butane: add systemd user unit example

[tormath1]

it's not that easy to create a systemd user unit from butane, I think it's nice to document it.

[krnowak]

Looks alright, but I don't understand why user units need the symlinks to be created by ignition, while system units don't.

[krnowak]

So user units require explicit creation of the symlink? As in: it's not enough to specify WantedBy in the unit file, like it's done for system units?

[tormath1]

Good point. I removed the symlink and the unit is not enabled. 🤔

[krnowak]

Meh, I think that this "gotcha" for user units should be spelled out in the example description. But I don't know if this is because it's a user unit, or it's because it's placed in /etc.

[pothos]

Ignition creates a preset file for systemd to enable the units. For user units, this should be under /etc/systemd/user-preset/ but Ignition doesn't know about that, only /etc/systemd/system-preset/.

[pothos]

Anyway there is no way to set enabled: true for the user unit because it's created as a file here.

[pothos]

Small hint, I think you could also add one of these Upholds= drop-ins for default.target.wants but the symlink is better^^

[tormath1]

I have two issues with this configuration actually:

1. We can see the logs / the status with systemctl --user status hello.service but we can't get logs with journalctl --user --unit hello.service because core as a UID < 1000 (see: journalctl --user --unit and --user-unit differ systemd/systemd#26742)
2. Even if we define the unit path to be /etc/systemd/user it lands into /etc/xdg/systemd/user which is not a direct issue as systemd looks into this path but can be disturbing to see systemctl --user cat hello.service showing this path...

[tormath1]

I think we can go ahead with this PR. We won't fix today this journalctl --user -u hello.service issue and it's tracked. The goal of this PR is to show how to create a systemd user unit, one can still get the logs with systemctl --user status hello.service