Treating bodies as IO streams is only part of the problem. Today in Rack to stream responses, the rack app returns:

    [-1, {}, []]

Which is interpreted by the app server like Thin, EventMachine as a deferrable response. The problem with this approach is that there's no way to process response headers asynchronously. Consider:

    class Middleware
      def initialize(app)
        @app = app
      end
      
      def call(env)
        status, headers, body = @app.call(env)
        # This wouldn't work because rack returns [-1, {}, []] for an async response.
        if headers['Content-Type'] == 'json'
          Logger.info "Its JSON!"
        end
        status, headers, body
      end
    end

The logger line would never evaluate because of the `[-1, {}, []]` response.

The Goliath web server is the closest thing I've seen in Ruby land that considers these problems (https://github.com/postrank-labs/goliath/wiki/Middleware).

Does anybody know if there's a Rack 2.0 spec in the works that takes care of async request/response cycles?