[PATCH] Cygwin: fchmodat: add limited support for AT_SYMLINK_NOFOLLOW

[Ken Brown]

On 1/27/2021 7:40 AM, Corinna Vinschen via Cygwin-patches wrote:
> On Jan 26 16:30, Ken Brown via Cygwin-patches wrote:
>> Allow fchmodat with the AT_SYMLINK_NOFOLLOW flag to succeed on
>> non-symlinks.  Previously it always failed, as it does on Linux.  But
>> POSIX permits it to succeed on non-symlinks even if it fails on
>> symlinks.
>>
>> The reason for following POSIX rather than Linux is to make gnulib
>> report that fchmodat works on Cygwin.  This improves the efficiency of
>> packages like GNU tar that use gnulib's fchmodat module.  Previously
>> such packages would use a gnulib replacement for fchmodat on Cygwin.
> 
> Wait, what?  So if Cygwin behaves like Linux, gnulib treats fchmodat
> as non-working?  So what does gnulib do on a Linux system?  Does it
> use its own fchmodat there, too?

Apparently so.  Here's a comment from gnulib's test program for fchmodat:

               /* Test whether fchmodat+AT_SYMLINK_NOFOLLOW works on non-symlinks.
                  This test fails on GNU/Linux with glibc 2.31 (but not on
                  GNU/kFreeBSD nor GNU/Hurd) and Cygwin 2.9.  */

I agree that it's strange.

Ken