Export plugin not working with GdExtensions

[piiertho]

Godot version

4.1.1-stable

System information

macOS 13.4.1 arm64

Issue description

Implementing _export_file, _export_begin and _export_end in a gdextension export plugin does nothing.
In engine we have this code:

EditorExportPlatform::ExportNotifier::ExportNotifier(EditorExportPlatform &p_platform, const Ref<EditorExportPreset> &p_preset, bool p_debug, const String &p_path, int p_flags) {
	HashSet<String> features = p_platform.get_features(p_preset, p_debug);
	Vector<Ref<EditorExportPlugin>> export_plugins = EditorExport::get_singleton()->get_export_plugins();
	//initial export plugin callback
	for (int i = 0; i < export_plugins.size(); i++) {
		export_plugins.write[i]->set_export_preset(p_preset);
		if (export_plugins[i]->get_script_instance()) { //script based
			PackedStringArray features_psa;
			for (const String &feature : features) {
				features_psa.push_back(feature);
			}
			export_plugins.write[i]->_export_begin_script(features_psa, p_debug, p_path, p_flags);
		} else {
			export_plugins.write[i]->_export_begin(features, p_debug, p_path, p_flags);
		}
	}
}

We can see it will call _export_begin_script if a script instance is attached to the export plugin, otherwise it calls _export_begin. _export_begin_script does a GDVIRTUAL_CALL call, so it works within a script language. On another side _export_begin rely on classic cpp virtual call.
In the GdExtension case, we have no script instance attached, so we end up in classic cpp vtable case, but gdextensions does not use cpp vtable for inheritance. This ends up with no call to gdextension implementation.

Currently, the only work around to make a gdextension export plugin work is to create a script proxy for it, here is my example.
gdextension:

namespace godot {
    class FmodEditorExportPlugin : public EditorExportPlugin {
        GDCLASS(FmodEditorExportPlugin, EditorExportPlugin)

    public:
        void _export_begin(const PackedStringArray &features, bool is_debug, const String &path, uint32_t flags) override;
        void _export_file(const String& path, const String& type, const PackedStringArray& features) override;

        String _get_name() const override;

        static void _bind_methods();

        FmodEditorExportPlugin() = default;
        ~FmodEditorExportPlugin() = default;
    };
}

Proxy script:

@tool
class_name FmodEditorExportPluginProxy
extends FmodEditorExportPlugin

I think we should always rely on GDVIRTUAL_CALL and avoid check on script instance, so that both script and gdextensions are handled.

Steps to reproduce

Create a gdextension export plugin and add it using editor plugin.

Minimal reproduction project

N/A

[dsnopek]

@alessandrofama Thanks for reporting the bug and all your investigation!

If you have time to test if PR #80999 fixes the issue for you, that would be much appreciated :-)

[dsnopek]

Here's an MRP that I made to test the linked PR: gdext-editor-plugin-test.zip