--backup does not keep empty directories in --backup-dir

[mgutt]

rsync deletes empty directories instead of moving them to the --backup-dir if --backup is used.

Version

root@atum:/mnt/disk1/test# rsync -V
rsync  version 3.4.1  protocol version 32

Test

# mkdir /tmp/src
# mkdir /tmp/dst
# mkdir /tmp/dst/sub
# mkdir /tmp/dst/sub/empty_dir
# touch /tmp/dst/sub/file.txt
# mkdir /tmp/backup_dir
# find /tmp/dst
/tmp/dst
/tmp/dst/sub
/tmp/dst/sub/file.txt
/tmp/dst/sub/empty_dir
# rsync -r --itemize-changes --delete --backup --backup-dir="/tmp/backup_dir/" /tmp/src/ /tmp/dst
*deleting   sub/empty_dir/
*deleting   sub/file.txt
*deleting   sub/

Result:

# find /tmp/backup_dir/
/tmp/backup_dir/
/tmp/backup_dir/sub
/tmp/backup_dir/sub/file.txt

Expected result:

# find /tmp/backup_dir/
/tmp/backup_dir/
/tmp/backup_dir/sub
/tmp/backup_dir/sub/empty_dir
/tmp/backup_dir/sub/file.txt

Research

It seems this bug is already known since 2014:
https://bugzilla.samba.org/show_bug.cgi?id=10724#c3

I wonder if that commit was trying to fix another bug that exists in at least 3.1.0, and possibly other versions. When an empty directory is deleted from source but exists in target, and the --delete option is enabled, the empty directory is not placed in --backup-dir. Going to see if I can figure out what exactly the commit changed.

But it seems nobody fixed this or opened an issue for that.