class Rack::Lock

def call(env)
  @mutex.lock
  @env = env
  @old_rack_multithread = env[RACK_MULTITHREAD]
  begin
    response = @app.call(env.merge!(RACK_MULTITHREAD => false))
    returned = response << BodyProxy.new(response.pop) { unlock }
  ensure
    unlock unless returned
  end
end