[newlib-cygwin] Cygwin: fchmodat: add limited support for AT_SYMLINK_NOFOLLOW

[Ken Brown <kbrown@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=883abd9d7d51ca3cd089acda5917f9fb80a85745

commit 883abd9d7d51ca3cd089acda5917f9fb80a85745
Author: Ken Brown <kbrown@cornell.edu>
Date:   Tue Jan 26 15:54:05 2021 -0500

    Cygwin: fchmodat: add limited support for AT_SYMLINK_NOFOLLOW
    
    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.

Diff:
---
 winsup/cygwin/syscalls.cc | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/winsup/cygwin/syscalls.cc b/winsup/cygwin/syscalls.cc
index 8bb6ebae7..82ddad46d 100644
--- a/winsup/cygwin/syscalls.cc
+++ b/winsup/cygwin/syscalls.cc
@@ -4787,17 +4787,27 @@ fchmodat (int dirfd, const char *pathname, mode_t mode, int flags)
   tmp_pathbuf tp;
   __try
     {
-      if (flags)
+      if (flags & ~AT_SYMLINK_NOFOLLOW)
 	{
-	  /* BSD has lchmod, but Linux does not.  POSIX says
-	     AT_SYMLINK_NOFOLLOW is allowed to fail on symlinks; but Linux
-	     blindly fails even for non-symlinks.  */
-	  set_errno ((flags & ~AT_SYMLINK_NOFOLLOW) ? EINVAL : EOPNOTSUPP);
+	  set_errno (EINVAL);
 	  __leave;
 	}
       char *path = tp.c_get ();
       if (gen_full_path_at (path, dirfd, pathname))
 	__leave;
+      if (flags & AT_SYMLINK_NOFOLLOW)
+	{
+          /* BSD has lchmod, but Linux does not.  POSIX says
+	     AT_SYMLINK_NOFOLLOW is allowed to fail on symlinks.
+	     Linux blindly fails even for non-symlinks, but we allow
+	     it to succeed. */
+	  path_conv pc (path, PC_SYM_NOFOLLOW, stat_suffixes);
+	  if (pc.issymlink ())
+	    {
+	      set_errno (EOPNOTSUPP);
+	      __leave;
+	    }
+	}
       return chmod (path, mode);
     }
   __except (EFAULT) {}