Do you have dbus running in your user session?

Ping?

[ricardofunke@funkenote ~]$ ps aux | grep dbus
dbus 541 0.0 0.0 36328 5224 ? Ss Dez02 0:47 /usr/bin/dbus-daemon --system --address=systemd: --nofork --nopidfile --systemd-activation
gdm 656 0.0 0.0 34828 3836 ? Ss Dez02 0:00 /usr/bin/dbus-daemon --session --address=systemd: --nofork --nopidfile --systemd-activation
gdm 1316 0.0 0.0 34612 3656 ? S Dez02 0:00 /usr/bin/dbus-daemon --config-file=/etc/at-spi2/accessibility.conf --nofork --print-address 3
ricardo+ 1417 0.0 0.0 35792 4912 ? Ss Dez02 0:03 /usr/bin/dbus-daemon --session --address=systemd: --nofork --nopidfile --systemd-activation
ricardo+ 1428 0.0 0.0 34744 3636 ? S Dez02 0:01 /usr/bin/dbus-daemon --config-file=/etc/at-spi2/accessibility.conf --nofork --print-address 3
daemon 16420 0.0 0.0 76536 5320 ? S Dez03 0:00 /usr/lib/cups/notifier/dbus dbus://
ricardo+ 29848 0.0 0.0 10752 2192 pts/2 S+ 00:18 0:00 grep dbus

systemctl
{...}
dbus.service loaded active running D-Bus System Message Bus
{...}
dbus.socket loaded active running D-Bus System Message Bus Socket
{...}

I don't know what you're meaning, but this is what I could answer based on what I think you want to know.

Don't know the status of these during the update though.

I do not think that things are in place for offline updates to work in Arch Linux, no?

Here is related information from Fedora:
https://fedoraproject.org/wiki/Features/OfflineSystemUpdates

Well, I'm not the package maintainer, so I don't know, I'm only reporting the problem. Things used to work until last updates though.

I didn't find anything in this article that shows what or why it doesn't work in Arch or maybe I didn't understand your point.

It appears that this Process now needs plymouth to run. I have not found a way to use it without Plymouth.
I installed plymouth as described in the wiki. It works now (does not show me any progress anymore while updating ;( )

Here is a more general document about freedesktop's offline system updates:
http://freedesktop.org/wiki/Software/systemd/SystemUpdates/

I still think that we do not support that. Probably gnome-software thinks we do... So two questions arise:
Why does it think we do support it?
How to prevent gnome-software from doing offline updates?

gnome-software is very unsupported; don't expect anything to work.

This isn't just a matter of bugs. The way it operates (automated offline upgrades) is contrary to how Arch upgrades, which may require user intervention before or after the upgrade in order to retain a usable system.

Offline updates worked before: the system rebooted, it installed the updates (without starting the display manager) and it rebooted again.

If I downgrade systemd to 227 offline updates work again, maybe there's a bug in systemd 228?

If gnome-software is very unsupported, why it's included in the official repositories?

Oh, offline updates did work? I missed that point and it surprises me.
Even more, I could not get it working myself. Can you give a step-by-step description what you do for an offline update? I do prefer pkcon (the packagekit console client, so no gnome-software, please - if possible).

$ sudo pkcon update -d
$ sudo pkcon offline-trigger
$ reboot

Looks like this really works... (Me is surprised! :) Well, kind of... :-p

Can you try to add "Requires=dbus.service" to packagekit-offline-update.service and test again, please? The file should look something like that:

[Unit]
Description=...
Requires=dbus.service
OnFailure=reboot.target
[...]

Should we enable this service before this change?

It is already enabled for system-update.target (see symlink /usr/lib/systemd/system/system-update.target.wants/packagekit-offline-update.service).

So no, just edit the file, prepare and trigger the offline update and reboot.

I've tried adding "Requires=dbus.service" to packagekit-offline-update.service and it worked.

Requires=dbus.socket would be more appropriate. In any case, this is also an upstream problem.

Already sent upstream (with dbus.socket... ;)
https://github.com/hughsie/PackageKit/pull/99