> is it unsafe, etc.?

As the readme notes, it's completely unsafe if you perform any mutation operation.

As to the performance hit, I'd expect almost none when not in use for long-running processes: it spawns a thread, and that thread will then wait on a socket accept, no connection, no resources spent. You'll still be paying in memory for the server thread, but that's it (the server thread only creates two objects, an rlcompleter.Completer instance and a function).

We've been using rfoo to do this. It doesn't seem to impact performance when you're not using it. When you are using it, it's unsafe if you do unsafe things. We primarily use it for debugging by passively inspecting the messed-up portion of the server's state.