Global main thread ID

[TitanNano]

In #1043 I need to identify the main thread without accessing the related engine API.

By storing the current thread ID during initialization, we can identify the main thread from that point on. I only found one other use of the main thread ID in godot-ffi but having access to the ThreadId of the main-thread could also be useful to user code.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-1045

[Bromeon]

Thanks, good idea!

I'm not sure if I'm doing premature optimization here, but since initialization happens exactly once, I'm not a huge fan of all future code needing to obtain a lock to read the thread ID. This might even be slower than thread::current().id(), depending on how that is implemented/cached.

In the library we have ManualInitCell<T> for this purpose. The invariant is that the value is only accessed after initialization, which means it needs to be unsafe, which is a bit annoying for ergonomics. In Debug mode, misuse would still be caught.

Thoughts? If it's not used in hot paths, we could keep things simple with OnceLock.

[TitanNano]

Thoughts? If it's not used in hot paths, we could keep things simple with OnceLock.

Yeah, I think that is a good idea. I definitely need to check the main thread ID every time a new async task is created, and it's hard to say if that will end up in someone's hot path. It being unsafe is unfortunate, but manageable.

[TitanNano]

@Bromeon this probably also fits better into the global module than meta right?

[Bromeon]

@Bromeon this probably also fits better into the global module than meta right?

Not really, that module is for global symbols from Godot itself.

Maybe init/mod.rs?
This functionality is only needed in godot-core, not in godot-ffi?

[TitanNano]

Maybe init/mod.rs?
This functionality is only needed in godot-core, not in godot-ffi?

Hmm, yes, I think it could be used in godot-ffi as well. I'll move it there and expose the fn main_thread_id() from godot-core::init as well? I think this could be useful for user-code too, so I think it should be public.

[Bromeon]

Hmm, yes, I think it could be used in godot-ffi as well. I'll move it there and expose the fn main_thread_id() from godot-core::init as well? I think this could be useful for user-code too, so I think it should be public.

Good idea!

Can you add the panic docs under its own section # Panics? That would also keep the brief description of main_thread_id short. You can see how it renders in the link from the bot: https://godot-rust.github.io/docs/gdext/pr-1045/godot/init/index.html -- this will be updated with a latency of several minutes, whenever you push.

Do you think it also makes sense to add a function is_main_thread() -> bool accompanying it?

[TitanNano]

Do you think it also makes sense to add a function is_main_thread() -> bool accompanying it?

Looks like a convenient shortcut, I have added it.

[Bromeon]

Thanks a lot!