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

[John Firebaugh <john.firebaugh@gmail.com>]

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 https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/4554
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.

Thoughts?