Cancellation with Future and Promise in Scala

Question

This is a followup to my previous question.

Suppose I have a task, which executes an interruptible blocking call. I would like to run it as a Future and cancel it with failure method of Promise.

I would like the cancel to work as follows:

• If one cancels the task before it finished I would like the task to finish "immediately", interrupting the blocking call if it has already started and I would like the Future to invoke onFailure.

• If one cancels the task after the task finished I would like to get a status saying that the cancel failed since the task already finished.

Does it make sense? Is it possible to implement in Scala? Are there any examples of such implementations?

Answer 1

scala.concurrent.Future is read-only, so one reader cannot mess things up for the other readers.

It seems like you should be able to implement what you want as follows:

def cancellableFuture[T](fun: Future[T] => T)(implicit ex: ExecutionContext): (Future[T], () => Boolean) = {
  val p = Promise[T]()
  val f = p.future
  p tryCompleteWith Future(fun(f))
  (f, () => p.tryFailure(new CancellationException))
}

val (f, cancel) = cancellableFuture( future => {
  while(!future.isCompleted) continueCalculation // isCompleted acts as our interrupted-flag

  result  // when we're done, return some result
})

val wasCancelled = cancel() // cancels the Future (sets its result to be a CancellationException conditionally)

Answer 2

Here is the interruptable version of Victor's code per his comments (Victor, please correct me if I misinterpreted).

object CancellableFuture extends App {

  def interruptableFuture[T](fun: () => T)(implicit ex: ExecutionContext): (Future[T], () => Boolean) = {
    val p = Promise[T]()
    val f = p.future
    val aref = new AtomicReference[Thread](null)
    p tryCompleteWith Future {
      val thread = Thread.currentThread
      aref.synchronized { aref.set(thread) }
      try fun() finally {
        val wasInterrupted = (aref.synchronized { aref getAndSet null }) ne thread
        //Deal with interrupted flag of this thread in desired
      }
    }

    (f, () => {
      aref.synchronized { Option(aref getAndSet null) foreach { _.interrupt() } }
      p.tryFailure(new CancellationException)
    })
  }

  val (f, cancel) = interruptableFuture[Int] { () =>
    val latch = new CountDownLatch(1)

    latch.await(5, TimeUnit.SECONDS)    // Blocks for 5 sec, is interruptable
    println("latch timed out")

    42  // Completed
  }

  f.onFailure { case ex => println(ex.getClass) }
  f.onSuccess { case i => println(i) }

  Thread.sleep(6000)   // Set to less than 5000 to cancel

  val wasCancelled = cancel()

  println("wasCancelled: " + wasCancelled)
}

With Thread.sleep(6000) the output is:

latch timed out
42
wasCancelled: false

With Thread.sleep(1000) the output is:

wasCancelled: true
class java.util.concurrent.CancellationException

Answer 3

Twitter's futures implement cancellation. Have a look here:

https://github.com/twitter/util/blob/master/util-core/src/main/scala/com/twitter/util/Future.scala

Line 563 shows the abstract method responsible for this. Scala's futures currently do not support cancellation.

Answer 4

You can use Monix library instead of Future

https://monix.io