ln: --relative: fix updating of existing symlinks

Don't dereference an existing symlink being replaced.
I.E. generate the symlink relative to the symlink's containing dir,
rather than to some arbitrary place it points to.

* src/ln.c (convert_abs_rel): Don't consider the final component
of the symlink name when canonicalizing, as we want to avoid
dereferencing the final component.
* tests/ln/relative.sh: Add a test case.
* NEWS: Mention the fix.
Resolves http://bugs.gnu.org/14116

1 files changed, 8 insertions, 6 deletions

diff --git a/src/ln.c b/src/ln.c
index 1aa147324..2489b9ac7 100644
--- a/src/ln.c
+++ b/src/ln.c
@@ -132,22 +132,24 @@ target_directory_operand (char const *file)
 static char *
 convert_abs_rel (const char *from, const char *target)
 {
-  char *realtarget = canonicalize_filename_mode (target, CAN_MISSING);
+  /* Get dirname to generate paths relative to.  We don't resolve
+     the full TARGET as the last component could be an existing symlink.  */
+  char *targetdir = dir_name (target);
+
+  char *realdest = canonicalize_filename_mode (targetdir, CAN_MISSING);
   char *realfrom = canonicalize_filename_mode (from, CAN_MISSING);
 
   /* Write to a PATH_MAX buffer.  */
   char *relative_from = xmalloc (PATH_MAX);
 
-  /* Get dirname to generate paths relative to.  */
-  realtarget[dir_len (realtarget)] = '\0';
-
-  if (!relpath (realfrom, realtarget, relative_from, PATH_MAX))
+  if (!relpath (realfrom, realdest, relative_from, PATH_MAX))
     {
       free (relative_from);
       relative_from = NULL;
     }
 
-  free (realtarget);
+  free (targetdir);
+  free (realdest);
   free (realfrom);
 
   return relative_from ? relative_from : xstrdup (from);