Rack::Response#body vs ActionDispatch::Response#body

The definitions of #body in Rack::Response and ActionDispatch::Response are competing and causing performance problems with streaming responses. Rack::Response provides attr_accessor :body, while ActionDispatch::Response overrides that as follows:

    def body       str = ''       each { |part| str << part.to_s }       str     end

This is problematic in instances where a streaming body is used, due to the definition of Rack::Response#close:

    def close       body.close if body.respond_to?(:close)     end

Given these definitions, executing the Rack protocol on an ActionDispatch::Response instance (calling '#each', then '#close') causes the body to be enumerated twice -- once via the expected call to #each, and a second time via the call chain #close -> #body -> #each. See #4554 render :text => proc { ... } regression - Ruby on Rails - rails for a user report of this problem.

This could be fixed by using the instance variable directly:

    def close       @body.close if @body.respond_to?(:close)     end

However, I think ActionDispatch::Response's #body override is dangerous and violates the principle of least surprise by removing the symmetry between #body and #body=. IMHO, it would be better to remove the override. If the concatenating behavior is necessary, supply a method named #body_text or a similar name that makes it clear that it disables streaming. This option causes numerous Rails test failures, however, as many tests are written to expect #body to return a string.


I agree that would fix this particular issue. My reluctance with that is that AD::Response#body would remain a dangerous method on its own, because it does not obey the semantics established by the base class. It's a Liskov Substitution Principle violation -- AD::Response cannot be safely substituted where Rack::Response is expected.

ActionDispatch::Response has two modes:

  • A mode for building up a response that will then be passed to a Rack server. The only interface on this object is #each
  • A mode for taking a tuple of [status, headers, body] and making it available for inspection using the Response API. The #body method is for the second mode, which is mostly used in testing and debugging. The fact that #close calls the #body method is a mistake, causing a (slow) debugging method to get called in the (fast) Rack response mode.

We should probably be using a different method for the debugging version of body, if Rack is calling #close on the result from #body (maybe something like string_body). Alternatively, we may want to create a debugging response object, and a to_something method to convert a response to that, but I worry that it has untold potential for breakage.

Yehuda Katz Architect | Strobe (ph) 718.877.1325