Apple can touch it, they choose not to. They choose to make exclusively proprietary software.

No they don't. They use and license various pieces of software that's under BSD and other similar licenses. For instance, they released Swift at the end of 2015. It's on GitHub and has an open development process. Plus, they use GPLv2, just not GPLv3.

I was specifically referring to their kernel when talking about "exclusively proprietary software" (which is what matters in discussions about porting btrfs). Sure, they've liberated some software but their entire stack is essentially proprietary.

I believe that https://opensource.apple.com/source/xnu/xnu-3248.20.55/ is the kernel source for the current version of of Mac OS X 'El Capitan,' and I believe that https://opensource.apple.com/source/xnu/xnu-3248.20.55/bsd/h... is the source for the current HFS+ filesystem. Is that not correct?

Much of the operating system is proprietary, but I didn't think the kernel or HFS+ filesystem were included in that.

I was under the impression that the released sources for the kernel are not complete (they are missing critical features), but I'm not sure whether or not this is the case with the repos you linked. To be clear, you still need proprietary software to compile it (so it's not practically free under the FSF definition).

> Plus, they use GPLv2, just not GPLv3.

Also, btrfs is GPLv2. So that agrees with my point -- if they didn't mind using free software they could use that code.