Bug Enumerator (StackOverflowError in a Promise)

bugStackOverflowErrorPromise.scala

package bugs

import play.api.libs.iteratee._
import scala.util.Failure
import scala.util.Success
import scala.concurrent.Future
import scala.concurrent.Promise

object StackOverflowErrorBug {
  import scala.concurrent.ExecutionContext.Implicits.global

  trait Cursor {
    def iterator :Iterator[String]

    def hasNext :Boolean

    def next :Future[Cursor]

    def i: Int = 0

    def n(cursor: Cursor) =
      if(cursor.iterator.hasNext) {
        Future(Some((cursor,Some(cursor.iterator.next))))
      } else if (cursor.hasNext) {
        val fut = //cursor.next.map(c => Some((c,None)))
          Future(Some(DefaultCursor(cursor.i + 1) -> None))
        print(fut + ",")
        fut
      } else {
        Future(None)
      }

    def enumerate = {
      CustomEnumerator.unfoldM(this) { cursor =>
        n(cursor)
      }.andThen(Enumerator.eof).onDoneEnumerating{
        println("done")
      } &> Enumeratee.collect {
        case Some(e) => e
      }
    }

    object CustomEnumerator {
      def unfoldM[S,E](s:S)(f: S => Future[Option[(S,E)]] ): Enumerator[E] = checkContinue1(s)(new TreatCont1[E,S]{

        def apply[A](loop: (Iteratee[E,A],S) => Future[Iteratee[E,A]], s:S, k: Input[E] => Iteratee[E,A]):Future[Iteratee[E,A]] = f(s).flatMap {
          case Some((newS,e)) => {
            // if we don't create this intermediate promise, then a stackoverflowerror is eventually thrown
            // original code ->
            // loop(k(Input.El(e)),newS)
            // <- original code
            val promise = Promise[play.api.libs.iteratee.Iteratee[E,A]]()
            loop(k(Input.El(e)),newS).onComplete {
              case Success(s) =>
                promise.success(s)
              case Failure(f) =>
                promise.failure(f)
            }
            promise.future
          }
          case None => Future(Cont(k))
        }
      })

      trait TreatCont1[E,S]{

        def apply[A](loop: (Iteratee[E,A],S) => Future[Iteratee[E,A]], s:S, k: Input[E] => Iteratee[E,A]):Future[Iteratee[E,A]]

      }

      def checkContinue1[E,S](s:S)(inner:TreatCont1[E,S]) = new Enumerator[E] {

        def apply[A](it: Iteratee[E, A]): Future[Iteratee[E, A]] = {

          def step(it: Iteratee[E, A], state:S): Future[Iteratee[E,A]] = it.fold{
              case Step.Done(a, e) => Future(Done(a,e))
              case Step.Cont(k) => inner[A](step,state,k)
              case Step.Error(msg, e) => Future(Error(msg,e))
          }
          step(it,s)
        }

      }

      def loop(cursor: Cursor) :Future[Option[(Option[String], Cursor)]] = {
        if(cursor.iterator.hasNext)
          Future(Some(Some(cursor.iterator.next) -> cursor))
        else if(cursor.hasNext)
          cursor.next.map(c => Some(None -> c))
        else Future(None)
      }
    }
  }

  case class DefaultCursor(override val i: Int) extends Cursor {
    val iterator = {
      val r = (for(j <- 0 to 1) yield i + "" + j)
      r.toIterator
    }

    def hasNext = i < 5000

    def next = {
      Future(DefaultCursor(i + 1))
    }
  }

  case class FlattenedCursor(cursor: Future[Cursor]) extends Cursor {
    val iterator = Iterator.empty

    def hasNext = true

    def next = cursor
  }

  // should print "done: <some result>" at the end
  def test = {
    val enumerator = FlattenedCursor(Future(DefaultCursor(0))).enumerate
    val fut = enumerator.apply(Iteratee.foreach({ e =>
      //println(e)
    }))
    val ff = Iteratee.flatten(fut).run

    ff.onComplete {
      case e =>
        println("done: " + e)
    }
  }
}

build.sbt

name := "TestIteratees"

version := "1.0"

scalaVersion := "2.10.0-RC1"

resolvers += "Typesafe repository snapshots" at "http://repo.typesafe.com/typesafe/snapshots/"

resolvers += "Typesafe repository releases" at "http://repo.typesafe.com/typesafe/releases/"

libraryDependencies ++= Seq(
  "play" % "play-iteratees_2.10" % "2.1-RC1"
)

not minimal enough? here might be a smaller one:

def tata(f:Future[Int]):Future[Int] = f.flatMap(i => if (i < 0)  tata(Future(i+1)) else Future(i))
// small enough number, -750 is enough in the console
tata(-1000)

java.lang.IllegalStateException: Promise already completed.
at scala.concurrent.Promise$class.complete(Promise.scala:55)
at scala.concurrent.impl.Promise$DefaultPromise.complete(Promise.scala:58)
at scala.concurrent.Promise$class.failure(Promise.scala:107)
at scala.concurrent.impl.Promise$DefaultPromise.failure(Promise.scala:58)
at scala.concurrent.Future$$anonfun$flatMap$1.liftedTree3$1(Future.scala:283)
at scala.concurrent.Future$$anonfun$flatMap$1.apply(Future.scala:277)
at scala.concurrent.Future$$anonfun$flatMap$1.apply(Future.scala:274)
at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:29)
at scala.concurrent.forkjoin.ForkJoinTask$AdaptedRunnableAction.exec(ForkJoinTask.java:1417)
at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:262)
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.pollAndExecAll(ForkJoinPool.java:915)
at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:980)
at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1478)
at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:104)

You're right :)