rack_drain.rb

require 'thread'
class RackDrain
  @@queue = Queue.new

  def self.start_request
    self.queue << :request
  end

  def self.end_request
    self.queue.pop
  end

  def self.queue
    @@queue
  end

  def self.drain
    @block_requests = true
    loop do
      break if queue.empty?
      sleep 0.01
    end
  ensure
    @block_requests = false
  end

  def self.block_requests?
    @block_requests
  end

  def initialize(app)
    @app = app
  end

  def call(env)
    RackDrain.start_request
    return block_request if RackDrain.block_requests?
    @app.call(env)
  ensure
    RackDrain.end_request
  end

  def block_request
    [503, {}, []]
  end
end