rack middleware parse request parameters twice

No, it's not.

Test something like:

    Rack::Request.any_instance.expects(:POST).once.returns({})
    @dispatcher.call({"REQUEST_METHOD" => "POST", "rack.input" => StringIO.new(""), "REQUEST_URI" => "/"})

where dispatcher is ActionController::Dispatcher. I'll provide normal test case later.

The problem because of Rack::MethodOverride has that code

        req = Request.new(env)
        method = req.POST[METHOD_OVERRIDE_PARAM_KEY] ||
          env[HTTP_METHOD_OVERRIDE_HEADER]

It uses Rack::Request, instead of ActionController::Request. ActionController::Request saves itself into rack env, so doesn't parse arguments twice. Rack::Request parses arguments every time POST is invoked.

To fix that problem locally, I monkey-patched Rack::MethodOverride to use ActionController::Request.

    def POST
      if @env["rack.request.form_input"].eql? @env["rack.input"]
        @env["rack.request.form_hash"]
      elsif form_data?
        @env["rack.request.form_input"] = @env["rack.input"]
        unless @env["rack.request.form_hash"] =
            Utils::Multipart.parse_multipart(env)
          form_vars = @env["rack.input"].read

          # Fix for Safari Ajax postings that always append \0
          form_vars.sub!(/\0\z/, '')

          @env["rack.request.form_vars"] = form_vars
          @env["rack.request.form_hash"] = Utils.parse_query(form_vars)

          begin
            @env["rack.input"].rewind if @env["rack.input"].respond_to?(:rewind)
          rescue Errno::ESPIPE
            # Handles exceptions raised by input streams that cannot be rewound
            # such as when using plain CGI under Apache
          end
        end
        @env["rack.request.form_hash"]
      else
        {}
      end
    end

Rack caches the result from Request#POST to @env["rack.request.form_hash"].