Hi, I'm the author of Artichoke.

My expectation is that MRB_API can be implemented in a thread-safe way. The API manipulates the interpreter state (in the case of mruby, an `mrb_state`). The interpreter state itself and the implementation of the API can be thread safe or they can not be.

I don't think mruby even assumes the presence of threads on target systems, so it is completely reasonable that the implementation is not thread safe.