Future's flatmap holds on to too much in it's closure... here's a simple fixed. Based on code from: https://github.com/scala/scala/blob/1f4a52b4ed9457863e00fe16d18705b6c6cd5db9/src/library/scala/concurrent/Future.scala#L271

gistfile1.scala

  def flatMap[S](f: T => Future[S])(implicit executor: ExecutionContext): Future[S] = {
    val p = Promise[S]()

    onComplete {
      case f: Failure[_] => p complete f.asInstanceOf[Failure[S]]
      case Success(v) => {
        val tempP = p
        try {
          f(v).onComplete({
            case f: Failure[_] => tempP complete f.asInstanceOf[Failure[S]]
            case Success(v) => tempP success v
          })(internalExecutor)
        } catch {
          case NonFatal(t) => tempP failure t
        }
      }
    }(executor)

    p.future
  }