What has priority - owner/user vs group permission?

Question

I have just run into the case in which the owner-group of a file has more permissions than the owner-user of a file.

user1@pc:/tmp$ ls testfile -l
----rw---- 1 user1 user1  9 Okt 16 13:16 testfile

Since the user user1 has no permissions to to read the file I get this

user1@pc:/tmp$ cat testfile
cat: testfile: Permission denied

This suprised me as user1 is member of the group user1 which has permission to read the file.

Interesstingly when doing this:

root@pc:/tmp$ addgroup user2 user1
Adding user `test' to group `ress' ...
Adding user test to group ress
Done.
root@pc:/tmp$ su user2
user2@pc:/tmp$ cat testfile
content of testfile
user2@pc:/tmp$

I can read testfile's content.

It seesms the permissions granded (or not) on the user-owner level take precedence over anything later like the permissions existing due to group membership.

My question is if there is a reference to this behaviour I experience in my linux system (that is that not having user-permissions takes away group-permissions)

Also is there a use case for this behaviour?

Solution

The file permissions specifically do not allow read, write or execute of that file to the owner (user1).

If you were to change the owner to another user, then you would be able to read the file under the group permissions.

Excert from File system permissions wiki page

Classes
...
The effective permissions are determined based on the user's class. For example, the user who is the owner of the file will have the permissions given to the owner class regardless of the permissions assigned to the group class or others class.