Make tokio / async-io optional task execution engines

[hanabi1224]

Make tokio / async-io optional task execution engines behind optional features

Objective

• Be able to use tokio as task engine in user code.
• It also fixes Calling .await inside async in AsyncComputeTaskPool.spawn() causes tokio panic #2532

Solution

• Leverage crate async-global-executor which uses compatible Task framework (async-executor) and has capabilities to switch to other task execution engines.
• Code change to bevy is very minimal
• It only adds one thin shim crate to the dependency tree on default feature set.
• This also re-exports on_block method from bevy_tasks crate under bevy::tasks namespace, so that users don't need to depend on a task engine explicitly.

[elpiel]

I'd love to have this option! The tonic default transport uses tokio and is unusable at the moment.

[alice-i-cecile]

This is a compelling little opt-in feature, and the design is very clean. I like the block_on re-export a lot.

The changes here are very scoped.

[alice-i-cecile]

bors try

[bors]

try

Merge conflict.

[alice-i-cecile]

@hanabi1224 Sorry that this has been left in limbo for so long. If you're around, could you rebase this?

If we don't hear back by the time this is ready to merge, we'll do so ourselves (and credit you as the author regardless).

[hymm]

You can run tokio futures on bevy's executor today with https://docs.rs/async-compat/latest/async_compat/. What's the advantage of this approach over that?

[hanabi1224]

@alice-i-cecile No problem, I just resolved all conflicts and have all CI jobs passing

[james7132]

This seems like a great change for those looking for tokio support or in general better async task performance.

However, I don't see any initialization code at all or anything that enqueues tasks added to TaskPools to the global executors. If the intent is just to allow global executors to run alongside the Bevy TaskPools, this change is sufficient, though I'm concerned with how we're adding yet more threads to the engine by default, but if we want to leverage them for running the core of bevy itself, there will need to be changes with how TaskPool is structured.

[james7132]

Are these two features mutually exclusive? If they are, this docs should cover that mutual exclusion.

[Pessi94]

Hello, is this feature still being considered? It would be nice to be able to use tokio with bevy

[alice-i-cecile]

Yes, it's being considered, but not something that's particularly pressing. @mockersf, how do you feel about this? Would you vote to start the bypass timer? I'm not immediately sure I would, but I want your opinion.

[mockersf]

The PR looks mostly good but it could use some cleanups and conflict resolution before doing that.

@hanabi1224 are you still around? Sorry for the long delay, could you update the PR? Otherwise we can mark it as available for adoption

[alice-i-cecile]

Closed in favor of #6137.