ScopedValue structured forking/forwarding API

[Alan Bateman]

(Your mail appears to be from June 18 but seems to have only been 
delivered to loom-dev on July 7).

On 18/06/2025 18:05, Nikita Bobko wrote:
> :
>
> ## Use cases
>
> *Use case 1.* The API would be useful for any third-party libraries
> that implement structured concurrency. kotlinx.coroutines is an
> example of such a library.
An initial question for you:  Would I correct to say that the 
currentThread may change across suspension points when using these 
coroutines? Just asking as it isn't immediately clear if these 
coroutines are compatible with ThreadLocals and other APIs/features that 
depend on Thread identity.

In any case, the summary is that scoped values can only be safely 
inherited into "child threads" in structured contexts. Inheritance is 
one of the privileged operations and primitives that are needed to 
support structured use.  Right now the only API exposed that uses this 
is StructuredTaskScope (STS). We need a lot more feedback and real world 
usage before re-visiting the lower level construct that allows other API 
be developed.

There has been some experiments with inheriting into "child tasks", 
which I suspect is closer to what you are asking for. In particular 
there was initial interest into inheriting into the tasks supporting the 
stages of a parallel stream pipeline.  The main challenges with the 
experiments were in the areas of performance (esp. CountedCompleter) and 
reliable recovery (e.g. StackOverflowError). These experiments are 
parked right now. I read your mail as looking to reboot these explorations.

-Alan