Symptoms
After upgrade of Linux distribution on NG webhost websites hosted there started to throw errors like:
Unknown: open(/var/lib/php/session/sess_de124443145c6cfd2fc8bafcf3608b61, O_RDWR) failed: Permission denied (13) in Unknown on line 0
Session save path directory /var/lib/php/session seem to be set correctly:
# ll -d /var/lib/php/session
drwxrwx--- 2 root apache 4096 1. Sep 16:59 /var/lib/php/session
open_basedir directive allows php to write there.
Cause
Permissions on target directory - /var/lib/php/session - had been reset during installation of Linux distribution packages or by some automated script not related to OA.
Apache on Odin Automation Linux Hosting NG is launched under apache user, but since it serves requests from multitude of users, server and client permissions are separated. So effective owner of files and directories, where end-users' websites should have access to must be set to pemcln.
Resolution
Change group ownership of /var/lib/php/session to pemcln:
# chown root:pemcln /var/lib/php/session