Flaky futures test

[Lukas Rytz]

Hi,

The test jvm/t8689.scala fails from time to time (for example, see here: https://scala-ci.typesafe.com/job/scala-2.11.x-validate-test/481/console)

It was added in https://github.com/scala/scala/pull/4289/files

Any ideas?

Lukas

[Jason Zaugg]

I can reproduce the failure locally:

scala> object Test {
     |   def main(args: Array[String]): Unit = {
     |     import scala.concurrent._
     |     import ExecutionContext.Implicits.global
     |     val source1 = Promise[Int]()
     |     val source2 = Promise[Int]()
     |     var done = new java.util.concurrent.atomic.AtomicBoolean(false)
     |     source2.completeWith(source1.future).future.onComplete {
     |       case _ => done.set(true)
     |     }
     |     source2.tryFailure(new TimeoutException)
     |     source1.success(123)
     |     assert(done.get())
     |   }
     | }
defined object Test

scala> (1 to 1000) foreach {i => print("."); Test.main(null)}
.....................................java.lang.AssertionError: assertion failed
  at scala.Predef$.assert(Predef.scala:151)
  at Test$.main(<console>:19)
  at $anonfun$1.apply$mcVI$sp(<console>:9)
  at scala.collection.immutable.Range.foreach$mVc$sp(Range.scala:166)
  ... 33 elided

scala> (1 to 1000) foreach {i => print("."); Test.main(null)}
...............java.lang.AssertionError: assertion failed
  at scala.Predef$.assert(Predef.scala:151)
  at Test$.main(<console>:19)
  at $anonfun$1.apply$mcVI$sp(<console>:9)
  at scala.collection.immutable.Range.foreach$mVc$sp(Range.scala:166)
  ... 33 elided

​

-jason

[Lukas Rytz]

thanks. i also tried with a loop, but could not reproduce

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[Viktor Klang]

The problem is the lack of HB edge between the onComplete callback executing the `done.set(true)` and the assertion that it is true.

--

Cheers,

√

[Viktor Klang]

Or rather, that was a bit misworded: There's no guarantee that the callback has executed within the Implicits.globals by the time the current thread executes that assertion.

--

Cheers,

√

[Viktor Klang]

So, since Implicits.global uses Daemon threads the main thread may exit before the thread pool has a chance to output the success-printout.

--

Cheers,

√

[Viktor Klang]

However, the important part of the test is to verify that that callback is executed, so we need to augment it to something like:

+object Test {

+  def main(args: Array[String]): Unit = {

+    import scala.concurrent._

+    import ExecutionContext.Implicits.global

+    val source1 = Promise[Int]()

+    val source2 = Promise[Int]()

+    val done = Promise[Unit]()

+    source2.completeWith(source1.future).future.onComplete {

+      case _ =>

+         print("success")

+         done.success(())

+    }

+    source2.tryFailure(new TimeoutException)

+    source1.success(123)

+    import duration._

+    Await.result(done.future, 30.seconds)

+  }

+}

--

Cheers,

√

[Jason Zaugg]

Great minds think alike :)

-jason