Stop mutating body response

If @app.call returns an object that is saved (for e.g., in a constant), the mutation results in a continuing cycle of wrapping the body in Rack::BodyProxy, eventually leading to SystemStackError
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
On branch fix-return-response-mutation-rack-logger - Tue  3 Apr 2018 19:54:28 PDT by Geoff Lee <geoff.lee@lendesk.com>

railties/lib/rails/rack/logger.rb

@@ -35,9 +35,9 @@ module Rails
           instrumenter = ActiveSupport::Notifications.instrumenter
           instrumenter.start "request.action_dispatch", request: request
           logger.info { started_request_message(request) }
-          resp = @app.call(env)
-          resp[2] = ::Rack::BodyProxy.new(resp[2]) { finish(request) }
-          resp
+          status, headers, body = @app.call(env)
+          body = ::Rack::BodyProxy.new(body) { finish(request) }
+          [status, headers, body]
         rescue Exception
           finish(request)
           raise

railties/test/rack_logger_test.rb

@@ -14,14 +14,21 @@ module Rails
 
         attr_reader :logger
 
-        def initialize(logger = NULL, taggers = nil, &block)
-          super(->(_) { block.call; [200, {}, []] }, taggers)
+        def initialize(logger = NULL, app: nil, taggers: nil, &block)
+          app ||= ->(_) { block.call; [200, {}, []] }
+          super(app, taggers)
           @logger = logger
         end
 
         def development?; false; end
       end
 
+      class TestApp < Struct.new(:response)
+        def call(_env)
+          response
+        end
+      end
+
       Subscriber = Struct.new(:starts, :finishes) do
         def initialize(starts = [], finishes = [])
           super
@@ -72,6 +79,15 @@ module Rails
           end
         end
       end
+
+      def test_logger_does_not_mutate_app_return
+        response = []
+        app = TestApp.new(response)
+        logger = TestLogger.new(app: app)
+        assert_no_changes('response') do
+          logger.call('REQUEST_METHOD' => 'GET')
+        end
+      end
     end
   end
 end