[PATCH] Add renameat2 function [BZ #17662]

[Paul Eggert]

Joseph Myers wrote:

> We've had complaints in glibc about fallbacks that introduce races (e.g.
> <https://sourceware.org/bugzilla/show_bug.cgi?id=9813> for pselect).  Do
> you have reason to believe that this race is somehow different and
> introducing it will never cause problems for users?

"Never" is a pretty strong word, and I don't know that it's true here. I do know 
that the uses of Gnulib renameat2 don't have any problems with races that they 
wouldn't have with their own fallbacks, and I haven't yet run into a potential 
case that would differ from this.

If the requirement is "never", then let's go with another flag 
(RENAME_NONATOMIC, say), that can be ORed in to allow renameat2 to behave 
non-atomically. This flag would not be passed to the Linux kernel syscall; it 
would affect only the user-mode fallback code.