Hi, I stumbled upon this behaviour. Is this really...

# coroutines

[Anton Kapelyushok]

Hi, I stumbled upon this behaviour. Is this really a bug or am I missing something?

https://github.com/Kotlin/kotlinx.coroutines/issues/3757

[Trevor Stone]

This looks like expected (but confusing) behavior.

the cancel() is called on a CoroutineScope. The this being called on is the surrounding launch . When this happens the flow is going to stop collecting, but the other launched coroutine scope is unaffected. The flow will stop collecting and call close on the channel, but according to the docs

Closing a channel after this function has suspended does not cause this suspended send invocation to abort, because closing a channel is conceptually like sending a special "close token" over this channel.

When you call coroutineScope { cancel() } what actually is happening is you are using the suspend block inside of collect, creating a coroutine scope, and calling cancel on that. That scope cancellation will then propagate up the suspending function calls.


To prove this, add a delay(100) between the channel send, and use the cancel() version. You should see your expected error

[Trevor Stone]

All of that said, you should avoid passing channels around, esp ones from channelFlow builders like this.

[Trevor Stone]

TLDR: 2nd send suspends before cancellation due to cancel() being called on the launch's coroutinescope. Using coroutineScope{cancel()} cancels the suspending function immediately so the second send doesn't get to suspend

[Anton Kapelyushok]

It seems that send gets suspended in both cases:
https://play.kotlinlang.org/#eyJ2ZXJzaW9uIjoiMS44LjIxIiwicGxhdGZvcm0iOiJqYXZhIiwiYXJncy[…]gpXG4gICAgICAgICAgICAgICAgfVxuICAgICAgICB9XG4gICAgfVxufSJ9

I see it as following:
in case with coroutineScope{cancel()} ProducerCoroutine gets canceled, which leads to channel.cancel() call which wakens up suspended channel.send()
in case with cancel() call, launch coroutine gets canceled, which then cancels all its children, in this case ProducerCoroutine, but it calls channel.close() instead, which has no effect on suspended channel.send()

[Trevor Stone]

https://pl.kotl.in/cg648v6Ni