Is there a cross-platform way to make a process based socket server in C++?

Question

It's something that seems deceptively simple, but comes with a lot of nasty details and compatibility problems. I have some code that kinda works on Linux and... sorta works on Windows but it's having various problems, for what seems like a common and simple problem. I know async is all the rage these days, but I have good reasons to want a process per connection.

I'm writing a server that hosts simulation processes. So each connection is long-running and CPU intensive. But more importantly, these simulators (Ngspice, Xyce) have global state and sometimes segfault or reach unrecoverable errors. So it is essential that each connection has its own process so they can run/crash in parallel and not mess with each other's state.

Another semi-important detail is that the protocol is based on Capnp RPC, which has a nice cross-platform async API, but not a blocking one. So what I do is have my own blocking accept loop that forks a new process and then starts the Capnp event loop in the new process.

So I started with a simple accept loop, added a ton of ifdefs to support windows, and then added fork to make it multiprocess and then added a SIGCHLD handler to try to avoid zombie processes. But Windows doesn't have fork, and if many clients disconnect simultaneously I still get zombies.

My current code lives here: https://github.com/NyanCAD/SimServer/blob/1ba47205904fe57196498653ece828c572579717/main.cpp

I'm fine with either some more ifdefs and hacks to make Windows work and avoid zombies, or some sort of library that either offers a ready made multiprocess socket server or functionality for writing such a thing. The important part is that it can accept a socket in a new process and pass the raw FD to the Capnp event loop.