Hi everyone :wave: Can anybody explain me what happens with second `Deferred` in `select` when the f...

# coroutines

[genovich]

Hi everyone 👋
Can anybody explain me what happens with second Deferred in select when the first one returns value?

select {
   deferred1.onAwait { it }
   deferred2.onAwait { it }
}

[elizarov]

Nothing happens to it.

[genovich]

So, should I cancel it manually, if I want to get result from one coroutine exclusively?

[elizarov]

Yes, you should cancel it yourself if you don’t need its result. You can just cancel them all (no harm in cancelling a completed one)

[genovich]

Thank you, I thought that I was wrong doing like this.

[genovich]

What do You think about this construction?
It allows us to cancel all async operations without iterating through them.
Developer still can forget to call async with job, but think that this is more reliable than cancel every Deferred.

val job = Job()
val select = select {
    async(job) { suspend1() }.onAwait { it }
    async(job) { suspend2() }.onAwait { it }
}
job.cancelChildren()

[elizarov]

@genovich This construction opts out of the structured concurrency. As usual with all such approaches it has a bug. If one suspend function crashes, then exception is throws and the other leaks, continues to work forever. If a parent coroutine is cancelled, then select throws CancellationException, but both async jobs continue to work

[elizarov]

You can do a similar trick, though, but with the structured concurrency:

val result = coroutineScope {
    select {
        async { suspend1() }.onAwait { it }
        async { suspend2() }.onAwait { it }
    }.also { 
        coroutineContext.cancelChildren()  
    }
}

[genovich]

Oh, yes, I forget that I should connect the job to current context, and totally forget about exceptions.
Your solution looks great, thank you.