waitForProcess currently calls waitpid with a safe FFI call, which means that

• you need to use -threaded to call this in a multithreaded program
• one OS thread is needed per waitProcess

instead we could implement this using the IO manager: have waitForProcess communicate with the IO manager, which wakes up the client when the appropriate SIGCHLD is received. Note that this means hooking SIGCHLD so we'd interact badly if the program itself wanted to use SIGCHLD.

I think it was Duncan Coutts who mentioned this idea to me.