Getting a timeout error when testing a stream with mapAsync stage

[Jakub Kahovec]

Hi,

when using Akka Streams (2.5.9)  I've recently bumped into a problem when testing a stream with a mapAsync stage. It unexpectedly ends with an assertion failing on "timeout (3 seconds) during expectMsg while waiting for OnComplete"

To demonstrate it I've created a simple example. 

The code below works as expected.

val sourceUnderTest = Source(1 to 2).mapAsync(2)(i => Future.successful(i * 2))

val c = TestSubscriber.manualProbe[Int]()
val p = sourceUnderTest.to(Sink.fromSubscriber(c)).run()
val sub = c.expectSubscription()

sub.request(2)
c.expectNextN(2)   // List(2,4)
c.expectComplete()   // akka.stream.testkit.TestSubscriber$ManualProbe@fc258b1

However, when I add another mapping function (bold code), the test fails with a timeout. When I call additional c.request(1) (commented code) it ends correctly. So it looks like the mapping function adds an additional item into the stream, which seems strange.

  

val sourceUnderTest = Source(1 to 2).mapAsync(2)(i => Future.successful(i * 2).map(identity))

val c = TestSubscriber.manualProbe[Int]()
val p = sourceUnderTest.to(Sink.fromSubscriber(c)).run()
val sub = c.expectSubscription()

sub.request(2)
c.expectNextN(2)   // List(2,4)
// c.request(1)
c.expectComplete()   // ends with java.lang.AssertionError: assertion failed: timeout (3 seconds) during expectMsg while waiting for OnComplete

Can anyone explain this strange behaviour ?

Thanks

Jakub

[Patrik Nordwall]

It is allowed to ”delay” delivery of the complete signal when there is no demand. This behavior is undefined, i.e. some stages deliver it immediately, some only when demand is requested. This is as expected and in such test you have to request enough to be sure to get the completed signal.

/Patrik

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[Jakub Kahovec]

I see. Thank you Patrik for the explanation.

Jakub

[Jakub Kahovec]

I had another thought about this behaviour and two things came to my mind. First, it might be worth mentioning this somewhere in the documentation about testing as other users might also experience this and I think didn't see this mentioned anywhere in the docs. And second, is it safe to use expectComplete at all if the behaviour is undefined and any such change might break the tests unexpectedly ?

Jakub

On Friday, February 16, 2018 at 9:55:45 PM UTC+1, Patrik Nordwall wrote:

[Patrik Nordwall]

Doc improvement is welcome. If you add request(1) before the expectComplete it’s not undefined, then you are supposed to get it.

/Patrik

[Patrik Nordwall]

btw, probably easier to use .runWith(TestSink.probe) in that example, or .runWith(Sink.seq).futureValue

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Patrik Nordwall
Akka Tech Lead
Lightbend -  Reactive apps on the JVM
Twitter: @patriknw

[Jakub Kahovec]

Thank you. So I'll add the request(1) before the expectComplete(). I actually use the .runWih(TesSink.probe) in my test, I used the TestSubscriber.manualProbe just in the demonstration example to be able to print the intermediate results.

Jakub

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