Weird Behavior of Scala Future and Thread.sleep

Question

I'm currently writing codes to extend the Future companion object. One function I want to implement is

Any

//returns the future that computes the first value computed from the list. If the first one fails, fail.

def any[T](fs: List[Future[T]]): Future[T] = {

  val p = Promise[T]()



  fs foreach { f => { 

    f onComplete { 

      case Success(v) => p trySuccess v 

      case Failure(e) => p tryFailure e

    } 

  } }



  p.future

}

I tried to test my code with

  test("A list of Futures return only the first computed value") {

    val nums = (0 until 10).toList

    val futures = 

      nums map { n => Future { Thread.sleep(n*1000); n } }



    val v = Await.result(Future.any(futures), Duration.Inf)



    assert(v === 0)

  }

But the returned value is 1, not 0. When I switched sleeping time to

n*1000

to

(n+1)*1000

, it works fine(returns 0).

Is there any special effect when called sleep on 0?

Answer 1

Thread.sleep is a blocking operation in your Future but you are not signaling to the ExecutionContext that you are doing so, so the behavior will vary depending on what ExecutionContext you use and how many processors your machine has. Your code works as expected with ExecutionContext.global if you add blocking:

nums map { n => Future { blocking { Thread.sleep(n*1000); n } } }

Weird Behavior of Scala Future and Thread.sleep

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