Change streams with Kotlin coroutines

[huberchrigu]

Currently there seems to be no way to consume mongo event streams in a Kotlin coroutines setup. See the following test:

@DataMongoTest
@TestConstructor(autowireMode = TestConstructor.AutowireMode.ALL)
@Import(TestcontainersConfiguration::class)
class MongoAggregateChangesTest(private val mongoTemplate: ReactiveMongoTemplate) {
    @Test
    fun testFlux() { // success
        runTest { onHit ->
            mongoTemplate.changeStream<TestDocument>().listen()
                .subscribe { onHit.apply(it.body!!) }
        }
    }

    @Test
    fun testLaunch() { // fails
        runTest { onHit ->
            launch {
                mongoTemplate.changeStream<TestDocument>().listen()
                    .subscribe { onHit.apply(it.body!!) }
            }
        }
    }

    @Test
    fun testCoroutine() { // fails
        runTest { onHit ->
            launch {
                mongoTemplate.changeStream<TestDocument>().listen().asFlow()
                    .collect { onHit.apply(it.body!!) }
            }
        }
    }

    private fun runTest(listener: suspend CoroutineScope.(EventHandler) -> Unit) {
        runBlocking {
            val check = mutableSetOf<Int>()
            listener(DefaultEventHandler(check))
            Flux.range(0, 100).flatMap { mongoTemplate.save(TestDocument(it)) }.subscribe { println("Saved: ${it.id}") }

            await().atMost(Duration.ofSeconds(3)).until { check.size == 100 }
        }
    }

    class TestDocument(val id: Int)

    class DefaultEventHandler(private val check: MutableSet<Int>) : EventHandler {
        override fun apply(document: TestDocument) {
            val id = document.id
            println("Received: $id")
            check.add(id)
        }
    }

    fun interface EventHandler {
        fun apply(document: TestDocument)
    }
}

In my project, I cannot use event streams as flux, as the events need to be mapped by suspend functions. Is there any way to use event streams with coroutines? asFlow() seems to cause issues not only in this test, but also in projects where the flow is consumed by controller methods.

I think this would require a dedicated CoroutineMongoTemplate that makes use of the Kotlin MongoClient.