systemd: suggest using `--runtime` on `systemctl edit`

`systemctl edit foobar.service` attempts to write into
`/etc/systemd/system`. This is impossible on purpose, for such a
configuration change the `configuration.nix(8)` should be modified.

However, sometimes it's still a handy feature, for instance when
debugging a broken service. For such a purpose,
`systemctl edit --runtime` is still working on NixOS and an even better
solution because it only overrides the unit temporarily by placing the
override config into `/run` rather than `/etc`.

Unfortunately, this flag isn't well-known in my experience. So inspired
by PEP668[1], RaitoBezarius nerd-sniped me into implementing a similar
warning:

    $ ./result/bin/systemctl edit libvirtd.service
    The unit-directory '/etc/systemd/system' is read-only on NixOS, so it's not possible to edit system-units directly. Use 'systemctl edit --runtime' instead.

This only applies to system-level units. Overrides for user-units are
written into `$XDG_CONFIG_HOME/systemd/user` which is not managed by
NixOS (even when using home-manager it's possible to write into that
directory).

[1] https://peps.python.org/pep-0668/

0 files changed, 0 insertions, 0 deletions