Maybe that's what's happening right now :-) If I would build something like this, I'd go for an interpreter with a stack you manage yourself to handle infinite

Maybe that's what's happening right now :-) If I would build something like this, I'd go for an interpreter with a stack you manage yourself to handle infinite loops... using a timeout, not by solving the halting problem, random smartass!. But I over-engineer everything so don't trust me on that. They probably use a timeout on the process too. Actually I would think using "dangerous" (i.e. filesystem) code is the real danger they'd have to defend against.