Can someone help me understand why I cannot replace `awaitAll(one, two)` by `one.await(); two.await...

# coroutines

[Jonathan]

Can someone help me understand why I cannot replace awaitAll(one, two) by one.await(); two.await() in this code?

-.kt

[marstran]

What happens when you do it?

[Jonathan]

it will hangs

[Jonathan]

As far as I understand, "two" reference a child job, and it should make the parent fail as soon as it throw its exception.

[Jonathan]

And as soon as the parent is failed the job "one" should be cancelled

[Jonathan]

making "one.await" throw CancelleationException

[Jonathan]

but that's not what we can observe. the job "one" doesn't seem to be cancelled, and the one.await hangs

[Vsevolod Tolstopyatov [JB]]

it’s a tricky place and we don’t yet decided how to design it 🙂

You have three coroutines: foo, one and two. async does not cancel its parent.
foo is waiting for one (which sleeps forever), while two already failed, but this exception is not delivered, because no one called await on it.

Complexity behind awaitAll implementation is exactly to resolve this problem

[Jonathan]

ah... of course it makes sense! Thank you @Vsevolod Tolstopyatov [JB]

[Jonathan]

(my mistake was assuming async cancel the parent on failure)

[Vsevolod Tolstopyatov [JB]]

You are welcome.
The first thing which comes to mind is to cancel parent when async fails, but then in

try {
async { throw MyException() }.await()
} catch (e: MyException) {
}

catch block won’t be executed because cancellation throws CancellationException 🙂

[Vsevolod Tolstopyatov [JB]]

but for GlobalScope.async catch block will be executed and that’s where this mental model breaks 😞

[codyoss]

huh, this is a little confusing. Is there a good article or any documentation that explains this more? Just trying to understand a little better….

[Vsevolod Tolstopyatov [JB]]

@codyoss https://github.com/Kotlin/kotlinx.coroutines/blob/master/coroutines-guide.md
+ all public API is documented

[Vsevolod Tolstopyatov [JB]]

https://github.com/Kotlin/kotlinx.coroutines/issues/552

[codyoss]

ah thanks, It has been a while since I have looked at that readme. Seems like it has gotten some love recently. Thanks!