Kotlin coroutines: flatMapLatest emits nothing

Question

There is the sourceFlow and I use flatMapLatest to produce new flow by value from sourceFlow. And there is no value in collect(). This code prints nothing.

fun main() {

    fun createNewFlow(): MutableSharedFlow<String> = MutableSharedFlow()
    runBlocking {
        val sourceFlow = MutableSharedFlow<Int>()

        // flow for transform
        val set = mapOf(
            1 to createNewFlow(),
            2 to createNewFlow(),
            3 to createNewFlow()
        )


        launch {
            // get flow from set by number from sourceFlow
            val res = sourceFlow.flatMapLatest { set[it]!! }
            res.collect {
                println(it)
            }
        }

        launch {
            sourceFlow.emit(1)
            set[1]!!.emit("1")
            set[1]!!.emit("1")
            sourceFlow.emit(2)
            set[2]!!.emit("2")
            set[2]!!.emit("2")
        }
    }
}

If I change flatMapLatest to flatMapConcat I see 1 twice, it is obviously because flow from set[1] is not completed, but I'm waiting for 1, 1, 2, 2, 3, 3 when I use flatMapLatest. What is wrong with my sample?

UPD: I just added delay to get result of println. So, this is really a race in the code

launch {
        delay(3000)
        sourceFlow.emit(1)
        set[1]!!.emit("1")
        set[1]!!.emit("1")
    }

Answer 1

As @louis-wasserman noted in the comments, it is a race (though not a thread-unsafe data race).

flatMapLatest internally collects (and cancels) the sub-flows. By default, MutableSharedFlow does not replay elements before the subscriptions.

So there is a race between the two coroutines. If the first coroutine becomes a subscriber after the second coroutine calls emit, no value will be received. You can specify the replay parameter to make sure the collect calls receive elements before the subscription.