What is the right way to test code that uses `Disp...

# kotest

[Osip Fatkullin]

What is the right way to test code that uses Dispatchers.Main from coroutines under the hood?
Environment: I use Kotest 5.6.2 and have enabled coroutineTestScope and timeout = 5.seconds
When I call Dispatchers.setMain right before test, everything is OK:

class MySpec : FeatureSpec({

    val dispatcher = StandardTestDispatcher()

    feature("...") {
        Dispatchers.setMain(dispatcher)

        scenario("...") {
            // It will crash if Main dispatcher was not set before
            launch(Dispatchers.Main) {
                delay(100)
            }.join()

            dispatcher.scheduler shouldBe testCoroutineScheduler // PASSED!
        }

        Dispatchers.resetMain()
    }
})

But when I set Main dispatcher in beforeTest or beforeSpec, I get `TimeoutCancellationException`:

class MySpec : FeatureSpec({

    val dispatcher = StandardTestDispatcher()

    beforeTest { Dispatchers.setMain(dispatcher) }
    afterTest { Dispatchers.resetMain() }

    feature("...") {
        scenario("...") {
            // It will crash if Main dispatcher was not set before
            launch(Dispatchers.Main) {
                delay(100)
            }.join()

            dispatcher.scheduler shouldBe testCoroutineScheduler // PASSED!
        }
    } // CRASH after the test
})

---

Timed out waiting for 5000 ms
kotlinx.coroutines.TimeoutCancellationException: Timed out waiting for 5000 ms
	at app//kotlinx.coroutines.TimeoutKt.TimeoutCancellationException(Timeout.kt:184)
    ...
    at io.kotest.engine.test.TestInvocationInterceptor.intercept(TestInvocationInterceptor.kt:31)
	at io.kotest.engine.test.TestCaseExecutor$execute$3$1.invokeSuspend(TestCaseExecutor.kt:97)
    ...

What am I doing wrong?

[Lukasz Kalnik]

I think it's better using the CoroutineDispatcher provided by the coroutineContext inside the lifecycle hooks (or tests):

beforeTest { Dispatchers.setMain(coroutineContext[CoroutineDispatcher]!!) }

[Lukasz Kalnik]

Although this passes for me as well (note coroutineTestScope = true at the beginning of the spec):

class MySpec : FeatureSpec({

    coroutineTestScope = true

    val dispatcher = StandardTestDispatcher()

    beforeTest { Dispatchers.setMain(dispatcher) }
    afterTest { Dispatchers.resetMain() }

    feature("...") {
        scenario("...") {
            // It will crash if Main dispatcher was not set before
            launch(Dispatchers.Main) {
                delay(100)
            }.join()
        }
    }
})

[Osip Fatkullin]

Sorry, missed your answer.
I think the problem exists only if you have specified timeout:

class KotestConfig : AbstractProjectConfig() {
    override val coroutineTestScope = true
    override val timeout = 5.seconds
}

But I have no idea why

[Lukasz Kalnik]

Maybe, I don't specify timeout in my tests.

[Osip Fatkullin]

My guess is that if I change Main dispatcher, Kotest uses virtual time not only inside tests but for test timeouts too. So it just skips time until TimeoutCancellationException happen.

[Osip Fatkullin]

Interesting. If I specify invocationTimeout instead of timeout, it works without TimeoutCancellationException . The difference I found between these options is that timeout applies to test containers, but invocationTimeout is not.

[Osip Fatkullin]

My guess about virtual time was right. Newer version of kotlinx.coroutines.test prints more clear exception message:

Timed out after 5s of _virtual_ (kotlinx.coroutines.test) time. To use the real time, wrap 'withTimeout' in 'withContext(Dispatchers.Default.limitedParallelism(1))'
kotlinx.coroutines.TimeoutCancellationException: Timed out after 5s of _virtual_ (kotlinx.coroutines.test) time. To use the real time, wrap 'withTimeout' in 'withContext(Dispatchers.Default.limitedParallelism(1))'

I’ve filed issue on GitHub: https://github.com/kotest/kotest/issues/3703