Question: what are nesting factories used for?

[weltkante]

I'm currently studying the codebase to evaluate how useful a port of the deadlock resolution to async frameworks on different runtimes would be and am a bit confused on the purpose of nestingFactories.

From what I can tell they contain a set of JoinableTaskFactory of its ambient tasks. This is calculated whenever a JoinableTask is launched.

The publically visible call sites to code which consumes nestingFactories are SynchronizationContext.Post and SwitchToMainThread. The usage of nestingFactories seems to be that it will post to any UnderlyingSynchronizationContext in the hierarchy of ambient tasks.

So whats the point of all this effort? It is optimized to not allocate in the common case of only having a single JoinableTaskFactory, so obviously there is some issue with having multiple TaskFactories it tries to solve? There are two scenarios I can think of that lead to having multiple factories:

• Creating multiple instances of JoinableTaskFactory for the same JoinableTaskContext (including JoinableTaskCollection)
• Creating multiple instances from distinct JoinableTaskContext (different UI threads?)

Since UnderlyingSynchronizationContext forwards to the JoinableTaskContext the first case does not really solve any issues does it? It just posts multiple times to the same SC, so it actually is an unnecessary performance overhead if anything, isn't it? I'd have expected the set of nesting JoinableTaskContext to be stored, not the set of nesting JoinableTaskFactory.

If the factories are for different UI threads then I'm wondering if this code is even correct in the first place, why would it post to every underlying thread when targeting a specific JoinableTask? Doesn't this basically mean code can run on arbitrary threads which happen to be in the chain launching a JoinableTask?

So, any idea what the nesting factories code is there for? Any scenario it actually helps with?

[AArnott]

FYI @sharwell actually did port this to Java several years ago. At least, I think he did, but now I can't find anything on an Internet search to back that up.