config.boot.kernelPackages.wireguard evaluates to null on machine
closure having a > 5.6 Linux kernels, hence making the evaluation of
this test fail.
Wireguard is now part of the mainline Linux kernel, we do not need to
to add it via a additional kernel module anymore for this test.
The original idea for this test was, on top of providing a networkd
test, to provide newcomers with a sample configuration they could use
to get started with networkd.
That's precisely why we were doing this systemd tmpfile dance in the
first place. It was a convenient way to create a runtime file with a
specific mode and owner.
Sadly, this tmpfile rule made the test flaky. There's a race condition
between the wireguard interface configured by systemd-networkd and
systemd-tmpfiles-setup.
Sometimes, networkd is going to try loading the wireguard private key
file *before* the said file gets created by systemd-tmpfiles.
A perfect solution here would be to create a "After" dependency
between wg0.netdev and systemd-tmpfiles-setup.service. Sadly, it is
currently impossible to create such a dependency between a
networkd-specific unit and a service.
We're removing this tmp file in favor of pointing networkd directly to
the Nix store. This is clearly something that shouldn't be done in the
real world for a private file: the store is world-readable. However,
this is the only way I found to fix this test flakiness for now.
In `systemd-243` the option `FwMark` in the `[WireGuard]` section of
a `.netdev`-unit has been renamed to `FirewallMark`[1]. Due to the
removal of deprecated options in our `networkd` module[2] the evaluation
of this test doesn't work.
Renaming the option to its new name fixes the issue.
[1] 1c30b174ed
[2] e9d13d3751
