cp -up: preserve all hard links

* src/copy.c (copy_internal): With --update (-u), this function would
return early once it found that the destination is not older than the
source, *without* recording the source-dev/ino--to--dest_name mapping.
That mapping is required in order to preserve src hard links in the
destination tree, so when using cp with --update and --preserve=links
(perhaps via -p or -a), cp could fail to preserve one hard link
per inode when at least one of the hard-linked names already exists
in the destination tree.
Reported by Odd Harry Mannsverk in http://debbugs.gnu.org/8419.
* tests/cp/preserve-link: New file.  Exercise the flaw/fix.
* tests/Makefile.am (TESTS): Add it.
* NEWS (Bug fixes): Mention it.

NEWS

@@ -8,6 +8,12 @@ GNU coreutils NEWS                                    -*- outline -*-
   I.E. for skipped files, the original ownership is output, not the new one.
   [bug introduced in sh-utils-2.0g]
 
+  cp -u -p would fail to preserve one hard link for each up-to-date copy
+  of a src-hard-linked name in the destination tree.  I.e., if s/a and s/b
+  are hard-linked and dst/s/a is up to date, "cp -up s dst" would copy s/b
+  to dst/s/b rather than simply linking dst/s/b to dst/s/a.
+  [This bug appears to have been present in "the beginning".]
+
   printf '%d' '"' no longer accesses out-of-bounds memory in the diagnostic.
   [bug introduced in sh-utils-1.16]
 

src/copy.c

@@ -1628,6 +1628,17 @@ copy_internal (char const *src_name, char const *dst_name,
                      end up removing the source file.  */
                   if (rename_succeeded)
                     *rename_succeeded = true;
+
+                  /* However, we still must record that we've processed
+                     this src/dest pair, in case this source file is
+                     hard-linked to another one.  In that case, we'll use
+                     the mapping information to link the corresponding
+                     destination names.  */
+                  earlier_file = remember_copied (dst_name, src_sb.st_ino,
+                                                  src_sb.st_dev);
+                  if (earlier_file)
+                    goto create_hard_link;
+
                   return true;
                 }
             }
@@ -1948,6 +1959,7 @@ copy_internal (char const *src_name, char const *dst_name,
         }
       else
         {
+        create_hard_link:;
           /* We want to guarantee that symlinks are not followed.  */
           bool link_failed = (linkat (AT_FDCWD, earlier_file, AT_FDCWD,
                                       dst_name, 0) != 0);

tests/Makefile.am

@@ -341,6 +341,7 @@ TESTS =						\
   cp/parent-perm-race				\
   cp/perm					\
   cp/preserve-2					\
+  cp/preserve-link				\
   cp/preserve-slink-time			\
   cp/proc-short-read				\
   cp/proc-zero-len				\

tests/cp/preserve-link

@@ -0,0 +1,40 @@
+#!/bin/sh
+# Exercise the fix for http://debbugs.gnu.org/8419
+
+# Copyright (C) 2011 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/>.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+print_ver_ cp
+
+same_inode()
+{
+  local u v
+  u=$(stat --format %i "$1") &&
+    v=$(stat --format %i "$2") && test "$u" = "$v"
+}
+
+mkdir -p s t/s || framework_failure_
+touch s/f t/s/f || framework_failure_
+ln s/f s/link || framework_failure_
+
+# This must create a hard link, t/s/link, to the existing file, t/s/f.
+# With cp from coreutils-8.12 and prior, it would mistakenly copy
+# the file rather than creating the link.
+cp -au s t || fail=1
+
+same_inode t/s/f t/s/link || fail=1
+
+Exit $fail