Rack API is awkward

class FooApplication
  class ErbPage
    def to_a
      head = "the head tag"
      sleep(2)
      body = "the body tag"
      sleep(2)
      [head, body]
    end
  end

  def call(env)
    [200, {}, ErbPage.new.to_a]
  end
end

class FakeRack
  def serve(application)
    status, headers, body = application.call({})
    p :status  => status
    p :headers => headers

    body.each do |string|
      p string
    end

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

app  = FooApplication.new
rack = FakeRack.new

rack.serve app

$ time ruby foo.rb                                       
{:status=>200}
{:headers=>{}}
"the head tag"
"the body tag"

real    0m4.008s
user    0m0.003s
sys     0m0.003s

class ResponseTimer
  def initialize(app)
    @app = app
  end

  def call(env)
    now                        = Time.now
    status, headers, body      = @app.call(env)
    headers['X-Response-Took'] = Time.now - now

    [status, headers, body]
  end
end

app   = FooApplication.new
timer = ResponseTimer.new app
rack  = FakeRack.new

rack.serve timer

$ time ruby foo.rb 
{:status=>200}
{:headers=>{"X-Response-Took"=>3.999937}}
"the head tag"
"the body tag"

real    0m4.010s
user    0m0.004s
sys     0m0.004s

class FooApplication
  class ErbPage
    def each
      head = "the head tag"
      yield head

      sleep(2)

      body = "the body tag"
      yield body

      sleep(2)
    end
  end

  def call(env)
    [200, {}, ErbPage.new]
  end
end

$ time ruby foo.rb
{:status=>200}
{:headers=>{"X-Response-Took"=>1.1e-05}}
"the head tag"
"the body tag"

real    0m4.032s
user    0m0.027s
sys     0m0.016s

class ResponseTimer
  def initialize(app)
    @app = app
  end

  def call(env)
    now                        = Time.now
    status, headers, body      = @app.call(env)

    newbody = []
    body.each { |str| newbody << str }
    headers['X-Response-Took'] = Time.now - now

    [status, headers, newbody]
  end
end

class ResponseTimer
  class TimerProxy
    def initialize(body)
      @now     = Time.now
      @body    = body
    end

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

      $stderr.puts({'X-Response-Took' => (Time.now - @now)})
    end

    def each(&block)
      @body.each(&block)
    end
  end

  def initialize(app)
    @app = app
  end

  def call(env)
    status, headers, body = @app.call(env)

    [status, headers, TimerProxy.new(body)]
  end
end

$ time ruby foo.rb 
{:status=>200}
{:headers=>{}}
"the head tag"
"the body tag"
{"X-Response-Took"=>4.000268}

real    0m4.044s
user    0m0.029s
sys     0m0.015s

class ResponseTimer
  def initialize(app)
    @app = app
  end

  def call(env)
    status, headers, body      = @app.call(env)
    body.extend(Module.new {
      now = Time.now

      define_method(:close) do
        super if defined?(super)

        $stderr.puts({'X-Response-Took' => (Time.now - now)})
      end
    })

    [status, headers, body]
  end
end

class EndOfLife
  attr_reader :callbacks

  def initialize(app)
    @app       = app
    @callbacks = []
  end

  def call(env)
    status, headers, body = @app.call(env)
    body.extend(Module.new {
      attr_accessor :eol

      def close
        super if defined?(super)
        eol.callbacks.each { |cb| cb.call }
      end
    })
    body.eol = self

    [status, headers, body]
  end
end

app = FooApplication.new
eol = EndOfLife.new app
eol.callbacks << lambda { puts "it finished!" }

rack  = FakeRack.new

rack.serve eol