Race-free process creation in the GNU C Library

Yes, a pidfd_getpid() *syscall* is clearly missing. It might not have been really needed before (modulo system libraries lacking appropriate (if non-portable) functionality to actually return you the pidfd *and* pid on clone and/or fork), but now that you can send pidfds around the system over sockets, a syscall is clearly very desirable, it seems to be the best way to solve the underlying problem.

While it looks at first glance that it would be "easy" to write one, that's for someone already used to working in that area of the kernel -- there are likely permission checks one need to get perfectly right to not create a security mishap, namespace concerns, etc. Experience in the specific area of the kernel you're working with almost always help a lot on the quality of the first public version of a patch, and faster acceptance in mainline for non-controversial changes.