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`.

This was originally applied in 36ef112a47,
but was reverted due to 1115959a8d causing
evaluation errors on hydra.

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

@@ -767,7 +767,15 @@ auth required pam_succeed_if.so uid >= 1000 quiet
       </para></listitem>
      </itemizedlist>
     </para>
-   </listitem>
+  </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 = {};
@@ -1045,44 +1053,49 @@ 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;
+    }
 
-    users.users.systemd-network.group = "systemd-network";
+    (mkIf config.systemd.network.enable {
 
-    systemd.additionalUpstreamSystemUnits = [
-      "systemd-networkd.service" "systemd-networkd-wait-online.service"
-    ];
+      users.users.systemd-network.group = "systemd-network";
 
-    systemd.network.units = mapAttrs' (n: v: nameValuePair "${n}.link" (linkToUnit n v)) cfg.links
-      // mapAttrs' (n: v: nameValuePair "${n}.netdev" (netdevToUnit n v)) cfg.netdevs
-      // mapAttrs' (n: v: nameValuePair "${n}.network" (networkToUnit n v)) cfg.networks;
+      systemd.additionalUpstreamSystemUnits = [
+        "systemd-networkd.service" "systemd-networkd-wait-online.service"
+      ];
 
-    environment.etc = unitFiles;
+      systemd.network.units = mapAttrs' (n: v: nameValuePair "${n}.netdev" (netdevToUnit n v)) cfg.netdevs
+        // mapAttrs' (n: v: nameValuePair "${n}.network" (networkToUnit n v)) cfg.networks;
 
-    systemd.services.systemd-networkd = {
-      wantedBy = [ "multi-user.target" ];
-      restartTriggers = attrNames unitFiles;
-      # prevent race condition with interface renaming (#39069)
-      requires = [ "systemd-udev-settle.service" ];
-      after = [ "systemd-udev-settle.service" ];
-    };
-
-    systemd.services.systemd-networkd-wait-online = {
-      wantedBy = [ "network-online.target" ];
-    };
-
-    systemd.services."systemd-network-wait-online@" = {
-      description = "Wait for Network Interface %I to be Configured";
-      conflicts = [ "shutdown.target" ];
-      requisite = [ "systemd-networkd.service" ];
-      after = [ "systemd-networkd.service" ];
-      serviceConfig = {
-        Type = "oneshot";
-        RemainAfterExit = true;
-        ExecStart = "${config.systemd.package}/lib/systemd/systemd-networkd-wait-online -i %I";
+      systemd.services.systemd-networkd = {
+        wantedBy = [ "multi-user.target" ];
+        restartTriggers = attrNames unitFiles;
+        # prevent race condition with interface renaming (#39069)
+        requires = [ "systemd-udev-settle.service" ];
+        after = [ "systemd-udev-settle.service" ];
       };
-    };
 
-    services.resolved.enable = mkDefault true;
-  };
+      systemd.services.systemd-networkd-wait-online = {
+        wantedBy = [ "network-online.target" ];
+      };
+
+      systemd.services."systemd-network-wait-online@" = {
+        description = "Wait for Network Interface %I to be Configured";
+        conflicts = [ "shutdown.target" ];
+        requisite = [ "systemd-networkd.service" ];
+        after = [ "systemd-networkd.service" ];
+        serviceConfig = {
+          Type = "oneshot";
+          RemainAfterExit = true;
+          ExecStart = "${config.systemd.package}/lib/systemd/systemd-networkd-wait-online -i %I";
+        };
+      };
+
+      services.resolved.enable = mkDefault true;
+    })
+  ];
 }

nixos/tests/networking.nix

@@ -655,6 +655,31 @@ 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 mapAttrs (const (attrs: makeTest (attrs // {