gistfile1.builder

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

  def call(env)
    background(env) { ... }
    @app.call env
  end

  def background(env, &block)
    if env['rack.run_once'] then yield
    elsif defined? EventMachine then EventMachine.next_tick(&block)
    elsif env['rack.multithread'] or not supports_fork? then Thread.new(&block)
    else fork(&block)
    end
  end

  def supports_fork?
    return @@supports_fork unless @@supports_fork.nil?
    @@supports_fork = begin
      fork { }
      true
    rescue NotImplementedError
      false
    end
  end
end