Out of memory in test with an active ticker channel

[lukasz-kalnik-gcx]

I have a presenter with injected dispatcher, which performs a task periodically based on a ticker Channel:

import kotlinx.coroutines.channels.ticker

class Presenter(
    // other dependencies...
    coroutineContext: CoroutineContext = Dispatchers.Default
) {

    val scope = CoroutineScope(coroutineContext)
    
    init {
        scope.launch {
            val tickerChannel = ticker(delayMillis = 30_000L, initialDelayMillis = 0L, coroutineContext)
            for (ticker in tickerChannel) { requestState() }
        }
    }
}

In the test I inject an UnconfinedTestDispatcher and use runTest but the test doesn't fail with unfinished coroutines. It shows a java.lang.OutOfMemoryError: Java heap space after some time:

class PresenterTest {

    val testScope = TestScope(UnconfinedTestDispatcher())

    @Test
    fun `test requesting state`() = testScope.runTest {
        val presenter = Presenter(
            // other mocked dependencies...
            testScope.coroutineContext
        )
        
        // verify some behavior of the mocked dependencies 
    }
}

JUnit Jupiter 5.7.2
Coroutines 1.6.0
Kotlin 1.6.10

[qwwdfsad]

Could you please post a self-contained Presenter source? Consuming from a channel is prohibited in non-suspending contexts, and Presenter init block is non-suspending context

[qwwdfsad]

From what I can say now -- tickerChannel never completes, so if your requestState creates memory pressure, then it eventually will lead to OOM

[lukasz-kalnik-gcx]

Could you please post a self-contained Presenter source? Consuming from a channel is prohibited in non-suspending contexts, and Presenter init block is non-suspending context

Sorry, I forgot to wrap consuming from the ticker channel in CoroutineScope.launch. I fixed it now in my example.

[lukasz-kalnik-gcx]

From what I can say now -- tickerChannel never completes, so if your requestState creates memory pressure, then it eventually will lead to OOM

Shouldn't runTest detect though that there is a non-canceled coroutine at the end of the test and just fail the test?

[lukasz-kalnik-gcx]

More background: the example is obviously simplified. In the real presenter we use a custom CoroutineScope attached to presenter lifecycle callbacks. It holds a Job reference and cancels the Job when presenter is destroyed.

What I did wrong in my test is I forgot to call presenter.onDestroy() at the end of the test. With this the ticker coroutine gets canceled.
I was wondering though if runTest shouldn't detect the "hanging" coroutine at the end of the test. Or do I understand it wrong?

[qwwdfsad]

In general, it should.

But tickers are not integrated into structured concurrency (that's why they are obsolete) and thus runTest cannot detect them -- coroutine backing ticker is a top-level one, not registered anywhere.

You can workaround that by using ticker(context = yourScope.coroutineContext), but ideally it should be automatic for tickers. See #540

[lukasz-kalnik-gcx]

Yes, I tried ticker(context = scope.coroutineContext) but it still didn't get detected by runTest that the coroutine was running.
Thank you for your fast and helpful responses, I appreciate them very much!

[lukasz-kalnik-gcx]

BTW is there an alternative to ticker() if they are obsolete?