nixosTests.systemd-networkd: fix test flakiness

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.

1 files changed, 5 insertions, 4 deletions

diff --git a/nixos/tests/systemd-networkd.nix b/nixos/tests/systemd-networkd.nix
index 162970e67537..d5fb2431dbad 100644
--- a/nixos/tests/systemd-networkd.nix
+++ b/nixos/tests/systemd-networkd.nix
@@ -7,16 +7,17 @@ let generateNodeConf = { lib, pkgs, config, privk, pubk, peerId, nodeId, ...}: {
       virtualisation.vlans = [ 1 ];
       environment.systemPackages = with pkgs; [ wireguard-tools ];
       boot.extraModulePackages = [ config.boot.kernelPackages.wireguard ];
-      systemd.tmpfiles.rules = [
-        "f /run/wg_priv 0640 root systemd-network - ${privk}"
-      ];
       systemd.network = {
         enable = true;
         netdevs = {
           "90-wg0" = {
             netdevConfig = { Kind = "wireguard"; Name = "wg0"; };
             wireguardConfig = {
-              PrivateKeyFile = "/run/wg_priv";
+              # NOTE: we're storing the wireguard private key in the
+              #       store for this test. Do not do this in the real
+              #       world. Keep in mind the nix store is
+              #       world-readable.
+              PrivateKeyFile = pkgs.writeText "wg0-priv" privk;
               ListenPort = 51820;
               FirewallMark = 42;
             };