ScriptLanguageExtension methods can be called from multiple threads

[TitanNano]

I have encountered this while doing reimports of some assets.

The method ScriptLanguageExtension::frame will be called for every frame in the main loop. In parallel, the engine might call ScriptLanguageExtension::handles_global_class_type from the import thread, which will cause a panic as frame is &mut self.

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The quickest workaround is to not implement ScriptLanguageExtension::frame.

[Bromeon]

The method ScriptLanguageExtension::frame will be called for every frame in the main loop. In parallel, the engine might call ScriptLanguageExtension::handles_global_class_type from the import thread, which will cause a panic as frame is &mut self.

Or maybe expose it without binding &mut self, similar to #[func(gd_self)] in other places?

[TitanNano]

@Bromeon this is certainly an option but if someone wants to perform mutable operations in fn frame() it would still panic in certain situations due to another thread holding an immutable reference.

I think (at least with experimental-threads) access to the class instance storage should block instead of panic.

[Bromeon]

@TitanNano What's the situation around this after recent SiMut changes?

[TitanNano]

@Bromeon unfortunately the changes to ScriptInstance are completely unrelated to this.

Godot (particularly the editor) accesses ScriptLanguage singletons across multiple threads. Since GdCell panics on the second mutable borrow, this is an issue. It would be more ergonomic if GdCell would block the other thread until it becomes available instead of panicking.