How does a friendica instance get the "contact id" of a visitor? / Need SQL help

Hi

I am digging in the problem that restricted image / images shared with a specific group or user are not accessible, see github.com/friendica/friendica…

Now I think that I found at least two issues:

First:
The allow_cid filed in the photo table seems not correctly be set. For an image shared with followers only, the result is:

 select `accessible`,allow_cid,allow_gid from photo where `resource-id` = '100228443963b9a03d31378698994057';
+------------+-----------+-----------+
| accessible | allow_cid | allow_gid |
+------------+-----------+-----------+
|          1 | <3>       | <~>       |
|          1 | <3>       | <~>       |
|          1 | <3>       | <~>       |
+------------+-----------+-----------+

Second:
The getRemoteContactID method returns a value > 0 iff the page is reloaded completely (ctrl + f5) otherwise 0.

Accordingly, I have two questions:

• Does anyone know how the "visitor" is identified, in more detail their contact-id (cid), on the local instance?

• To the mysql pros: Can someone provide me the sql command to update the allow_cid filed in the photo table? This would make by debugging efforts much easier :-) .

Thank you very much, helping me to support <3