[swift-evolution] Contextualizing async coroutines

[Daniel Vollmer]

Hello,

> On 31. Aug 2017, at 20:35, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
> 
> # `onResume` hooks
> 
> Relying on coroutine context alone still leaves responsibility wholly on suspending APIs to pay attention to the coroutine context and schedule the continuation correctly. You'd still have the expression problem when coroutine-spawning APIs from one framework interact with suspending APIs from another framework that doesn't understand the spawning framework's desired scheduling policy. We could provide some defense against this by letting the coroutine control its own resumption with an "onResume" hook, which would run when a suspended continuation is invoked instead of immediately resuming the coroutine. That would let the coroutine-aware dispatch_async example from above do something like this, to ensure the continuation always ends up back on the correct queue:

I do feel the “correct” scheduling should be done by the actual scheduler / run-loop system, so we’re not paying the context switch cost twice. Of course this means that the run-time / OS needs enough information to do this correctly.

To be honest, the enqueuing of messages to actors seems quite straight-forward, but I’m having trouble envisioning the draining / processing of the messages.

	Daniel.