Similar to the `user` option, the added `group` option sets the group of
the executing process. If not `null`, it also sets `DynamicUser=false`.
In case `user` is set to `null` (the default), systemd would run the
service as root implicitly. As this is dangerous and most certainly not
what users want, we force them to set `user = "root"` explicitly if
that's really their intention. That's achieved through an assertion.
Older versions of the github-runner package might not have the
`nodeRuntimes` argument yet causing an error as the NixOS module always
tries to override the argument.
The commit makes sure we only override `nodeRuntimes` if the configured
package has a `nodeRuntimes` argument.
Nothing the script `config.sh` does prior to the final call to
`Runner.Listener configure` is relevant for the systemd service.
Particularly, we don't need (nor want) any of the artifacts the `env.sh`
script creates.
Purge contents of `workDir` as root to also allow the removal of files
marked as read-only. It is easy to create read-only files in `workDir`,
e.g., by copying files from the Nix store.
The `serviceOverrides` module option is commonly used to loosen the
systemd unit's hardening. This commit merges the `serviceConfig` with
`mkMerge` instead of using the update operator `//` which discards all
existing values on conflict. To avoid a breaking change which requires
defining each option with a higher priority (e.g., through `mkForce`),
this commit prefixes hardening values with `mkDefault`.
Notable exceptions are list hardening options which use `mkBefore`
instead of `mkDefault`. This allows for easy extension of the existing
settings. Resetting redefinitions are still possible through `mkForce`.
On some occasions, the GitHub runner service encounters errors which are
deemed retryable but result in the runner's termination. To signal a
retryable error, the runner exits with status code 2:
https://github.com/actions/runner/blob/40ed7f8/src/Runner.Common/Constants.cs#L146
To account for that behavior, this commit sets
`RestartForceExitStatus=2` which results in a service restart regardless
of using an ephemeral runner or not.
