nixos/kea: Allow specifying custom config file

nixos/modules/services/networking/kea.nix

@@ -9,20 +9,26 @@ with lib;
 let
   cfg = config.services.kea;
 
+  xor = x: y: (!x && y) || (x && !y);
   format = pkgs.formats.json {};
 
-  ctrlAgentConfig = format.generate "kea-ctrl-agent.conf" {
+  chooseNotNull = x: y: if x != null then x else y;
+
+  ctrlAgentConfig = chooseNotNull cfg.ctrl-agent.configFile (format.generate "kea-ctrl-agent.conf" {
     Control-agent = cfg.ctrl-agent.settings;
-  };
-  dhcp4Config = format.generate "kea-dhcp4.conf" {
+  });
+
+  dhcp4Config = chooseNotNull cfg.dhcp4.configFile (format.generate "kea-dhcp4.conf" {
     Dhcp4 = cfg.dhcp4.settings;
-  };
-  dhcp6Config = format.generate "kea-dhcp6.conf" {
+  });
+
+  dhcp6Config = chooseNotNull cfg.dhcp6.configFile (format.generate "kea-dhcp6.conf" {
     Dhcp6 = cfg.dhcp6.settings;
-  };
-  dhcpDdnsConfig = format.generate "kea-dhcp-ddns.conf" {
+  });
+
+  dhcpDdnsConfig = chooseNotNull cfg.dhcp-ddns.configFile (format.generate "kea-dhcp-ddns.conf" {
     DhcpDdns = cfg.dhcp-ddns.settings;
-  };
+  });
 
   package = pkgs.kea;
 in
@@ -45,6 +51,17 @@ in
             '';
           };
 
+          configFile = mkOption {
+            type = nullOr path;
+            default = null;
+            description = ''
+              Kea Control Agent configuration as a path, see <link xlink:href="https://kea.readthedocs.io/en/kea-${package.version}/arm/agent.html"/>.
+
+              Takes preference over <link linkend="opt-services.kea.ctrl-agent.settings">settings</link>.
+              Most users should prefer using <link linkend="opt-services.kea.ctrl-agent.settings">settings</link> instead.
+            '';
+          };
+
           settings = mkOption {
             type = format.type;
             default = null;
@@ -73,6 +90,17 @@ in
             '';
           };
 
+          configFile = mkOption {
+            type = nullOr path;
+            default = null;
+            description = ''
+              Kea DHCP4 configuration as a path, see <link xlink:href="https://kea.readthedocs.io/en/kea-${package.version}/arm/dhcp4-srv.html"/>.
+
+              Takes preference over <link linkend="opt-services.kea.dhcp4.settings">settings</link>.
+              Most users should prefer using <link linkend="opt-services.kea.dhcp4.settings">settings</link> instead.
+            '';
+          };
+
           settings = mkOption {
             type = format.type;
             default = null;
@@ -122,6 +150,17 @@ in
             '';
           };
 
+          configFile = mkOption {
+            type = nullOr path;
+            default = null;
+            description = ''
+              Kea DHCP6 configuration as a path, see <link xlink:href="https://kea.readthedocs.io/en/kea-${package.version}/arm/dhcp6-srv.html"/>.
+
+              Takes preference over <link linkend="opt-services.kea.dhcp6.settings">settings</link>.
+              Most users should prefer using <link linkend="opt-services.kea.dhcp6.settings">settings</link> instead.
+            '';
+          };
+
           settings = mkOption {
             type = format.type;
             default = null;
@@ -172,6 +211,17 @@ in
             '';
           };
 
+          configFile = mkOption {
+            type = nullOr path;
+            default = null;
+            description = ''
+              Kea DHCP-DDNS configuration as a path, see <link xlink:href="https://kea.readthedocs.io/en/kea-${package.version}/arm/ddns.html"/>.
+
+              Takes preference over <link linkend="opt-services.kea.dhcp-ddns.settings">settings</link>.
+              Most users should prefer using <link linkend="opt-services.kea.dhcp-ddns.settings">settings</link> instead.
+            '';
+          };
+
           settings = mkOption {
             type = format.type;
             default = null;
@@ -214,6 +264,10 @@ in
   }
 
   (mkIf cfg.ctrl-agent.enable {
+    assertions = [{
+        assertion = xor (cfg.ctrl-agent.settings == null) (cfg.ctrl-agent.configFile == null);
+        message = "Either services.kea.ctrl-agent.settings or services.kea.ctrl-agent.configFile must be set to a non-null value.";
+    }];
 
     environment.etc."kea/ctrl-agent.conf".source = ctrlAgentConfig;
 
@@ -252,6 +306,10 @@ in
   })
 
   (mkIf cfg.dhcp4.enable {
+    assertions = [{
+        assertion = xor (cfg.dhcp4.settings == null) (cfg.dhcp4.configFile == null);
+        message = "Either services.kea.dhcp4.settings or services.kea.dhcp4.configFile must be set to a non-null value.";
+    }];
 
     environment.etc."kea/dhcp4-server.conf".source = dhcp4Config;
 
@@ -295,6 +353,10 @@ in
   })
 
   (mkIf cfg.dhcp6.enable {
+    assertions = [{
+        assertion = xor (cfg.dhcp6.settings == null) (cfg.dhcp6.configFile == null);
+        message = "Either services.kea.dhcp6.settings or services.kea.dhcp6.configFile must be set to a non-null value.";
+    }];
 
     environment.etc."kea/dhcp6-server.conf".source = dhcp6Config;
 
@@ -336,6 +398,10 @@ in
   })
 
   (mkIf cfg.dhcp-ddns.enable {
+    assertions = [{
+        assertion = xor (cfg.dhcp-ddns.settings == null) (cfg.dhcp-ddns.configFile == null);
+        message = "Either services.kea.dhcp-ddns.settings or services.kea.dhcp-ddns.configFile must be set to a non-null value.";
+    }];
 
     environment.etc."kea/dhcp-ddns.conf".source = dhcpDdnsConfig;