Adding xstat support

[Eric Blake]

Here's the best link I could quickly find summarizing the recent kernel 
discussions:

http://groups.google.com/group/linux.kernel/browse_thread/thread/65f244ddf6829edf/4273489273fb9797?lnk=raot&pli=1

As far as I can tell, the proposed xstat patch has not yet hit the 
kernel proper, so we don't want to code to an interface that isn't yet 
set in stone.  But once xstat() does hit, it offers the potential to 
provide some large speedups to programs like ls(1) and find(1), by 
letting the caller choose how much work the kernel must do rather than 
forcing the kernel to completely populate struct stat only for the 
caller to then ignore that work.  Meanwhile, cygwin is considering 
adding support for xstat() once the Linux interface is set in stone, 
where it has the potential for even more significant speedups.

At any rate, it seems like it should be fairly simple to write a gnulib 
wrapper for xstat() - on platforms that lack the kernel call, simply 
call the full stat().  And it also seems like it would be nice to start 
thinking about which utilities can start being written to use xstat() 
instead of [l]stat() for faster performance in the case where kernel 
support exists.

--
Eric Blake   address@hidden    +1-801-349-2682
Libvirt virtualization library http://libvirt.org