bytes_to_var_with_objects fails to decode custom classes on threads in v4.3.beta3

[yukinogatari]

Tested versions

Regression introduced in v4.3.beta3.official [82cedc8]
Works as expected in v4.3.beta2.official [b75f048]

System information

Godot v4.3.beta3 - Windows 10.0.19045 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 3070 (NVIDIA; 31.0.15.4633) - AMD Ryzen 9 7900X 12-Core Processor (24 Threads)

Issue description

Deserializing custom classes with var_to_bytes_with_objects sometimes fails when run on a separate thread. Attempting to do so inconsistently results in the engine spitting out a bunch of Can't load script at path: 'res://custom_resource.gd'. errors.

Steps to reproduce

1. Run the MRP in v4.3.beta3.official [82cedc8]
2. Click the Run Threaded button: it fails to properly deserialize the data ~60% of the time
3. Click the Run Unthreaded button: it produces no deserialization failures

Minimal reproduction project (MRP)

threaded-deserialization-bug.zip

[AThousandShips]

Are you able to test with this PR?

• Batch of fixes for WorkerThreadPool and ResourceLoader #94169

CC @RandomShaper could you take a look

[yukinogatari]

That PR does indeed appear to fix the problem!

[RandomShaper]

Fixed by #94526.