contrib/systemd/labgrid-exporter: improve service

The labgrid exporter needs to talk to the coordinator (crossbar), so
start after the network-online.target instead of after the
network.target. Also add a weak requirement for that target to make sure
it is actually started.

Adding `PYTHONUNBUFFERED=1` [1] to the environment forces the stdout and
stderr streams to be unbuffered, so all logs end up in the journal
instantly.

Try to read environment variables from `/etc/environment`. This is the
place where `LG_CROSSBAR` should be defined. Don't fail if that file does
not exist, though.

Soften the restart criteria by using `on-failure` instead of `on-abort`,
which covers more failure scenarios, see [2]. The exporter exits with
exit code 100 if it loses the connection to the coordinator. While this
is already covered by `on-failure`, add it nonetheless for documentation
purposes and users that might adjust `Restart=`. Also increase the time
to sleep before restarting the exporter on failure to prevent system load
on failure loops.

Replace explicit `User=`/`Group=` by `DynamicUser=yes`. The labgrid user
might not exist and there is no requirement for a specific user anyway.
Since the exporter does not write any state, drop
`CacheDirectory=`/`CacheDirectoryMode=` altogether. The caching
directories on the exporter host are created by labgrid's
`contrib/systemd/tmpfiles.d/labgrid.conf`. This directory is used by
labgrid's ManagedFile, which is not part of the exporter.

[1] https://docs.python.org/3/using/cmdline.html#envvar-PYTHONUNBUFFERED
[2] https://www.freedesktop.org/software/systemd/man/systemd.service.html#Restart=

Signed-off-by: Bastian Krause <[email protected]>
(cherry picked from commit 0a2deef)

contrib/systemd/labgrid-exporter.service

@@ -1,16 +1,19 @@
 [Unit]
 Description=Labgrid Exporter
-After=network.target
+After=network-online.target
+Wants=network-online.target
 
 [Service]
+Environment="PYTHONUNBUFFERED=1"
+# Should contain LG_CROSSBAR configuration
+EnvironmentFile=-/etc/environment
 ExecStart=/path/to/labgrid/venv/bin/labgrid-exporter /etc/labgrid/exporter.yaml
-Restart=on-abort
-User=labgrid
-Group=labgrid
+Restart=on-failure
+RestartForceExitStatus=100
+RestartSec=30
+DynamicUser=yes
 # Adjust to your distribution (most often "dialout" or "tty")
 SupplementaryGroups=dialout
-CacheDirectory=labgrid
-CacheDirectoryMode=1775
 
 [Install]
 WantedBy=multi-user.target