Allow a custom exceptions app to set.

railties/lib/rails/application.rb

@@ -244,7 +244,7 @@ def default_middleware_stack
         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, ActionDispatch::PublicExceptions.new(Rails.public_path)
+        middleware.use ::ActionDispatch::ShowExceptions, config.exceptions_app || ActionDispatch::PublicExceptions.new(Rails.public_path)
         middleware.use ::ActionDispatch::DebugExceptions
         middleware.use ::ActionDispatch::RemoteIp, config.action_dispatch.ip_spoofing_check, config.action_dispatch.trusted_proxies
 

railties/lib/rails/application/configuration.rb

@@ -8,7 +8,7 @@ class Application
     class Configuration < ::Rails::Engine::Configuration
       attr_accessor :allow_concurrency, :asset_host, :asset_path, :assets,
                     :cache_classes, :cache_store, :consider_all_requests_local,
-                    :dependency_loading, :file_watcher, :filter_parameters,
+                    :dependency_loading, :exceptions_app, :file_watcher, :filter_parameters,
                     :force_ssl, :helpers_paths, :logger, :log_tags, :preload_frameworks,
                     :railties_order, :relative_url_root, :reload_plugins, :secret_token,
                     :serve_static_assets, :ssl_options, :static_cache_control, :session_options,
@@ -40,6 +40,7 @@ def initialize(*)
         @relative_url_root             = ENV["RAILS_RELATIVE_URL_ROOT"]
         @reload_classes_only_on_change = true
         @file_watcher                  = ActiveSupport::FileUpdateChecker
+        @exceptions_app                = nil
 
         @assets = ActiveSupport::OrderedOptions.new
         @assets.enabled                  = false

railties/test/application/middleware/exceptions_test.rb

@@ -45,6 +45,20 @@ def call(env)
       assert_equal 404, last_response.status
     end
 
+    test "uses custom exceptions app" do
+      add_to_config <<-RUBY
+        config.exceptions_app = lambda do |env|
+          ["404", { "Content-Type" => "text/plain" }, ["YOU FAILED BRO"]]
+        end
+      RUBY
+
+      app.config.action_dispatch.show_exceptions = true
+
+      get "/foo"
+      assert_equal 404, last_response.status
+      assert_equal "YOU FAILED BRO", last_response.body
+    end
+
     test "unspecified route when action_dispatch.show_exceptions is not set raises an exception" do
       app.config.action_dispatch.show_exceptions = false