Add ability to call code on rendering thread

[reduz]

As more users use compute in Godot 4, the way they do is most likely incompatible when running on separate threads and will start erroring soon as we improve the thread safety of the render thread.

To properly run code on the render thread, this function was added. Use like this:

func initialize_compute_code():
	....

func update_compute_code(custom_data):
	...

func _ready():
	RenderingServer.call_on_render_thread( initialize_compute_code )

func _process():
	RenderingServer.call_on_render_thread( update_compute_code.bind(with_data) )

[clayjohn]

For clarity, this is only needed for compute code that needs to synchronize with the main RenderingDevice. Compute code running on a local rendering device can be managed from any thread

[yankscally]

I just ran into this issue earlier, to me it was behaving unexpectedly because I assumed the RenderingServer was not on a seperate thread and calling code in order did not produce the results I expected. Also I had a bad case of 'didn't read docs' today.

here is my report:
#79691

This change would definitely be appreciated now that I understand the problem!

[lyuma]

@yankscally to be clear, the problem you are trying to solve is synchronizing texture resources for CPU readback?
As in, get_viewport().get_texture().get_image()

I just want to clarify the issue of not being able to directly access the node from another thread. Note that Node apis may not be called from another thread. Let's for the moment assume you have converted the ViewportTexture into a RID that can be safely accessed from the render thread.

I'm not sure about the performance of doing a synchronous CPU readback of texture data from the rendering thread. I'd definitely be worried about calling get_image() regularly, but in your case you are specifically doing this to get a screenshot, so I assume a stall / stutter is expected? (though ideally there would be a way to do this without a stutter)
I'll wait for someone more involved with rendering to respond on the question of synchronous readback of Image data

[reduz]

@lyuma That is a different use case unrelated to this PR, how to do it it is documented in the function itself.

[BastiaanOlij]

@reduz I was wondering about something, and this may not be possible, but seeing the thread group logic you added to nodes, would it be possible to mark a node as being run on the rendering thread?

I guess this would add a lot more difficulty in supporting functions like _ready and require a whole lot more implementation than a "simple" call queue on the rendering thread, but just thought I'd mention it as a discussion topic.

Other than that, fully approve of this approach.

[clayjohn]

Looks fine to me!

Note for merging, this should not be cherrypicked. But can be merged any time for 4.2

[YuriSizov]

Thanks!