Stream: wg-async-foundations

Topic: Async Lifecycles

David Barsky (Jan 15 2020 at 20:00, on Zulip):

(Quick notes: @nikomatsakis proposed something like this and I fleshed out portions of it with @tmandry. Eliza Weisman helped me flesh this out.)

I wrote up a rough proposal for a Lifecycle trait, which is meant to support certain types of RAII guards in asynchronous contexts. Specifically, it exposes methods for a type to be notified if was yielded/resumed in a future or a generator. I have a draft which isn't complete, but is shareable:

David Barsky (Jan 15 2020 at 20:03, on Zulip):

I wanted to share this here before I posted a more fleshed-out proposal on internals.

Aaron Hill (Jan 16 2020 at 20:07, on Zulip):

@David Barsky :

It would be implemented as a fundamental trait, like Drop. This implies a blanket implementation for all types. 

Note that Drop is not implemented for all types. In fact, the pin-project macro relies on this fact in order to to prevent user-written Drop impls of certain annotated types.

Instead, the compiler has the notion of "drop glue", where types that are !Drop (e.g String) nevertheless "do something" when dropped (invoking drop glue for fields)

If Lifecycle is impemented for all types (e.g. T: Lifecycle always holds), then this would need to rely on specialization in some way

Aaron Hill (Jan 16 2020 at 20:09, on Zulip):

If we Lifecycle to work in the same way as as Drop, we we need to introduce a notion of Lifecycle glue. This would require making Lifecycle even more deeply integrated into the core language

David Barsky (Jan 16 2020 at 20:10, on Zulip):

@Aaron Hill oh, that's _really_ cool—I didn't know that. in that case, Lifecycle should follow Drop's footsteps, or it is a "magic" trait like Deref which is implemented on only _some_ types.

Aaron Hill (Jan 16 2020 at 20:10, on Zulip):

Did you mean Drop instead of Deref?

David Barsky (Jan 16 2020 at 20:11, on Zulip):

that was a confusingly worded sentence :)

Aaron Hill (Jan 16 2020 at 20:11, on Zulip):

AFAIK, Deref is only magical in that it has syntactic sugar, and can be automatically invoked

David Barsky (Jan 16 2020 at 20:11, on Zulip):

what i'm trying to get to is that this trait should be minimal and opt-in.

Aaron Hill (Jan 16 2020 at 20:11, on Zulip):

But "Deref glue" doesn't exist

David Barsky (Jan 16 2020 at 20:12, on Zulip):

and ideally, would reduce how much work is needed to support to it.

David Barsky (Jan 16 2020 at 20:12, on Zulip):

ideally, Lifecycle shouldn't rely on specialization or negative bounds.

David Barsky (Jan 16 2020 at 20:13, on Zulip):

it's meant to be more of a hint to the compiler along the lines of: "if you see a type implementing a trait named Lifecycle, do this codegen"

Aaron Hill (Jan 16 2020 at 20:19, on Zulip):

On the design level, I'm concerned about how implicit this is. From the perspective of an async fn author, arbitrary code is being injected both before and after every .yield.

With Drop, you look at the indentation of a variable to get a hint as to where it's drop glue will run (assuming it's not moved out of). With Lifecycle, you essentially need to keep a mental stack of all locals, which you consult whenever you see a .yield

Aaron Hill (Jan 16 2020 at 20:22, on Zulip):

I think the explicit approach of annotating the function makes things a lot more readable. I think composing futures is much easier to understand (analogous to the composition of functions) in terms of existing concepts. The Lifecycle trait is in many ways a "new" concept that all users need to be aware of in order to understand the behavior of their code

David Barsky (Jan 16 2020 at 20:42, on Zulip):

Thanks for the design feedback—the "implicitness" is a concern for me as well, and I'm not 100% sold on the utility of this. I'd like to fully understand if some sort of task locals/structured concurrency could address the scoping of spans without language changes.

David Barsky (Jan 30 2020 at 17:30, on Zulip):

I realized that Node's Async Hooks is some prior work similar to this proposal:

Last update: Feb 25 2020 at 04:20UTC