Bring config.allow_concurrency back

Since the Rack::Lock still exists in development,
let's provide a way to disable it explicitly.

railties/lib/rails/application.rb

@@ -305,22 +305,8 @@ def reload_dependencies? #:nodoc:
     def default_middleware_stack #:nodoc:
       ActionDispatch::MiddlewareStack.new.tap do |middleware|
         app = self
-        if rack_cache = config.action_dispatch.rack_cache
-          begin
-            require 'rack/cache'
-          rescue LoadError => error
-            error.message << ' Be sure to add rack-cache to your Gemfile'
-            raise
-          end
-
-          if rack_cache == true
-            rack_cache = {
-              metastore: "rails:/",
-              entitystore: "rails:/",
-              verbose: false
-            }
-          end
 
+        if rack_cache = load_rack_cache
           require "action_dispatch/http/rack_cache"
           middleware.use ::Rack::Cache, rack_cache
         end
@@ -337,12 +323,14 @@ def default_middleware_stack #:nodoc:
           middleware.use ::ActionDispatch::Static, paths["public"].first, config.static_cache_control
         end
 
-        middleware.use ::Rack::Lock unless config.cache_classes
+        middleware.use ::Rack::Lock unless allow_concurrency?
         middleware.use ::Rack::Runtime
         middleware.use ::Rack::MethodOverride
         middleware.use ::ActionDispatch::RequestId
-        middleware.use ::Rails::Rack::Logger, config.log_tags # must come after Rack::MethodOverride to properly log overridden methods
-        middleware.use ::ActionDispatch::ShowExceptions, config.exceptions_app || ActionDispatch::PublicExceptions.new(Rails.public_path)
+
+        # Must come after Rack::MethodOverride to properly log overridden methods
+        middleware.use ::Rails::Rack::Logger, config.log_tags
+        middleware.use ::ActionDispatch::ShowExceptions, show_exceptions_app
         middleware.use ::ActionDispatch::DebugExceptions, app
         middleware.use ::ActionDispatch::RemoteIp, config.action_dispatch.ip_spoofing_check, config.action_dispatch.trusted_proxies
 
@@ -368,6 +356,40 @@ def default_middleware_stack #:nodoc:
       end
     end
 
+    def allow_concurrency?
+      if config.allow_concurrency.nil?
+        config.cache_classes
+      else
+        config.allow_concurrency
+      end
+    end
+
+    def load_rack_cache
+      rack_cache = config.action_dispatch.rack_cache
+      return unless rack_cache
+
+      begin
+        require 'rack/cache'
+      rescue LoadError => error
+        error.message << ' Be sure to add rack-cache to your Gemfile'
+        raise
+      end
+
+      if rack_cache == true
+        {
+          metastore: "rails:/",
+          entitystore: "rails:/",
+          verbose: false
+        }
+      else
+        rack_cache
+      end
+    end
+
+    def show_exceptions_app
+      config.exceptions_app || ActionDispatch::PublicExceptions.new(Rails.public_path)
+    end
+
     def build_original_fullpath(env) #:nodoc:
       path_info    = env["PATH_INFO"]
       query_string = env["QUERY_STRING"]

railties/lib/rails/application/configuration.rb

@@ -5,7 +5,7 @@
 module Rails
   class Application
     class Configuration < ::Rails::Engine::Configuration
-      attr_accessor :asset_host, :assets, :autoflush_log,
+      attr_accessor :allow_concurrency, :asset_host, :assets, :autoflush_log,
                     :cache_classes, :cache_store, :consider_all_requests_local, :console,
                     :eager_load, :exceptions_app, :file_watcher, :filter_parameters,
                     :force_ssl, :helpers_paths, :logger, :log_formatter, :log_tags,
@@ -20,6 +20,7 @@ class Configuration < ::Rails::Engine::Configuration
       def initialize(*)
         super
         self.encoding = "utf-8"
+        @allow_concurrency             = nil
         @consider_all_requests_local   = false
         @filter_parameters             = []
         @filter_redirect               = []

railties/test/application/middleware_test.rb

@@ -96,6 +96,12 @@ def app
       assert !middleware.include?("Rack::Lock")
     end
 
+    test "removes lock if allow concurrency is set" do
+      add_to_config "config.allow_concurrency = true"
+      boot!
+      assert !middleware.include?("Rack::Lock")
+    end
+
     test "removes static asset server if serve_static_assets is disabled" do
       add_to_config "config.serve_static_assets = false"
       boot!