Implement a single-threaded emulation of DeferredWorkQueue

[cittyinthecloud]

DeferredWorkQueue is an API that mods can use to call things on the main loader thread. Since we're single threaded, this version just runs the code can creates completed futures.

[kitlith]

Two potential issues, depending on how this is usually used.

Nothing is preventing a mod from creating a new thread itself (or use a mixin in code that gets called in another thread), and then trying to use DeferredWorkQueue to schedule things on thr main thread. That would not work as expected with this implementation.

If a mod is actually trying to defer execution (i.e. do a bit of work, then schedule itself again so that other stuff can run) it would also not work as intended.

I'm gonna admit that I don't know how this is used in practice, but I suspect they include these problem cases.

[ZNixian]

Looks good to me.

[ZNixian]

I can imagine a plausible race condition where mods assume the payload hasn't finished executing. In those cases, if they exist, it would be reasonable to file bug reports against those mods IMO.
(And as a note, please don't reply to this if you think it might lead to bikeshedding)

[cittyinthecloud]

@kitlith This is only usable during loading on Forge, so spawning threads is highly unlikely.

[kitlith]

Okay, then add a comment to the effect of that, explaining that it is only used during loading, (akin to the forge comment) and for bonus points add an explicit error when used outside of loading and LGTM.

[Rongmario]

Looks good, any tests being done yet?

[TheGlitch76]

Would it be possible to check if this is on the main thread as a safety net? Concurrency errors are a pain in the ass to solve.

[TheGlitch76]

@kitlith do you still have any issues with this PR?

[kitlith]

Looks good to me!