Hi! Is there any known cases when `GlobalScope.launch(Dispatchers.Unconfined)` will actually dispatc...

# coroutines

[Vsevolod Ganin]

Hi! Is there any known cases when GlobalScope.launch(Dispatchers.Unconfined) will actually dispatch? I stumbled upon some case when GlobalScope.launch(start = CoroutineStart.UNDISPATCHED, context = Dispatchers.Unconfined) != GlobalScope.launch(Dispatchers.Unconfined) but I can’t quite figure what’s causing it

[octylFractal]

if you can create a test case, I might have some ideas, but I think those two should be equivalent from what I know

[Vsevolod Ganin]

I’m trying but no luck in reproducing it in simple environment yet unfortunately. I will get back if I manage to.

[Vsevolod Ganin]

Okay, I managed to pinpoint it. This has something to do with Android Dispatchers.Main.immediate parent context. Simple test:

GlobalScope.launch(Dispatchers.Main.immediate) {
            GlobalScope.launch(Dispatchers.Unconfined) {
                println("$coroutineContext: before")
            }
            println("$coroutineContext: after")
        }

It prints:

[CoroutineId(3), "coroutine#3":StandaloneCoroutine{Active}@229841e, Main [immediate]]: after
[CoroutineId(4), "coroutine#4":StandaloneCoroutine{Active}@f2294ff, Unconfined]: before

And if I use start = CoroutineStart.UNDISPATCHED it works correctly.

[octylFractal]

I think the key here is DispatchedContinuation<*>.executeUnconfined -- when you are already in an event loop (which I believe Dispatchers.Main.immediate counts as well) it will not immediately execute, but re-dispatch for later to avoid stack overflows

[octylFractal]

CoroutineStart.UNDISPATCHED is a little more strict and does not use the event loop to avoid stack overflows

[Vsevolod Ganin]

Strange. Using Dispatchers.Main yield correct result:

[CoroutineId(29), "coroutine#29":StandaloneCoroutine{Active}@c6ed5c2, Unconfined]: before
[CoroutineId(25), "coroutine#25":StandaloneCoroutine{Active}@65e4fd3, Main]: after

[octylFractal]

correct, because the non-immediate one does not use the undispatched event loop

[Vsevolod Ganin]

But do Main.immediate and Unconfined share the same event loop to avoid stack overflows? Is it not a private implementation detail?

[octylFractal]

yes, they share the same event loop. I believe it is technically an implementation detail of how coroutines are dispatched when CoroutineDispatcher.isDispatcherNeeded returns false, which means that they share it because it is the same mechanism

[Vsevolod Ganin]

I see. Thank you for your time and help