Haiku (the OS) is looking OK in beta5. POSIX is OK, and there’s been a lot of ports of Linux things. Looking a little into seeing how complex it might be.
The rack source should compile mostly, but rack depends on several 3rd party libraries. The dependencies need porting too.
I don’t code any more but I was the guy who for a couple of years, I got the graphic PaulStretch working on OSX. So I’ve some idea of the underbrush you’ll have to hack through to get it working.