I am learning Kotlin by reading the <Programming Kotlin book>. In the section about coroutine cancel...

# coroutines

[Mikael Ståldal]

I am learning Kotlin by reading the Programming Kotlin book. In the section about coroutine cancellations (page 319-320), it suggest to wrap long running blocking code with async { doStuff() }.await() to make the coroutine cancellable (see code/async/cancelandsuspension.kts in https://media.pragprog.com/titles/vskotlin/code/vskotlin-code.zip). Is that really a good advice? It seems like the long running blocking code is not cancelled, it is simply detached from the parent coroutine and will continue to run to completion in backround, and will block the completion of the surrounding runBlocking.

fun getResponse(ms: Int): String {
    val s = URL("<http://httpstat.us/200?sleep=$ms>").readText()
    println("${LocalTime.now()} Got: $s")
    return s
}

suspend fun fetchResponse() = coroutineScope {
    try {
        println(async { getResponse(4000) }.await())
    } catch (ex: CancellationException) {
        println("${LocalTime.now()} fetchResponse: ${ex.message}")
    }
}

fun main() {
    println("${LocalTime.now()} before runBlocking")
    runBlocking {
        val job = launch(Dispatchers.Default) {
            launch { fetchResponse() }
        }

        println("${LocalTime.now()} Let it run...")
        delay(2000)
        println("${LocalTime.now()} OK, that's enough, cancel")
        job.cancel()
        println("${LocalTime.now()} end of runBlocking")
    }
    println("${LocalTime.now()} after runBlocking")
}

[uli]

Nothing much to add. Cancelation is cooperative and no async/await will magically make your long running code cancelable.
Wether it is good advise depends on your circumstances. Sometimes the top goal is to avoid successful completion. i.e. on android you don’t want the response from your readtext to be forwarded to a dead activity.
What’s missing from my pov is a word of warning that cancelation in that case can not actually abort the blocking code.

[ephemient]

runInterruptible can interrupt typical Java blocking code

[Mikael Ståldal]

Not my example with reading from java.net.URL it seems.

[ephemient]

it translates cancellations into Thread.interrupt() which stops Thread.sleep(), lock acquisition, and some I/O. if it doesn't stop URL.getText() then there's no way of stopping it in Java either

[ephemient]

perhaps you could try `ExecutorService.submit().await()`; that should at least abort the await even if it can't stop the operation

[Mikael Ståldal]

Not sure what you mean with aborting the await 🤔
Even the original example I posted aborts the await.