Comment 22 for bug 1626651

The reason why it is fast under upstart is that with upstart the entire session is running in the logind scope/session, in particular unity-settings-daemon; and then pkexec causes

Okt 06 22:10:46 donald pkexec[9943]: pam_systemd(polkit-1:session): Cannot create session: Already running in a session

This is why the entire pam_systemd, systemd --user etc. for root are entirely skipped.

Under the "user session" paradigm that we get with dbus-user-session, most session processes (including gnome-terminal-server and unity-settings-daemon) run *outside* of the logind session/scope and thus not technically within a PAM session. That's why pam_systemd actually does start a new full session which makes this slow.

In conclusion: Under an upstart session we never actually had a "full" new interactive PAM session for the target user anyway, as this was *already* a PAM session. Thus changing pkexec to common-session-noninteractive is actually doing (mostly) the same as we previously did in 16.04 under an upstart session. Thus I am much more convinced now that switching to common-session-noninteractive is not only correct (and also fixing this bug), but also not actually that regression prone.