How to "fail fast" when an agent's target is freed?

[hatmix]

I started this on Reddit, but it's perhaps too specific to LimboAI to get help there. For completeness, here is the link to the Reddit post.

I am using Godot 4.5.1 with LimboAI 1.5.2 (according to the version.txt). LimboAI does not show up in Godot's addon list, in case that's unexpected.

I'm trying to use a dynamic selector to reset behavior if the "target" node of the agent is freed. The tree looks like so:

The code for the "Clear var 'target' if not valid" task:

@tool
extends BTAction

## Specify var to be checked
@export var var_name: StringName


func _generate_name() -> String:
	return "Clear var \"%s\" if not valid" % var_name


func _tick(_delta: float) -> Status:
	var check_var: Variant = blackboard.get_var(var_name)
	if check_var == null or is_instance_valid(check_var):
		# Fail b/c we don't want to change our plan when there's no target
		# or a valid target
		return FAILURE
	else:
		# Clear var and return success to re-evaluate what to do
		blackboard.set_var(var_name, null)
		return SUCCESS

The sequences work for the happy path, but if 2 agents target the same node, one wins and the other causes an exception when trying to access the previously freed node.

The LimboAI debugger panel makes it look like the "Clear var 'target' if not valid" task is not running every _tick (but none of them look like that, so it's difficult to tell). There is a recording available at https://youtube.com/watch?v=eP5F6kGbTEw&si=HTI9fOvpKmgXELjo

Why doesn't this work as expected? What's the "right" way to short-circuit the running task when it becomes invalid? TIA!

[limbonaut]

I see that there's a logical error in the example. You should be checking if the instance is NOT valid (missing logical not).

[hatmix]

Thanks for having a look!

The else: clause handles the NOT valid case. My understanding is that the task should return success in order to fulfill the dynamic selector and reset the behavior. Is that not so?

[limbonaut]

I meant this: In your code
if check_var == null or is_instance_valid(check_var):
should be
if check_var == null or not is_instance_valid(check_var):

[hatmix]

To check my understanding, I implemented the same tree in another library and it works as expected. IDK enough about LimboAI to know whether it's a fault in my understanding or potentiallly a bug in 1.5.2.