app.routes_reloader.route_sets << routes
         app.routes_reloader.external_routes.unshift(*external_paths)
+        routes.application = app
       end
     end
 

railties/lib/rails/engine/route_set.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+# :markup: markdown
+
+require "action_dispatch/routing/route_set"
+
+module Rails
+  class Engine
+    class RouteSet < ActionDispatch::Routing::RouteSet # :nodoc:
+      attr_accessor :application
+
+      def generate_url_helpers(supports_path)
+        application = self.application
+
+        method_missing = Module.new do
+          define_method(:method_missing) do |method_name, *args, &block|
+            if application && application.initialized? && application.routes_reloader.execute_unless_loaded
+              ActiveSupport.run_load_hooks(:after_routes_loaded, application)
+              public_send(method_name, *args, &block)
+            else
+              super(method_name, *args, &block)
+            end
+          end
+        end
+        super.tap do |mod|
+          mod.singleton_class.prepend(method_missing)
+        end
+      end
+    end
+  end
+end

railties/lib/rails/rack.rb

@@ -2,6 +2,7 @@
 
 module Rails
   module Rack
-    autoload :Logger, "rails/rack/logger"
+    autoload :Logger,     "rails/rack/logger"
+    autoload :LoadRoutes, "rails/rack/load_routes"
   end
 end

railties/lib/rails/rack/load_routes.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+module Rails
+  module Rack
+    class LoadRoutes < ActionDispatch::Once # :nodoc:
+      def initialize(app, routes_reloader)
+        super(app, proc { routes_reloader.execute_unless_loaded })
+      end
+    end
+  end
+end

railties/test/application/rack/load_routes_test.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+require "isolation/abstract_unit"
+require "active_support/log_subscriber/test_helper"
+require "rack/test"
+
+module ApplicationTests
+  module RackTests
+    class LoggerTest < ActiveSupport::TestCase
+      include ActiveSupport::Testing::Isolation
+      include ActiveSupport::LogSubscriber::TestHelper
+      include Rack::Test::Methods
+
+      setup do
+        build_app
+        require "#{app_path}/config/environment"
+      end
+
+      teardown do
+        teardown_app
+      end
+
+      test "loads routes on request" do
+        assert_equal(false, Rails.application.routes_reloader.loaded)
+
+        get "/blah"
+
+        assert_equal(true, Rails.application.routes_reloader.loaded)
+      end
+    end
+  end
+end

railties/test/isolation/abstract_unit.rb

@@ -265,6 +265,7 @@ def self.name; "RailtiesTestApp"; end
       @app.config.active_support.deprecation = :log
       @app.config.log_level = :info
       @app.config.secret_key_base = "b3c631c314c0bbca50c1b2843150fe33"
+      @app.config.middleware.delete Rails::Rack::LoadRoutes
 
       yield @app if block_given?
       @app.initialize!