SUMMARY

Using a database with more than one user needs appropriate permissions if you will allow others to tag images or even add images to the DB.

index.xml should be rw-rw-... for people in the group to be able to tag and save.
.thumbnails/* should either be rw-r--... or rw-rw-... (the fist allows only 1 person to update the thumbnails, the second allows that for all).

Unfortunately KPA changes the setting of index.xml to rw-r--...
of .thumbnails/thumbnailindex to rw-rw-...
and of .thumbnails/thumb-0 to rw-r--...

When other users access the DB and do changes, the ownership of files will change too. This is a problem, when at the same time the permissions are set to more restrictive settings than before (rw-rw- to rw-r--).

Here is a log (with the unnecessary lines deleted):

as@wshome5:~/Lokal/tmp/testkpa> ll -a
-rw-rw-r-- 1 as users 45056  5. Jun 17:34 exif-info.db
-rw-rw-r-- 1 as users 21038  5. Jun 17:35 index.xml
drwxrwxr-x 2 as users  4096  5. Jun 17:34 t1
drwxr-xr-x 2 as users  4096  5. Jun 17:35 .thumbnails
as@wshome5:~/Lokal/tmp/testkpa> chmod g-w .thumbnails/*
as@wshome5:~/Lokal/tmp/testkpa> ll .thumbnails/
-rw-r--r-- 1 as users 1082721  5. Jun 17:34 thumb-0
-rw-r--r-- 1 as users    6322  5. Jun 17:35 thumbnailindex
as@wshome5:~/Lokal/tmp/testkpa> cp ../Day201004/I09993* t1
as@wshome5:~/Lokal/tmp/testkpa> kphotoalbum -c index.xml

 --- view the new files as the original owner and save ---

as@wshome5:~/Lokal/tmp/testkpa> ll -a
-rw-r--r-- 1 as users 35138  5. Jun 17:38 index.xml   <== permissions have changed
drwxrwxr-x 2 as users 12288  5. Jun 17:37 t1
drwxr-xr-x 2 as users  4096  5. Jun 17:38 .thumbnails
as@wshome5:~/Lokal/tmp/testkpa> ll .thumbnails/
insgesamt 1772
-rw-r--r-- 1 as users 1801984  5. Jun 17:38 thumb-0
-rw-rw-r-- 1 as users   10922  5. Jun 17:38 thumbnailindex   <== permissions have changed

 --- Now I reset everything to rw-rw-... (also the thumbnails) and add a few images as another user (test)

as@wshome5:~/Lokal/tmp/testkpa> ll -a
-rw-r--r-- 1 test users  79638  5. Jun 17:52 index.xml   <== permissions AND owner have changed
-rw-rw-r-- 1 as   users   7268  5. Jun 17:48 index.xml~0003~.zip
-rw-r--r-- 1 test users  10048  5. Jun 17:52 index.xml~0004~.zip
drwxrwxr-x 2 as   users  20480  5. Jun 17:52 t1
drwxrwxr-x 2 as   users   4096  5. Jun 17:53 .thumbnails
as@wshome5:~/Lokal/tmp/testkpa> ll .thumbnails/
insgesamt 3352
-rw-r--r-- 1 test users 3403652  5. Jun 17:53 thumb-0           <== permissions and owner have changed
-rw-rw-r-- 1 test users   24722  5. Jun 17:53 thumbnailindex    <== owner has changed

 --- reset the ownership and permissions to "admin-only" settings (still letting others tag and save):

as@wshome5:~/Lokal/tmp/testkpa> ll -a
-rw-rw-r-- 1 as users  79638  5. Jun 17:52 index.xml
drwxr-xr-x 2 as users   4096  5. Jun 17:53 .thumbnails
as@wshome5:~/Lokal/tmp/testkpa> ll .thumbnails/
-rw-r--r-- 1 as users 3403652  5. Jun 17:53 thumb-0
-rw-r--r-- 1 as users   24722  5. Jun 17:53 thumbnailindex

 --- let the other user do stuff (add images and tag something)
 --- rebuilding the thumbnails fails (predictably) for the other user

as@wshome5:~/Lokal/tmp/testkpa> ll -a
-rw-r--r-- 1 test users  94846  5. Jun 18:14 index.xml      <== permissions and owner have changed

 --- look at the database as original owner (as) tag and save

as@wshome5:~/Lokal/tmp/testkpa> ll -a
-rw-r--r-- 1 as   users 143004  5. Jun 18:18 index.xml      <== owner has changed (magic!)
as@wshome5:~/Lokal/tmp/testkpa> ll .thumbnails/
insgesamt 4212
-rw-r--r-- 1 as users 4276567  5. Jun 18:18 thumb-0
-rw-rw-r-- 1 as users   29322  5. Jun 18:18 thumbnailindex  <== new thumbs have been generated (but permissions have changed again!)

OBSERVED RESULT

I see inconsistent settings in .thumbnails (the auto-settings are different for thumb-0 and thumbnailindex)
I see changes of ownership of these files and of the index.xml file
I see change in permissions and ownership of the index.xml file which seem to have no effect whatsoever (the magic lies in the backup files).

EXPECTED RESULT

When I set the permissions to some value, I expect them to stay the way I set them.
However, I understand that the permissions on the index file are kept in the backup files and each index file is in effect a new copy (thus the magic).
Allowing write access to a file, I do not expect the file to change ownership (and permissions).

I especially do not like the different settings for the content of .thumbnails (rw-r-- for the thumb-0 and rw-rw- for the thumbnailindex).
I could live with the "admin settings", because then nobody could trash the thumbnails. Normally this does not happen as the thumb-0 file is still write protected. Disaster is waiting to happen, if the thumb-0 file is just "full" and another thumb-1 files is created by the other user.

The "magic" with the index files does not work with the thumb-X files. Thus the other user will just "take them away from me" when I allow her write access.

CHANGE I WOULD WISH FOR

When the thumb-X files are RW for the group they must stay so (even if somebody else becomes the owner).
Changing the owner of existing files is not nice.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: openSuse Leap 15.2
KPA: v5.7.0-264-g36a530b7
KDE Frameworks 5.71.0
Qt 5.12.7 (kompiliert gegen 5.12.7)