Hello colleagues! I’ve just found that implementations of `raceN()` probably not handling exceptions...

# arrow

[genovich]

Hello colleagues!
I’ve just found that implementations of raceN() probably not handling exceptions in a right way.
According to the conversation I had a year ago, it seems to me that if one of functions will throw an exception, then another will not be cancelled.
Do you have an idea how to check such behaviour?

[simon.vergauwen]

Hey @genovich,

That is the idea behind the raceN operator. I just read your conversation with Roman, and the canceling is happening after select.

https://github.com/arrow-kt/arrow/blob/225e16f4f22e9458c1d8fe96ae5aa8242306fe17/ar[…]x-coroutines/src/commonMain/kotlin/arrow/fx/coroutines/Race2.kt

Here you can see that if Left wins, then it cancels Right.

[simon.vergauwen]

There are also a bunch of tests for this here.

https://github.com/arrow-kt/arrow/blob/225e16f4f22e9458c1d8fe96ae5aa8242306fe17/ar[…]routines/src/commonTest/kotlin/arrow/fx/coroutines/RaceNTest.kt

[simon.vergauwen]

probably not handling exceptions in a right way.

Oh, exceptions.

So if fa fails that it cancels fb? There is indeed test missing for that 🤔

We should definitely write them! They can be written in a similar fashion than the other tests I linked.

If you do write them, please raise a PR for this 🙏

[genovich]

Sure, thank you

[simon.vergauwen]

I just wrote a quick test to see if it works as expected, and this passes.

val latch = CompletableDeferred<Unit>()
val exitCase = CompletableDeferred<ExitCase>()
val res = Either.catch {
    raceN(
        {
            guaranteeCase({
                latch.complete(Unit)
                awaitCancellation()
            }) { require(exitCase.complete(it)) }
        },
        {
            latch.await()
            throw RuntimeException("")
        }
    )
}
require(res is Either.Left)
require(exitCase.await() is ExitCase.Cancelled)

[simon.vergauwen]

Is that the behavior you expect?

[simon.vergauwen]

So fa got canceled due to fb throwing an exception, and the result of raceN is the exception thrown by fb.

[genovich]

Yes, I thought that fa will not be cancelled when fb will throw an exception.

[genovich]

Looks like this was an issue for earlier versions of select

[genovich]

Or I just don't understand something about how coroutineScope works inside

[genovich]

Anyway, thank you for checking, glad to know that everything is working

[simon.vergauwen]

Arrow builds this around select. I don't think select cancels the other one out of the box. There are some gotchas CoroutineScope, and depending on the actual implementation of the CoroutineScope can also behave differently. That's why we prefer higher-level operators as we define in Arrow, then you can ignore these lower-level things.

[genovich]

Oh, I meant coroutineScope() function. I read it’s documentation and it says that all coroutines will be cancelled if one of them throw an exception