Dtyjlui

In ubuntu 18.04 with default desktop, behaviour of logout/login has changed:

Previously on an Ubuntu system, when I find that I need to add my user id to some group, it was sufficient to

sudo adduser ludwig docker # adds me to group docker

and then I had to logout and login again to make the group change effective.

I notice that with Ubuntu 18.04, after adding the group and logging out and back in, the list of effective groups is still unchanged.

As a workaround I rebooted the system, which is inconvenient (requires making the right selection in grub and re-entering the disk encryption password).

1) Why is the behaviour now like this?
2) Can I do something else short of rebooting?

(I know I can ssh into localhost and get the correct groups in the ssh session only. This is also too inconvenient.)

When "logging out" of the default desktop in ubuntu 18.04, some of the user's processes are not terminated immediately, but linger around. These are (observed by another user):

$ ps axu | grep ^ludwig

ludwig    26508  0.3  0.2  77052  8308 ?        Ss   23:32   0:00 /lib/systemd/systemd --user

ludwig    26509  0.0  0.0 261776  2968 ?        S    23:32   0:00 (sd-pam)

ludwig    26691  0.2  0.3 381288 12204 ?        S<l  23:32   0:00 /usr/bin/pulseaudio --start --log-target=syslog

ludwig    27352  0.0  0.0  49796  3756 ?        Ss   23:33   0:00 /usr/bin/dbus-daemon --session --address=systemd: --nofork --nopidfile --systemd-activation --syslog-only

When logging back in before these processes exit voluntarily, then no new login session is created, but the old one is reused. This is the reason why the new group membership is not visible, it is still the same old login session.

A workaround to avoid rebooting is to wait ~20 seconds after logging out and only then logging back in. The processes exit somewhere between 10 and 20 seconds after logging out.

Edit:
As reported in the comments below, sometimes the lingering processes will not quit even with waiting, and after logging back in, the group memberships have not been updated. I found that in this case it helps to

ps axu | grep ^ludwig | awk '{print $2}' | xargs kill -9

Replace ludwig with your user name. This kills all processes that belong to you. Use only when you're sure you have all your data in all your open programs saved.

The command loginctl terminate-user <user> worked for me. (Replace <user> with your user name) You probably shouldn't run this when logged in though as it will kill all your processes.

A workaround in the current shell is to run "su " to have the new group without having to reboot.

As I said, this trick has to be applied to each shell. That's not global.

The processes remaining for the user after logout are related to systemd

usrtest 4150 4150 4150 1.7 0.1 77140 8500 ? Ss 22:46 0:00 /lib/systemd/systemd --user

usrtest 4151 4150 4150 0.0 0.0 280232 3084 ? S 22:46 0:00 (sd-pam)

usrtest 4610 4610 4610 0.2 0.0 49796 3824 ? Ss 22:47 0:00 /usr/bin/dbus-daemon --session --address=systemd: --nofork --nopidfile --systemd-activation --syslog-only

usrtest 4328 4327 4327 0.5 0.1 386100 12060 ? S

these processes are indeed launched by systemd on behalf of the user.

So I did the following test:

cat /etc/systemd/system/user@1000.service
KillMode=control-group

systemctl --system show -p KillMode user@1000.service
KillMode=control-group

after a logout/login operation, it seems to change this logout behaviour
after (yet) another logout/login following a usermod -a -G, the 'groups' command returns the added group. I don't like it because if these processes are lingering after logout there is probably a good reason (closing operations seems probable), but this problem feels definitely related to systemd.