Cancel all scope children coroutines based on any child result

Question

Assume there are two suspend function which return a boolean. They need to be launched in parallel but cancel each other if false is returned.

suspend fun task1(): Boolean {
    delay(10000)
    return true
}
suspend fun task2(): Boolean {
    return false
}

The tasks can be run in parallel by launching them with async and cancel each other by throwing CancellationException inside a coroutineScope as shown below.

coroutineScope {
    setOf(
        async { task1() },
        async { task2() }
    ).forEach{ d ->
        d.await().also {
            if (it == false)
                throw CancellationException()
        }
    }
}

The problem is that the results are processed in the order of await calls, i.e. task2 result is processed only after task1 result and hence task2 won't ever cancel task1. It is possible to re-arrange this to let task2 cancel task1?

Answer 1

If you need to wait for more results at once in a parallel manner (instead of the sequential manner where your problem lies) you can use the select clause, like this:

coroutineScope {
        val tasks = setOf(
                async { task1() },
                async { task2() }
        )
        
        select {
            tasks.forEach {
                it.onAwait { finishedSuccessfully ->
                    if (!finishedSuccessfully)
                        tasks.forEach { task -> task.cancel() }
                }
            }
        }
    }