Add a mean to get the current threadpool

[lu-zero]

It is useful when you want to share the same threadpool across
threads outside the threadpool.

[cuviper]

I hesitate to expose ThreadPool like this because it leaves us with no clear owner of the pool. The Registry is shared because each thread in that pool also holds a handle, but it would be nice if ThreadPool remains a bit stronger, as the Drop semantics imply by calling registry.terminate(). There have also been requests to support a synchronized shutdown, which should naturally consume self by value, but that semantic doesn't work as well if ThreadPool is a shared handle.

Can you say more about what you want to do with this? Most of the free functions already work naturally with the current pool, whether that's global or something custom, so I wonder why you actually need the ThreadPool itself.

[lu-zero]

So far I used it to println!() the threadpool to make sure which is which.

[nikomatsakis]

Maybe we can return some kind of opaque id

[cuviper]

What would you be able to do with an opaque id?

It seems to me what's wanted is just to run things on the pool, not share ownership, so maybe we need some kind of ThreadPoolRef mirroring ThreadPool's &self methods? But then there are still ownership questions -- does this act like Weak and fail/panic if the real ThreadPool owner has dropped? (since the threads will then be exiting...)

[nikomatsakis]

@cuviper print it out so you can make sure which threadpool is which

[lu-zero]

It is also nice so I do not have the annoying pattern of

match maybe_threadpool {
  Some(pool) => pool.spawn(run),
  None => rayon::spawn(run),
}

I currently have in rav1e.

[nikomatsakis]

Doesn't rayon::spawn use the current threadpool, if any? I forget. Maybe we can make a function for that?

[lu-zero]

Doesn't rayon::spawn use the current threadpool, if any? I forget. Maybe we can make a function for that?

My usecase is to not have to special case between a custom pool and the global pool. Some users would enjoy setting the threadpool according to certain constraint, others would just use the global threadpool they already set for their needs or share a threadpool otherwise created.