assert from a thread does not function as expected

[tjbrockmeyer]

Godot version

v4.1.3.stable.official [f06b6836a]

System information

Godot v4.1.3.stable - Linux Mint 21.1 (Vera) - X11 - Vulkan (Mobile) - dedicated NVIDIA GeForce GTX 1070 (nvidia; 525.125.06) - AMD Ryzen 7 1700 Eight-Core Processor (16 Threads)

Issue description

When using assert from a thread, I expect that the execution would halt the same as when using assert on the main thread. Instead, it forces the caller of assert to return immediately with the "default" value (null for references, false for bool, 0 for int, etc). No error messages are created whatsoever, and execution is not halted. This can cause extremely hard-to-detect bugs that will disappear or transform in a release build. The documentation does not say anything about assert having different functionality on threads other than the main thread, only that you "must be running in the editor," which my assumption was that a new thread is, in fact, still running in the editor.

Steps to reproduce

Create a thread, call a function from inside that thread, call assert(false) from the called function. Then observe that there is no halt for the debugger, there are no error messages, the return value of the called function is the "default" value of the function's return type, and the thread ran to completion despite a failed assert call.

Minimal reproduction project

The following script is enough to reproduce this issue.

extends Node

func _ready() -> void:
	var thread = Thread.new()
	thread.start(_run_thread)
	thread.wait_to_finish()

func _run_thread():
	print('thread running...')
	var result = _fn()
	prints('result:', result)

func _fn() -> int:
	assert(false)
	return 200

Expected output:

thread running...
result: 200

Actual output:

thread running...
result: 0

[jsjtxietian]

I got a debug break using your code in Godot v4.2.rc (830f29422) - Windows 10.0.22621 - GLES3 (Compatibility) - NVIDIA GeForce RTX 3060 Ti (NVIDIA; 31.0.15.4601) - 12th Gen Intel(R) Core(TM) i7-12700F (20 Threads):

[AThousandShips]

Probably fixed by:

• Support threads in the script debugger #76582

So a duplicate of:

• Breakpoints do not work in Thread #2446
  Or
• Threads do not output errors to debugger #18330

[tjbrockmeyer]

Good to know that it is fixed. I didn't find those other issues when searching for assert. Awaiting the fix in the stable release 😄