Hi, can someone please explain why following takes...

# coroutines

[Milan Hruban]

Hi, can someone please explain why following takes 5000ms instead of 200ms? And how do I make the withContext call cancellable by timeout?

fun main() = runBlocking<Unit> {
    withTimeout(200) {
        withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
            Thread.sleep(5000)
        }
    }
}

[Zach Klippenstein (he/him) [MOD]]

Because coroutine cancellation is cooperative (opt-in) and doesn’t have any effect on actual blocking calls by default (i.e. sleep).

If sleep here represents an actual blocking call (like an InputStream read), you can use runInterrupting to interrupt the blocked thread when the coroutine is cancelled.

[Ian Lake]

delay(5000) is the coroutine equivalent to Thread.sleep(5000) that plays nicely with cancellation, etc.

[Milan Hruban]

@Zach Klippenstein (he/him) [MOD] thank you so much, that's it! Can't believe I have never seen it before

[Marc Knaup]

I would’ve used Dispatchers.Default but thinking about it IO actually makes more sense 😄

[Milan Hruban]

the Thread.sleep() call was there as a placeholder for any blocking call (database query in my particular case)

[gildor]

If your blocking call supports thread interruption, you can use this to integrate it with coroutine cancellation
https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/run-interruptible.html

But it works only if it supported by blocking code (which not very common, unfortunately), but it should fix cancellation for your Thread.sleep example above (just because sleep supports interruption)

[spand]

I had the exact same case a week ago. Since I could not rely on cancellation I had to "detach" the blocking call from the job like:

fun main() = runBlocking<Unit> {
    withTimeout(200) {
        GlobalScope.async(<http://Dispatchers.IO|Dispatchers.IO>) {
            Thread.sleep(5000)
        }.await()
    }
}

In effect "leaking" the threads.. and just letting them fail or complete in the background.