nix: Add NixOS module for villas-node

Signed-off-by: Steffen Vogel <post@steffenvogel.de>

flake.nix

@@ -208,5 +208,17 @@
 
     # Standard flake attribute specifying the formatter invoked on `nix fmt`
     formatter = forSupportedSystems (system: (pkgsFor system).alejandra);
+
+    # Standard flake attribute for NixOS modules
+    nixosModules = rec {
+      default = villas;
+
+      villas = {
+        imports = [(nixDir + "/module.nix")];
+        nixpkgs.overlays = [
+          self.overlays.default
+        ];
+      };
+    };
   };
 }

packaging/nix/module.nix

@@ -0,0 +1,57 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}: let
+  cfg = config.services.villas.node;
+
+  villasNodeCfg = pkgs.writeText "villas-node.conf" (builtins.toJSON cfg.config);
+in
+  with lib; {
+    options = {
+      services.villas.node = {
+        enable = mkEnableOption (lib.mdDoc "VILLASnode is a client/server application to connect simulation equipment and software.");
+
+        package = mkPackageOption pkgs "villas" {};
+
+        config = mkOption {
+          type = types.attrsOf types.anything;
+          default = {
+            nodes = {};
+            paths = [];
+            idle_stop = false; # Do not stop daemon because of empty paths
+          };
+
+          description = lib.mdDoc ''
+            Contents of the VILLASnode configuration file,
+            {file}`villas-node.json`.
+
+            See: https://villas.fein-aachen.org/docs/node/config/
+          '';
+        };
+      };
+    };
+
+    config = mkIf cfg.enable {
+      assertions = [
+        {
+          assertion = cfg.config != null && cfg.config != {};
+          message = "You must provide services.villas.node.config.";
+        }
+      ];
+
+      # configuration file indirection is needed to support reloading
+      environment.etc."villas/node.json".source = villasNodeCfg;
+
+      systemd.services.villas-node = {
+        description = "VILLASnode";
+        after = ["network.target"];
+        wantedBy = ["multi-user.target"];
+        serviceConfig = {
+          Type = "simple";
+          ExecStart = "${lib.getExe cfg.package} node /etc/villas/node.json";
+        };
+      };
+    };
+  }