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

3 files changed, 17 insertions, 6 deletions

diff --git a/NEWS b/NEWS
index 0c2daad3e..b9fc6d2d9 100644
--- a/NEWS
+++ b/NEWS
@@ -9,6 +9,10 @@ GNU coreutils NEWS                                    -*- outline -*-
   permissions.
   [This bug was present in "the beginning".]
 
+  ln --relative now updates existing symlinks correctly.  Previously it based
+  the relative link on the dereferenced path of an existing link.
+  [This bug was introduced when --relative was added in coreutils-8.16.]
+
 ** New features
 
   join accepts a new option: --zero-terminated (-z). As with the sort,uniq
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);
diff --git a/tests/ln/relative.sh b/tests/ln/relative.sh
index 0418b8a04..818da8392 100755
--- a/tests/ln/relative.sh
+++ b/tests/ln/relative.sh
@@ -29,4 +29,9 @@ test $(readlink usr/bin/foo) = '../lib/foo/foo' || fail=1
 ln -sr usr/bin/foo usr/lib/foo/link-to-foo
 test $(readlink usr/lib/foo/link-to-foo) = 'foo' || fail=1
 
+# Correctly update an existing link, which was broken in <= 8.21
+ln -s dir1/dir2/f existing_link
+ln -srf here existing_link
+test $(readlink existing_link) = 'here' || fail=1
+
 Exit $fail