install: avoid redundant metadata updates with -C

"install -C" (--compare) has always called chown/chmod on the
destination even after determining that copying was unnecessary,
ever since --compare was added in commit v7.0-182-gdac5f12c6.
need_copy() already verifies that the destination's mode, owner, and
group match (and, with --preserve-context, that the SELinux context
matches too) before skipping the copy, so re-applying them is
redundant, and can needlessly fail on a destination that cannot be
chown/chmod'd even though nothing would actually change, e.g. one
marked immutable with chattr +i.

* src/install.c (install_file_in_file): Skip metadata updates
if we already skipped the data, as previous checks ensure
that the metadata is already as required.
* tests/install/install-C-immutable.sh: Add a test.
(based on tests/rm/empty-immutable-skip.sh).
* tests/local.mk: Reference the new test.
* NEWS: Mention the improvement.

NEWS

@@ -67,6 +67,9 @@ GNU coreutils NEWS                                    -*- outline -*-
   'df', 'du', 'ls', 'od', 'pr', and 'sort' now escape invalid arguments in error
   messages for options expecting an integer.
 
+  'install -C' will now avoid updating file metadata when the destination
+  already has the appropriate ownership and permissions.
+
   'sort' will now better use available memory and parallel operation
   when reading from unknown sized inputs like pipes.
 

src/install.c

@@ -737,7 +737,9 @@ install_file_in_file (char const *from, char const *to,
                                  || ! S_ISREG (from_sb.st_mode))
       && ! change_timestamps (&from_sb, to, to_dirfd, to_relname))
     return false;
-  return change_attributes (to, to_dirfd, to_relname);
+
+  return copy_status == COPY_SKIPPED
+         || change_attributes (to, to_dirfd, to_relname);
 }
 
 /* Create any missing parent directories of TO,

tests/install/install-C-immutable.sh

@@ -0,0 +1,58 @@
+#!/bin/sh
+# Ensure "install -C" does not attempt to chown/chmod an unchanged,
+# immutable destination file.
+# Requires root access to do chattr +i, as well as a file system
+# that supports the immutable attribute (e.g., ext2/3/4, xfs, btrfs).
+
+# Copyright (C) 2026 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 <https://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ ginstall
+require_root_
+
+# These simple one-file operations are expected to work even in the
+# presence of this bug, and we need them to set up the rest of the test.
+chattr_i_works=1
+touch f || framework_failure_
+chattr +i f 2>/dev/null || chattr_i_works=0
+rm f 2>/dev/null
+test -f f || chattr_i_works=0
+chattr -i f 2>/dev/null || chattr_i_works=0
+rm f 2>/dev/null || chattr_i_works=0
+test -f f && chattr_i_works=0
+
+if test $chattr_i_works = 0; then
+  skip_ "chattr +i doesn't work on this file system"
+fi
+
+u1=1
+g1=1
+
+echo test > a || framework_failure_
+
+# Install once to create the destination, then make it immutable.
+ginstall -Cv -m0644 -o$u1 -g$g1 a b || framework_failure_
+chattr +i b || framework_failure_
+
+# A second "-C" install with an identical source, mode, owner and group
+# must be a no-op: it must not attempt to chown/chmod the destination,
+# since that would fail on an immutable file even though nothing about
+# it would actually change.
+ginstall -Cv -m0644 -o$u1 -g$g1 a b || fail=1
+
+chattr -i b
+
+Exit $fail

tests/local.mk

@@ -121,6 +121,7 @@ all_root_tests =				\
   tests/du/bind-mount-dir-cycle.sh		\
   tests/du/bind-mount-dir-cycle-v2.sh		\
   tests/id/setgid.sh				\
+  tests/install/install-C-immutable.sh		\
   tests/install/install-C-root.sh		\
   tests/ls/capability.sh			\
   tests/ls/no-cap.sh				\