mv: allow moving symlink onto same-inode dest with >= 2 hard links

Normally, mv detects a few subtle cases in which proceeding with a
same-file rename would, with very high probability, cause data loss.
Here, we have found a corner case in which one of these same-inode
tests makes mv refuse to perform a useful operation.  Permit that
corner case.
* src/copy.c (same_file_ok): Detect/exempt this case.
* tests/mv/symlink-onto-hardlink: New test.
* tests/Makefile.am (TESTS): Add it.
* NEWS (Bug fixes): Mention it.
Initially reported by: Matt McCutchen in http://bugs.gnu.org/6960.
Raised again by Anders Kaseorg due to http://bugs.debian.org/654596.
Improved-by: Paul Eggert.

2 files changed, 42 insertions, 0 deletions

diff --git a/tests/Makefile.am b/tests/Makefile.am
index 8b670fc40..a94aaa26f 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -491,6 +491,7 @@ TESTS =						\
   mv/part-symlink				\
   mv/partition-perm				\
   mv/perm-1					\
+  mv/symlink-onto-hardlink			\
   mv/to-symlink					\
   mv/trailing-slash				\
   mv/update					\
diff --git a/tests/mv/symlink-onto-hardlink b/tests/mv/symlink-onto-hardlink
new file mode 100755
index 000000000..2dac48401
--- /dev/null
+++ b/tests/mv/symlink-onto-hardlink
@@ -0,0 +1,41 @@
+#!/bin/sh
+# Ensure that mv works with a few symlink-onto-hard-link cases.
+
+# Copyright (C) 2012 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+print_ver_ mv
+
+touch f || framework_failure_
+ln f h || framework_failure_
+ln -s f s || framework_failure_
+
+# Given two links f and h to some important content, and a symlink s to f,
+# "mv s f" must fail because it might then be hard to find the link, h.
+# "mv s l" may succeed because then, s (now "l") still points to f.
+# Of course, if the symlink were being moved into a different destination
+# directory, things would be very different, and, I suspect, implausible.
+
+echo "mv: 's' and 'f' are the same file" > exp || framework_failure_
+mv s f > out 2> err && fail=1
+compare /dev/null out || fail=1
+compare exp err || fail=1
+
+mv s l > out 2> err || fail=1
+compare /dev/null out || fail=1
+compare /dev/null err || fail=1
+
+Exit $fail