[4.4 dev2] ResourceLoader: load() and load_threaded_request() causes hang when called from a non-main thread in certain cases

[raccoon-path]

Tested versions

• Reproducible: v4.4.dev2.official [97ef3c8]
• Not reproducible: v4.4.dev1 and earlier

System information

Vulkan 1.3.280 - Forward+ INVIDIA GeForce RTX 4070, AMD Ryzen 7 2700 3.20 GHz, Windows 10 Pro 22H2

Issue description

When calling load() or load_threaded_request() methods from a non-main thread, it sometimes hangs without throwing an error in certain cases in version 4.4-dev2, whereas it works fine in 4.4-dev1.

It's possible that a deadlock(?) occurs in certain cases during the resource loading and caching process, depending on the loading sequence and combination of resources in the scene(?)
P.S. Based on my observations, if a scene contains a 3D-GUI (created following the 'GUI in 3D' tutorial), the probability of hanging in 4.4-dev2 is close to 100%. Additionally, if a scene contains complex CSG geometry (with a large number of combiners), hanging also occurs, but rarely and random. In 4.4-dev1, I did not experience a single hang during the entire testing period.

Steps to reproduce

• Open project in 4.4-dev2. Run Level1, Level2 directly from editor (Always OK). Now, play and try to load levels from the main menu. The Level1 button works, Level2 loading hangs, Level2 (load_threaded_request) works. Transition between Level1 и Level2 - hangs (because loading initiated from non-main thread)
• Open the attached project in 4.4-dev1. Run the scenes by pressing buttons in main menu. The Level1, Level2, and Level2 (load_threaded_request) buttons work as expected, all loading fine and transitions between locations work correctly.

Minimal reproduction project (MRP)

240915_MRP_loadbug.zip

[AThousandShips]

This project is very much not minimal, please upload a minimal project with everything unnecessary stripped

[RandomShaper]

#96904 fixed this.

[raccoon-path]

I can confirm, problem fixed, v4.4.dev.custom_build #97077.