nixos/wg-quick: added support for configuration files

nixos/modules/services/networking/wg-quick.nix

@@ -10,6 +10,18 @@ let
 
   interfaceOpts = { ... }: {
     options = {
+
+      configFile = mkOption {
+        example = "/secret/wg0.conf";
+        default = null;
+        type = with types; nullOr str;
+        description = ''
+          wg-quick .conf file, describing the interface.
+          This overrides any other configuration interface configuration options.
+          See wg-quick manpage for more details.
+        '';
+      };
+
       address = mkOption {
         example = [ "192.168.2.1/24" ];
         default = [];
@@ -205,7 +217,7 @@ let
   writeScriptFile = name: text: ((pkgs.writeShellScriptBin name text) + "/bin/${name}");
 
   generateUnit = name: values:
-    assert assertMsg ((values.privateKey != null) != (values.privateKeyFile != null)) "Only one of privateKey or privateKeyFile may be set";
+    assert assertMsg (values.configFile != null || ((values.privateKey != null) != (values.privateKeyFile != null))) "Only one of privateKey, configFile or privateKeyFile may be set";
     let
       preUpFile = if values.preUp != "" then writeScriptFile "preUp.sh" values.preUp else null;
       postUp =
@@ -247,7 +259,12 @@ let
           optionalString (peer.allowedIPs != []) "AllowedIPs = ${concatStringsSep "," peer.allowedIPs}\n"
         ) values.peers;
       };
-      configPath = "${configDir}/${name}.conf";
+      configPath =
+        if values.configFile != null then
+          # This uses bind-mounted private tmp folder (/tmp/systemd-private-***)
+          "/tmp/${name}.conf"
+        else
+          "${configDir}/${name}.conf";
     in
     nameValuePair "wg-quick-${name}"
       {
@@ -265,9 +282,17 @@ let
 
         script = ''
           ${optionalString (!config.boot.isContainer) "modprobe wireguard"}
+          ${optionalString (values.configFile != null) ''
+            cp ${values.configFile} ${configPath}
+          ''}
           wg-quick up ${configPath}
         '';
 
+        serviceConfig = {
+          # Used to privately store renamed copies of external config files during activation
+          PrivateTmp = true;
+        };
+
         preStop = ''
           wg-quick down ${configPath}
         '';