Advice on semi-blocking implementation of `system` / `popen` / `posix_spawn` / `fork` / `vfork` / `exec` calls

I saw [approaches](https://browsix.org/) that running processes as [workers](https://github.com/tbfleming/em-shell). Is there a modern advice/recipes on how to do this? e.g. sometimes an always-parent-blocking call is okay and useful (as if we have a single-user machine without time-sharing betweeen processes). The goal would be to run busybox ash shell with some support of pipes and external commands.

E.g. what would be nice is some advice / easy hooks for implementing various kinds of process emulation. E.g. Module.execve call that the user could override in JavaScript