Extend PhysicsDirectSpaceState3D or PhysicsServer3D for easy state duplication / memoization

[warent]

Describe the project you are working on

Indie MOBA game

Describe the problem or limitation you are having in your project

PhysicsServer3D provides the tools needed to run physics against the current physics state, but it does not seem to easily support state freezing/exporting--a feature that is particularly important for handling lag compensation to run physics against past states.

The only way to do this now that I can think is to write a whole new PhysicsServer3DExtension which can export its current state on demand or handle some kind of custom internal buffer, which is pretty cumbersome.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

I'm not a physics or networking expert, so I can't speak to the best way to generalize this. My best guess is perhaps something like PhysicsDirectSpaceState3D.export_state() and PhysicsDirectSpaceState3D.import_state() would do the trick.

In this case, then it is easy to export state on demand (e.g. per server tick) to store in some state buffer to run retroactive physics on.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Noted above

If this enhancement will not be used often, can it be worked around with a few lines of script?

Not easily

Is there a reason why this should be core and not an add-on in the asset library?

It seems fundamentally integrated functionality

[warent]

I think it's related to this: #2821

But it's not clear that they would necessarily have the same solution

[warent]

Actually yeah after reading that proposal further, this is definitely a dupe. Consolidating!