[.NET] Fix serialization of delegates capturing variables

[paulloz]

WIP fix for #81903.

Let's use a similar example to the one described in the issue.

[Tool]
public partial class Scene : VBoxContainer
{
    public override void _Ready()
    {
        for (var i = 0; i < 4; ++i)
        {
            var button = new Button { Text = $"Click me!" };

            var buttonIndex = i;

            // Case 1
            button.Connect(Button.SignalName.Pressed, Callable.From(() => GD.Print($"Button #{buttonIndex} was clicked!"));
            // Case 2
            button.Connect(Button.SignalName.Pressed, Callable.From(() => WhenButtonClicked(buttonIndex)));

            AddChild(button);
        }
    }

    private void WhenButtonClicked(int buttonIndex)
    {
        GD.Print($"Button #{buttonIndex} was clicked!");
    }
}

As of right now, this PR completely fixes case 1 (crudely: captured variables aren't GodotObject).

On my way to fixing case 2 (here, we also capture this). I had to switch the serialization and call godotsharp_var_to_bytes() with p_full_objects = true. I'm not entirely sure about it, there might be security concerns? Everything now serializes fine (and thus, is properly unloaded). But once we try to deserialize the delegate back, this (in the example above) is considered as a VBoxContainer instead of the actual concrete class, resulting in an ArgumentException when we try to use its value. I kinda imagine it's because of the script being unloaded in between, but I can't really think of any good/simple solution here.

Hoping for someone to have a stroke of genius, or to rubber duck me in the right direction ❤️

I've also been wondering on and off if serializing these when reloading is actually something we want to do in this case.

• Bugsquad edit, closes: Capturing a variable in a Callable prevents assembly unloading #81903

[raulsntos]

I suspect closures weren't really tested thoroughly when this was implemented. This is not an area of the module that I'm very familiar with, so I'm trying to understand why it was implemented this way.

But there are definitely improvements in this PR compared to the current implementation, so if it fixes some cases I think that may be worth merging.

I've also been wondering on and off if serializing these when reloading is actually something we want to do in this case.

I also wonder if serializing these is necessary at all.

[paulloz]

I'm removing the cherrypick:4.2 tag, because this can't be cherry-picked without #78219.

[Chubercik]

If it's ready for review, it might be worth it to update the PR description :)

[paulloz]

If it's ready for review, it might be worth it to update the PR description :)

I think the actual back and forth in the comments is easier to understand than re-writing the history?

Long story short:

• We used to try to serialize a System.Type instead of our own representation of variant type like everywhere else.
• We used to have legacy code relying on IConvertible, but no Godot object implements that.

[raulsntos]

LGTM. Thanks!

[akien-mga]

Thanks!