nix: initiate NixOS module; add ncro to packages output

Signed-off-by: NotAShelf <raf@notashelf.dev>
Change-Id: I4948bbc2a53adbc64727a1dc935b750e6a6a6964

nix/module.nix

@@ -0,0 +1,67 @@
+self: {
+  config,
+  pkgs,
+  lib,
+  ...
+}: let
+  inherit (lib.modules) mkIf;
+  inherit (lib.options) mkOption mkEnableOption mkPackageOption;
+
+  format = pkgs.formats.yaml {};
+
+  cfg = config.services.ncro;
+  configFile = format.generate "ncro.yaml" cfg.settings;
+in {
+  options.services.ncro = {
+    enable = mkEnableOption "ncro, the Nix cache route optimizer";
+
+    package = mkPackageOption self.packages.${pkgs.stdenv.hostPlatform.system} ["ncro"] {};
+
+    settings = mkOption {
+      type = format.type;
+      default = {};
+      description = ''
+        ncro configuration as an attribute set. Keys and structure match the
+        YAML config file format; all defaults are handled by the ncro binary.
+      '';
+      example = {
+        logging.level = "info";
+        server = {
+          listen = ":8080";
+          cache_priority = 20;
+        };
+
+        upstreams = [
+          {
+            url = "https://cache.nixos.org";
+            priority = 10;
+          }
+          {
+            url = "https://nix-community.cachix.org";
+            priority = 20;
+          }
+        ];
+
+        cache = {
+          ttl = "2h";
+          negative_ttl = "15m";
+        };
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    systemd.services.ncro = {
+      description = "Nix Cache Route Optimizer";
+      wantedBy = ["multi-user.target"];
+      after = ["network.target"];
+      serviceConfig = {
+        ExecStart = "${lib.getExe cfg.package} -config ${configFile}";
+        DynamicUser = true;
+        StateDirectory = "ncro";
+        Restart = "on-failure";
+        RestartSec = "5s";
+      };
+    };
+  };
+}