Diverge Metal implementation from Rack::Cascade since we want the last app to return its headers and body if the status is a 404.

railties/lib/rails/rack/cascade.rb

@@ -1,31 +0,0 @@
-require 'active_support/ordered_hash'
-
-module Rails
-  module Rack
-    # Try a request on several apps; return the first non-404 response.
-    class Cascade
-      attr_reader :apps
-
-      def initialize(apps)
-        @apps = ActiveSupport::OrderedHash.new
-        apps.each { |app| add app }
-      end
-
-      def call(env)
-        @apps.keys.each do |app|
-          result = app.call(env)
-          return result unless result[0].to_i == 404
-        end
-        Metal::NotFoundResponse
-      end
-
-      def add(app)
-        @apps[app] = true
-      end
-
-      def include?(app)
-        @apps.include?(app)
-      end
-    end
-  end
-end

railties/lib/rails/rack/metal.rb

@@ -1,17 +1,12 @@
-require 'rails/rack/cascade'
+require 'active_support/ordered_hash'
 
 module Rails
   module Rack
-    module Metal
+    class Metal
       NotFoundResponse = [404, {}, []].freeze
       NotFound = lambda { NotFoundResponse }
 
-      class << self
-        def new(app)
-          Cascade.new(builtins + [app])
-        end
-
-        def builtins
+      def self.metals
         base = "#{Rails.root}/app/metal"
         matcher = /\A#{Regexp.escape(base)}\/(.*)\.rb\Z/
 
@@ -21,6 +16,20 @@ module Rails
           file.classify.constantize
         end
       end
+
+      def initialize(app)
+        @app = app
+        @metals = ActiveSupport::OrderedHash.new
+        self.class.metals.each { |app| @metals[app] = true }
+        freeze
+      end
+
+      def call(env)
+        @metals.keys.each do |app|
+          result = app.call(env)
+          return result unless result[0].to_i == 404
+        end
+        @app.call(env)
       end
     end
   end