Cancelling a non-cancelable coroutine in Kotlin

Question

Consider this non cancellable coroutine that works as its name implies.

fun main(args: Array<String>) = runBlocking {

    val nonCancellableJob = launch(Dispatchers.Default) {
        for (i in 1..1000) {
            if (i % 100 == 0) {
                println("Non cancellable iteration $i")
            }
        }
    }

    println("Cancelling non cancellable job...")
    nonCancellableJob.cancelAndJoin()
}

Now, if I get rid of the explicit dispatcher Dispatchers.Default and use the inherited one i.e. launch {...} the coroutine gets cancelled immediately without printing anything. It seems that a non cancelling coroutine is being cancelled! Is it a bug or what?

Pawel Pawel · Accepted Answer · 2020-12-10T11:59:35

When you don't use Dispatchers.Default then launch is inheriting dispatcher of runBlocking which seems to defer execution until needed.

Even though your coroutine cannot be canceled while running (since it doesn't have any activity check nor does it have suspension points) it's still possible to cancel it before it starts executing.

You can modify CoroutineStart of your launch builder to alter this behavior:

To execute it immediately when declared:

val nonCancellableJob = launch(start = CoroutineStart.UNDISPATCHED) { ... }

To prevent cancellation before it starts:

val nonCancellableJob = launch(start = CoroutineStart.ATOMIC) { ... }

Cancelling a non-cancelable coroutine in Kotlin

2 Answers