How to determine who or what caused a node to be freed?

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,
1

Godot Version

v4.4.1.stable.official [49a5bc7b6]

Question

I am trying to determine why a specific node is being freed. The issue is that when I attempt to debug, I only get one frame in the stack trace (the current line). The origin of the call is never available.

I’ve tried in _notification, and in a bound signal to exit_tree:

func _notification(what: int) -> void:
	if what == NOTIFICATION_PREDELETE:
		print("Node freed") ## Debug placed here
	
func exit_tree():
		print("Node freed") ## Debug placed here

The stack trace isn’t helpful Frame 0 - res://some_script.gd:122 in function '_notification'

What tools do I have to debug a node being prematurely freed (somebody is calling free or queue_free presumably)?

2

I managed to track it down. I was queue_freeing a slot note, adding a new slot node, and then attaching a node to the first available slot. The first available slot was the slot I earlier queue freed, which next tick went out of scope.

I would still like more robust debugging tools for this use case.

I also tried overriding queue_free() and free() in the object, to see if that allowed me more information, but sadly that’s not possible.

3

Rather than that, what you might want to do is not call queue_free() or free() directly, but call them through a debug passthrough:

# Maybe this is a global script called ObjDebug or something?  Untested...

# Camel case to differentiate from the builtins...

func TrackQueueFree(o: Object) -> void:
    o.set_meta(&"debug:queue_free", true)

func QueueFree(o: Object, src: Object, debug: String) -> void:
    if o.has_meta(&"debug:queue_free"):
        print("[DEB] QueueFree(%s, %s, %s)" % [o.to_string(), src.to_string(), debug])
    o.queue_free()

Then, let’s say you’ve got a bullet that you want to track. Its script might look like:

#[...]

func _ready() -> void:
    ObjDebug.TrackQueueFree(self)

func _process(delta: float) -> void:
    lifetime -= delta
    if lifetime <= 0.0:
        ObjDebug.QueueFree(self, self, "Lifetime expired")

And if the player can sweep their sword to destroy bullets, the player might have:

#[...]

    ObjDebug.QueueFree(bullet, self, "Hit with sword.")

The scheme uses metadata to make the telemetry selective, so you don’t get spammed with every queue_free() unless you want to. It adds a little overhead (this is where #ifdef or comptime or the like would be nice to have, but AFAIK we don’t…), but not much and only on object deletion, which is a relatively rare event.

4

Thanks for the idea!

I did consider replacing all the free/queue-free calls in the project with a tracked version. I had 18 I think, so it wouldn’t have been too terrible to retrofit.

In general I do with we had better capabilities in debugging a signal back to the call site.