nixos/networkd: respect systemd.network.links also with disabled systemd-networkd

This mirrors the behaviour of systemd - It's udev that parses `.link`
files, not `systemd-networkd`.

nixos/doc/manual/release-notes/rl-2003.xml

@@ -712,6 +712,14 @@ auth required pam_succeed_if.so uid >= 1000 quiet
      For further reference, please read <link xlink:href="https://github.com/NixOS/nixpkgs/pull/68953">#68953</link> or the corresponding <link xlink:href="https://discourse.nixos.org/t/predictable-network-interface-names-in-initrd/4055">discourse thread</link>.
     </para>
    </listitem>
+   <listitem>
+    <para>
+     The <link linkend="opt-systemd.network.links">systemd.network.links</link> option is now respected
+     even when <link linkend="opt-systemd.network.enable">systemd-networkd</command> is disabled.
+     This mirrors the behaviour of systemd - It's udev that parses <literal>.link</literal> files,
+     not <command>systemd-networkd</command>.
+    </para>
+   </listitem>
   </itemizedlist>
  </section>
 </section>

nixos/modules/system/boot/networkd.nix

@@ -355,6 +355,14 @@ let
   };
 
   linkOptions = commonNetworkOptions // {
+    # overwrite enable option from above
+    enable = mkOption {
+      default = true;
+      type = types.bool;
+      description = ''
+        Whether to enable this .link unit. It's handled by udev no matter if <command>systemd-networkd</command> is enabled or not
+      '';
+    };
 
     linkConfig = mkOption {
       default = {};
@@ -1045,7 +1053,14 @@ in
 
   };
 
-  config = mkIf config.systemd.network.enable {
+  config = mkMerge [
+    # .link units are honored by udev, no matter if systemd-networkd is enabled or not.
+    {
+      systemd.network.units = mapAttrs' (n: v: nameValuePair "${n}.link" (linkToUnit n v)) cfg.links;
+      environment.etc = unitFiles;
+    }
+
+    (mkIf config.systemd.network.enable {
 
       users.users.systemd-network.group = "systemd-network";
 
@@ -1053,12 +1068,9 @@ in
         "systemd-networkd.service" "systemd-networkd-wait-online.service"
       ];
 
-    systemd.network.units = mapAttrs' (n: v: nameValuePair "${n}.link" (linkToUnit n v)) cfg.links
+      systemd.network.units = mapAttrs' (n: v: nameValuePair "${n}.netdev" (netdevToUnit n v)) cfg.netdevs
-      // mapAttrs' (n: v: nameValuePair "${n}.netdev" (netdevToUnit n v)) cfg.netdevs
         // mapAttrs' (n: v: nameValuePair "${n}.network" (networkToUnit n v)) cfg.networks;
 
-    environment.etc = unitFiles;
-
       systemd.services.systemd-networkd = {
         wantedBy = [ "multi-user.target" ];
         restartTriggers = attrNames unitFiles;
@@ -1084,5 +1096,6 @@ in
       };
 
       services.resolved.enable = mkDefault true;
-  };
+    })
+  ];
 }

nixos/tests/networking.nix

@@ -654,6 +654,31 @@ let
             ), "The IPv6 routing table has not been properly cleaned:\n{}".format(ipv6Residue)
       '';
     };
+    # even with disabled networkd, systemd.network.links should work
+    # (as it's handled by udev, not networkd)
+    link = {
+      name = "Link";
+      nodes.client = { pkgs, ... }: {
+        virtualisation.vlans = [ 1 ];
+        networking = {
+          useNetworkd = networkd;
+          useDHCP = false;
+        };
+        systemd.network.links."50-foo" = {
+          matchConfig = {
+            Name = "foo";
+            Driver = "dummy";
+          };
+          linkConfig.MTUBytes = "1442";
+        };
+      };
+      testScript = ''
+        print(client.succeed("ip l add name foo type dummy"))
+        print(client.succeed("stat /etc/systemd/network/50-foo.link"))
+        client.succeed("udevadm settle")
+        assert "mtu 1442" in client.succeed("ip l show dummy0")
+      '';
+    };
   };
 
 in pkgs.lib.mapAttrs (pkgs.lib.const (attrs: makeTest (attrs // {