Add GDExtension function to get Object class name

[dsnopek]

This adds a function for GDExtension to get the class name of an Object, in order to support a fix to godot-cpp: godotengine/godot-cpp#1050

Please see that PR for the full details!

This is draft, because that PR is still a draft, and this PR may be updated before it's ready.

The other PR is out of draft, so taking this one out of draft too!

Back in draft, since I'm rebasing this on #76406

UPDATE: #76406 was merged, so I'm taking this out of draft!

[dsnopek]

Now that Godot 4.0 stable has happened, and merging this as-is would break the GDExtension ABI, I'd love some advice on how we plan on making those sort of changes in a "safe-ish" :-) way.

All we need to do here is add another function pointer to GDExtensionInterface. With GDNative in Godot 3.x, there was a "next" pointer at the end of some structures, so we could add more later. We don't appear to have something like that here?

If this hasn't been figured out yet, and there's a discussion about it, I'd love to be a part of that!

[dsnopek]

Converting this to a draft while PR #76406 is in progress. If that gets merged, then I'll update this PR to add the new function to GDExtension's interface using the new mechanism

[dsnopek]

I've updated this to be based on #76406

So, this'll need to wait until at least that one is merged before this can come out of draft again.

However, there is one more thing I'd like to add: this doesn't take into account classes that come from other GDExtensions. It should get the class name, and if that's a built in class or a class from the current extension, return it. But if the class is from another extension, it should return the class name of the built in class it descends from.

Update: This last bit is now in too!

[dsnopek]

Alright, I just got in the last thing I wanted to add here! So, now this one is just waiting on PR #76406 in order to come out of draft.

[dsnopek]

This is finally really ready for review! I've also put it on the agenda for the next GDExtension meeting :-)

[BastiaanOlij]

Isn't the class name already accessible through the macro functions added to the class?

cc @vnen

[dsnopek]

@BastiaanOlij:

Isn't the class name already accessible through the macro functions added to the class?

We can't use that in this case for two reasons:

1. This is before the wrapper class is created (in fact, we need it to make sure the correct wrapper class is created - see the companion godot-cpp PR which discusses the bug this is needed to fix)
2. We need the class name from the perspective of this particular GDExtension. If the class comes from this extension, we need the actual class name, but if it's from another extension we need the name of the built-in class it descends from

[BastiaanOlij]

@dsnopek ah I get it, damn thats funky

[dsnopek]

Discussed at the GDExtension meeting, and we think this solution makes sense. It is a problem that has also come up in the Python and Rust bindings.

[touilleMan]

🥳

[akien-mga]

Thanks!