bug#12632: file permissions checking mishandled when setuid

[Paul Eggert]

On 10/16/2012 09:36 AM, Eli Zaretskii wrote:
>> > I thought the point of Bug#12587 was that on Windows,
>> > 'stat' can be way slower than faccessat, so much
>> > so that Emacs appears unresponsive.
> Granted, I was talking only about Posix platforms.

I'd rather avoid the situation where we have code like this:

  #if defined WINDOWSNT || defined __sun
    use faccessat
  #elif defined __linux__
    use stat
  #else
    /* not sure which is faster */
  #endif

unless there's a reasonably significant performance advantage
in doing so.  The performance advantage of stat on GNU/Linux
is relatively minor, whereas the performance advantage of
faccessat on Windows is major.  Since it's simpler to use faccessat
everywhere, I installed a patch to do that as trunk bzr 110591
and am marking this bug as done.  In the unlikely event that we
run into significant performance issues because of this we can
easily switch to the more-complex approach.