[isr]

Have a look at tcl's ability to run multiple iterations of itself (nested interpreters) within the same process, and how those interpreters can be sandboxed & limited to run just what you allow, & nothing more.

If you were going to handle "untrusted input", that's how you would do it in tcl.

Note: this is separate from tcl's multithreading capability, where it can also run individual interpreters per thread , with thread safe channels between them. (btw, you can mix & match both approaches - multithreading & nested interpreters)