Race condition in Resource::connect_changed

[0x0ACB]

Tested versions

4.3

System information

Windows 11

Issue description

62d9ce6 changed Resource::connect_changed to delay signal connection if the ResourceLoader is currently loading. This leads to issues if a ResourceLoader creates a PackedScene with eg a MeshInstance3D or CollisionShape3D on a thread other than the main thread.

<core\object\object.cpp(1369): Object::connect> Condition "!p_callable.is_valid()" is true. Returning: ERR_INVALID_PARAMETER:
Cannot connect to 'changed': the provided callable is not valid: MeshInstance3D::_mesh_changed

<core\object\object.cpp(1369): Object::connect> Condition "!p_callable.is_valid()" is true. Returning: ERR_INVALID_PARAMETER:
Cannot connect to 'changed': the provided callable is not valid: Node3D::update_gizmos

Steps to reproduce

• create a loader that creates a PackedScene with MeshInstance3D and sets a mesh
• load a resource with the loader via `ResourceLoader::load_threaded_requet

Minimal reproduction project (MRP)

[RandomShaper]

An MRP would help immensely.

[0x0ACB]

This one triggers it quite reliably for me
project.zip

[RandomShaper]

My assessment so far:

• This is not what we usually call a race condition, as this behavior is deterministic.
• The only issue in a case like the one in the MRP is the warnings being printed, with no real damage. UPDATE: Not true.
• That said, I can see why this behavior would be a problem in a more complex loader that actually needs to change resources later than being assigned (e.g., the PrimitiveMesh changed later in the MRP).