Monkey patch Rack::Lint to allow string subclass body

actionpack/lib/action_controller/integration.rb

@@ -1,6 +1,7 @@
 require 'stringio'
 require 'uri'
 require 'active_support/test_case'
+require 'action_controller/rack_lint_patch'
 
 module ActionController
   module Integration #:nodoc:

actionpack/lib/action_controller/rack_lint_patch.rb

@@ -0,0 +1,36 @@
+# Rack 1.0 does not allow string subclass body. This does not play well with our ActionView::SafeBuffer.
+# The next release of Rack will be allowing string subclass body - http://github.com/rack/rack/commit/de668df02802a0335376a81ba709270e43ba9d55
+# TODO : Remove this monkey patch after the next release of Rack
+
+module RackLintPatch
+  module AllowStringSubclass
+    def self.included(base)
+      base.send :alias_method, :each, :each_with_hack
+    end
+
+    def each_with_hack
+      @closed = false
+
+      @body.each { |part|
+        assert("Body yielded non-string value #{part.inspect}") {
+          part.kind_of?(String)
+        }
+        yield part
+      }
+
+      if @body.respond_to?(:to_path)
+        assert("The file identified by body.to_path does not exist") {
+          ::File.exist? @body.to_path
+        }
+      end
+    end
+  end
+
+  begin
+    app = proc {|env| [200, {"Content-Type" => "text/plain", "Content-Length" => "12"}, [Class.new(String).new("Hello World!")]] }
+    response = Rack::MockRequest.new(Rack::Lint.new(app)).get('/')
+  rescue Rack::Lint::LintError => e
+    raise(e) unless e.message =~ /Body yielded non-string value/
+    Rack::Lint.send :include, AllowStringSubclass
+  end
+end

actionpack/test/controller/integration_test.rb

@@ -443,3 +443,23 @@ def test_failed_get
     assert_equal '', response.body
   end
 end
+
+class StringSubclassBodyTest < ActionController::IntegrationTest
+  class SafeString < String
+  end
+
+  class SafeStringMiddleware
+    def self.call(env)
+      [200, {"Content-Type" => "text/plain", "Content-Length" => "12"}, [SafeString.new("Hello World!")]]
+    end
+  end
+
+  def setup
+    @integration_session = ActionController::Integration::Session.new(SafeStringMiddleware)
+  end
+
+  def test_string_subclass_body
+    get '/'
+    assert_equal 'Hello World!', response.body
+  end
+end