Lazy async inside suspend function blocks forever?...

# coroutines

[Landerl Young]

Lazy async inside suspend function blocks forever?
I have such code:

fun main() = runBlocking {
        sth().await()
    }

    suspend fun sth() = coroutineScope {
        async(start = CoroutineStart.LAZY) {
            println("async execution")
        }
    }

The main function blocks forever, and the “async execution” is never printed. Could anyone give a hand, thanks.
😘

[gildor]

Yes, it’s expected behavior

[gildor]

this is how coroutineScope function works: suspend until all child coroutines are finished, this is main use case of this function

[gildor]

it may be not so obvious with lazy, because coroutine is not started, but lazy also can be started at any moment, so if coroutineScope will return it will leak child coroutine

[Landerl Young]

Thanks for the quick reply!

So in such case, when sth() returns, the coroutineScope is completed.
Am I actually calling await on a Deferred with completed coroutine context.

[gildor]

in this particular case sth() never returns because coroutineScope is suspended and not completed

[Landerl Young]

Oh I see, that is because deferred(it’s child coroutine) is not completed.

thanks~

[gildor]

Yes, it’s because async is not completed and you don’t have any code that start this child coroutine and complete

[Landerl Young]

Thanks so much.
I’ll dive into the source code a bit.

[gildor]

This is also described in documentation of this function
https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/coroutine-scope.html

[Landerl Young]

I misunderstood the purpose of this function. I thought it is designed to access current coroutine context from a suspend function. actually it’s not and there is a

public suspend inline val coroutineContext: CoroutineContext

for accessing context.

[gildor]

yes, coroutineContext is intrinsic that available in any suspend function