rsync --delete with empty source folder for fast snapshot deletion: Permissions of hardlinked files are changed to 644. Workaround?

[rsync at altfeld-im.de]

Context
-------

I am one of the active developers of the open source application "Back in Time"
which uses "rsync" as backend and I want to fix an open issue:

    "Back in Time"-Bug:
    https://github.com/bit-team/backintime/issues/994#issuecomment-1724211507

"Back in Time" uses "--link-dest" to reduce traffic and storage by hardlinking
unchanged files in the backups/snapshots and tries to keep the permissions
by using "--perms --group --owner" by default.

Older snapshots are then deleted according to a schedule.



Issue
-----

When deleting a complete snapshot folder with rsync using an empty source folder
(which is a best practice for faster deletion than "rm -f") the permissions of hardlinked
files are changed from eg. 444/-r--r--r-- to 644/-rw-r--r-- for all deleted files
with existing hardlinks:

    mkdir empty
    rsync -a --delete -s empty/ snapshot1/

This distorts the backup history.

There is a corrsponding 6-year-old issue in Bugzilla incl. a patch but the issue is still unfixed:

    https://bugzilla.samba.org/show_bug.cgi?id=12806



Questions
---------

1. Is there a reliable workaround ("--super" is proposed in the issue but may probably not always work)?
2. If a rsync developer is reading this:  Is there any chance to fix this?



Steps to reproduce
------------------

I have attached a bash script "setup.sh" as txt file which makes the issue fully reproducible (on Ubuntu 22.04).
-------------- next part --------------
#!/bin/bash
clear
echo
echo "Minimal reproducible example (MRE)"
echo "----------------------------------"
echo
echo "Tested with"
echo "- rsync  version 3.2.7  protocol version 31"
echo "- ext4 file system"
echo
rm -rf MRE
mkdir MRE
cd MRE
mkdir source
touch source/read_only.txt
chmod 444 source/read_only.txt

mkdir snapshot1
# rsync --recursive --times --devices --specials --hard-links --human-readable -s --links --perms --executability --group --owner source/* snapshot1/
rsync --perms --executability --group --owner source/* snapshot1/

mkdir snapshot2
# rsync --recursive --times --devices --specials --hard-links --human-readable -s --links --perms --executability --group --owner --link-dest=../snapshot1 source/* snapshot2/
rsync --perms --executability --group --owner --link-dest=../snapshot1 source/* snapshot2/
# stat snapshot1/read_only.txt 
echo "File stats BEFORE rsync --delete":
echo
stat snapshot2/read_only.txt

mkdir empty
# delete snapshot1 using an empty source directory (shall be the fastest way to delete a subdir)
# --debug=ALL
rsync -a --delete -s empty/ snapshot1/
# rm -f snapshot1/  # works as expected (unlink does neither change 'Changed' time nor file permissions)
# strace -c rm -rf snapshot1
echo
stat snapshot2/read_only.txt

echo
echo "Results (after deleting snapshot1 via rsync --delete):"
echo "1. The 'Change' time of the hardlinked file was updated"
echo "2. The hardlinked file has now different access rights (644 instead of 444 so it is writable again!)"
echo "This does NOT happen if 'rm -f snapshot1/' is used!"
echo

echo "Files in snapshot2:"
ls -l snapshot2