Similar to the (Open)AFS situation

I see lots of parallels with the OpenAFS situation. OpenAFS is a distributed file system that predates Linux and runs on lots of OSes. Maintaining a codebase that can compile kernel modules across several OSes and many versions of each (based on feature, not version number) is no small task. Integrating patches requires not breaking other OS builds. And there are licensing issues. There's no way to relicense the code, and some Linux kernel folks have been borderline hostile to the idea of even allowing the OpenAFS kernel module to be linked/loaded into the GPLed Linux kernel; there's no way to bring the existing OpenAFS kernel module in-tree. Yet somehow the OpenAFS core team manages to keep code working across versions of multiple OSes. I'm glad they do it, as I use/need OpenAFS, but I'm glad it's not my problem.