export_tool_button self-referencing resources

[chrysopoeia]

When adding an export tool button, the supplied lambda also gets added when running your project. This can cause a self-referencing issue where the resource will never be freed.

When closing your project you will see
ObjectDB instances were leaked at exit or resources still in use at exit

example:

@export_tool_button("My Button", "Callable") var mybutton = func():
    self.some_var = some_func()

work around this by setting those lambdas to null in the initializer:

func _init():
    if not Engine.is_editor_hint():
        mybutton = null

This is an unknown to new users!

What do you think? Should the lambda only be available while in the editor?

[HolonProduction]

As mentioned on godotengine/godot#121729 we should document this and should consider adding a warning about it. Beyond that there is nothing we can and should do, since there are ways you could utilize this behaviour that are correct.

[dalexeev]

This behavior is documented:

Note: Avoid storing lambda callables in member variables of RefCounted-based classes (e.g. resources), as this can lead to memory leaks. Use only method callables and optionally Callable.bind() or Callable.unbind().

You can use a getter instead:

@export_tool_button("My Button", "Callable") var mybutton:
    get: return func():
        self.some_var = some_func()

I also have a (closed) PR that allows you to create a shorthand. With it, you could do something like this (note the => instead of =):

@export_tool_button("My Button", "Callable") var mybutton => func():
    self.some_var = some_func()

[chrysopoeia]

Excellent answers, though I guess this is where the confusion on documentation comes in: tutorial

[HolonProduction]

Ah yeah, that explains why I didn't find it the first time 😅

[Calinou]

Pull request opened:

• Document caveat with @export_tool_button and RefCounted-inheriting classes in GDScript exports godot-docs#12197