make lchmodat friendlier

[Eric Blake]

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Jim, what do you think of this patch, so that lchmodat only fails with
ENOSYS when called on a symlink (when called on other file types, it would
now succeed)?

Also, should "openat.h" provide shortcuts named:

accessat() => faccessat(,0)
euidaccessat() => faccessat(,AT_EACCESS)
statat() => fstatat(,0)
lstatat() => fstatat(,AT_SYMLINK_NOFOLLOW)

to match the existing shortcuts of chownat, lchownat, etc.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden
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>From d4d4d1cae33f0dcc2829d3fbf12fa0b83bc3c53e Mon Sep 17 00:00:00 2001
From: Eric Blake <address@hidden>
Date: Sun, 6 Sep 2009 21:08:00 -0600
Subject: [PATCH] openat: with no lchmod, only fail lchmodat on symlinks

* lib/fchmodat.c (lchmod) [!HAVE_LCHMOD]: Perform lstat first, to
minimize failure cases.

Signed-off-by: Eric Blake <address@hidden>
---
 ChangeLog      |    4 ++++
 lib/fchmodat.c |   11 ++++++++++-
 2 files changed, 14 insertions(+), 1 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 06287d4..03e91c9 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,9 @@
 2009-09-06  Eric Blake  <address@hidden>

+       openat: with no lchmod, only fail lchmodat on symlinks
+       * lib/fchmodat.c (lchmod) [!HAVE_LCHMOD]: Perform lstat first, to
+       minimize failure cases.
+
        renameat: new module
        * modules/renameat: New file.
        * lib/at-func2.c: Likewise.
diff --git a/lib/fchmodat.c b/lib/fchmodat.c
index 53cafe0..11ace83 100644
--- a/lib/fchmodat.c
+++ b/lib/fchmodat.c
@@ -30,7 +30,16 @@
    system-supplied declaration.  */
 # undef lchmod
 # define lchmod lchmod_rpl
-static int lchmod (char const *f, mode_t m) { errno = ENOSYS; return -1; }
+static int
+lchmod (char const *f, mode_t m)
+{
+  /* This is slightly racy, but better than nothing.  */
+  struct stat st;
+  if (lstat (f, &st) == -1 || !S_ISLNK (st.st_mode))
+    return chmod (f, m);
+  errno = ENOSYS;
+  return -1;
+}
 #endif

 /* Solaris 10 has no function like this.
-- 
1.6.3.3.334.g916e1