C# Getting Node3D Position/GlobalPosition in Thread will always return zero

[Xyotic]

Godot version

Godot 4.1 stable

System information

Windows 10, RTX2080, i9-9900K

Issue description

Trying to access the position with node3D.Position or node3D.GlobalPosition inside a thread will always return Vector3(0,0,0) in 4.1
Worked without any issue in 4.0

Steps to reproduce

Add a Node to your scene and attach a script to it like this: (same code in mrp)

using Godot;
using System;
using System.Threading.Tasks;

public partial class Demonstration : MeshInstance3D
{
	public override void _Ready()
	{
		var currentPosition = this.Position;
		GD.Print(currentPosition); // will print the correct position
		
		Task.Run(async() => 
		{
			await Task.Delay(100);
			var currentPositionInThread = this.Position;
			GD.Print(currentPositionInThread); // will print zero
		});
	}
}

If step into this code you will see that the information of the position will be lost as soon as you enter the Task.Run() block

Minimal reproduction project

GetPositionIssue.zip

[raulsntos]

Accessing node members outside of the main thread is not allowed. The Node3D::get_position method has a thread guard:

godot/scene/3d/node_3d.cpp

Lines 508 to 511 in c3b0a92

	Vector3 Node3D::get_position() const {
	ERR_READ_THREAD_GUARD_V(Vector3());
	return data.local_transform.origin;
	}

• These thread guards were added in Refactor Node Processing to allow Scene Multithreading #75901.

You should be getting this error:

ERROR: This function in this node (/root/Node3D/MeshInstance3D) can only be accessed
       from either the main thread or a thread group. Use call_deferred() instead.

Take a look at the Thread-safe APIs documentation for more information.

---

You can follow the error's suggestion and use CallDeferred or Callable.CallDeferred to avoid this error, here's an example using a Callable:

using Godot;
using System;
using System.Threading.Tasks;

public partial class Demonstration : MeshInstance3D
{
	public override void _Ready()
	{
		var currentPosition = this.Position;
		GD.Print(currentPosition); // will print the correct position
		
		Task.Run(async() => 
		{
			await Task.Delay(100);
			Callable.From(() =>
			{
				var currentPositionInThread = this.Position;
				GD.Print(currentPositionInThread); // will print the correct position
			}).CallDeferred();
		});
	}
}

[Xyotic]

Thanks for clarification.
This change in 4.1 breaks a lot of my code.
Is there any way I can await getting the nodes position? i.e. awaiting CallDeferred?

[raulsntos]

Yes, you should also be able to await the process_frame signal on the SceneTree:

await ToSignal(GetTree(), SceneTree.SignalName.ProcessFrame);

The continuation will execute in the main thread.

[RedworkDE]

Also consider if the Task.Run is even necessary, for example _Ready could be made async directly:

public override async void _Ready()
{
	var currentPosition = this.Position;
	GD.Print(currentPosition); // will print the correct position
	
	await Task.Delay(100);
	var currentPositionAfterWait = this.Position;
	GD.Print(currentPositionAfterWait); // will print the correct position
}

[Xyotic]

Also consider if the Task.Run is even necessary, for example _Ready could be made async directly:

public override async void _Ready()
{
	var currentPosition = this.Position;
	GD.Print(currentPosition); // will print the correct position
	
	await Task.Delay(100);
	var currentPositionAfterWait = this.Position;
	GD.Print(currentPositionAfterWait); // will print the correct position
}

Would this not block the whole game? Considering you are awaiting on the main thread?

[RedworkDE]

That is the whole point of async, awaiting will not block the thread, instead it can continue doing other stuff.
See https://learn.microsoft.com/en-us/dotnet/csharp/asynchronous-programming/async-scenarios

[Xyotic]

Alright thanks for your suggestions @raulsntos & @RedworkDE
I will give those a shot
Closing this as this isnt a bug