Stability: systemd communication

[hug-dev]

Definition: systemd should receive the same status information from Parsec in future stable versions.
Enforcement: Status notifications must not be removed.

Enforcement Check

Parsec notifies systemd that it is ready/reloading/stopping using sd_notify. This behaviour should remain the same for future stable Parsec versions.

This can only be checked via testing/code review. We can also add warnings in the code, move the relevent files in different modules.

Testing

The systemd communication should be tested for different versions of Parsec.

[hug-dev]

All existing systemd communication should be tested. New tests should be added if more communication channels with systemd are used.
Similar code separation feature as in #392

[hug-dev]

The different notification sent to systemd are: "ready", "reloading" and "stopping".

Those status can be checked using systemctl --user start parsec and seen when the service is started (ready), when it is reloading (after a systemctl --user kill -s HUP parsec) and when it is stopping (after a systemctl --user stop parsec).

I have been reading through this, this and this that since our systemd unit file is not Type=notify, these status are only informational for the user when doing systemctl --user start parsec.

It is not possible to test the systemd integration with Docker. It might be enough to do systemd integration tests during the code freeze period, ahead of releases.