freeing `PhysicsServer3DExtension` crashs without any info

[MikeSchulze]

Godot version

v4.0.beta3.official [01ae26d31]

System information

windows 10

Issue description

I run in this issue while migration my unit test api. To test extraction of class names from engine classes i loop over all engine classes provided by ClassDB.get_class_list() My test aborts without any reason, no error in the console

After adding more log infos i find out freeing a instance of PhysicsServer3DExtension aborts function calls without any error.

Steps to reproduce

Run the code and you see the line prints("freed") is not executed.

func _ready():
    var instance := PhysicsServer3DExtension.new()
    prints(instance, instance.has_method("free"))
    # await get_tree().process_frame
    instance.free()
    prints("freed")
    await get_tree().process_frame

To force to see the "real" error i try to add an extra await process_frame Rerun the code and comment in the line # await get_tree().process_frame Now an error is shown

You can run over all engine classes and see the debuger crashes randomly. The line prints("--- Done ---") is never called

func _ready():
    for clazz_name in ClassDB.get_class_list():
        if not ClassDB.can_instantiate(clazz_name):
            continue
        var instance = ClassDB.instantiate(clazz_name)
        prints(clazz_name, instance)
        await get_tree().process_frame
        if not instance is RefCounted:
            prints("---> free()", instance, instance.has_method("free"))
            instance.free()
            prints("freed")
            await get_tree().process_frame
    prints("--- Done ---")

Minimal reproduction project

No response

[rburing]

This happens because the destructor sets the PhysicsServer3D singleton to nullptr.

To avoid this we could take the approach used by TextServer, which is to make the servers RefCounted:

https://github.com/godotengine/godot/blob/040f49ed6e71a6e7f23d763c4b56095cbf319ef7/servers/text_server.h#L46

and let the server manager (singleton) keep a reference to the primary implementation:

https://github.com/godotengine/godot/blob/040f49ed6e71a6e7f23d763c4b56095cbf319ef7/servers/text_server.h#L548

This would fix the issue, though I don't think it's very high priority, and we would have to find a good replacement for the ~400 occurrences of PhysicsServer3D::get_singleton() in the existing code base. I'm not a big fan of using an acronym like

https://github.com/godotengine/godot/blob/040f49ed6e71a6e7f23d763c4b56095cbf319ef7/servers/text_server.h#L574

instead of something more explicit. Maybe PhysicsServer3D::get_singleton() could remain and we make it call the manager instead, which would be a simple solution though it feels slightly dirty. cc @bruvzg for thoughts on the design/style.