Add NixOS module

Fixes #1. Still needs documentation though.

crane.nix

@@ -24,7 +24,7 @@
 let
   version = "0.1.0";
 
-  ignoredPaths = [ ".github" "target" "book" ];
+  ignoredPaths = [ ".github" "target" "book" "nixos" ];
 
   src = lib.cleanSourceWith {
     filter = name: type: !(type == "directory" && builtins.elem (baseNameOf name) ignoredPaths);

flake.nix

@@ -115,5 +115,19 @@
         inherit (cranePkgs) attic attic-client attic-server;
       };
     };
+
+    nixosModules = {
+      atticd = {
+        imports = [
+          ./nixos/atticd.nix
+        ];
+
+        services.atticd.useFlakeCompatOverlay = false;
+
+        nixpkgs.overlays = [
+          self.overlays.default
+        ];
+      };
+    };
   };
 }

nixos/atticd.nix

@@ -0,0 +1,138 @@
+{ lib, pkgs, config, ... }:
+
+let
+  inherit (lib) types;
+
+  cfg = config.services.atticd;
+
+  # unused when the entrypoint is flake
+  flake = import ../flake-compat.nix;
+  overlay = flake.defaultNix.overlays.default;
+
+  format = pkgs.formats.toml { };
+
+  checkedConfigFile = pkgs.runCommand "checked-attic-server.toml" {
+    configFile = cfg.configFile;
+  } ''
+    cat $configFile
+
+    export ATTIC_SERVER_TOKEN_HS256_SECRET_BASE64="dGVzdCBzZWNyZXQ="
+    ${cfg.package}/bin/atticd --mode check-config -f $configFile
+    cat <$configFile >$out
+  '';
+
+  hasLocalPostgresDB = let
+    url = cfg.settings.database.url;
+    localStrings = [ "localhost" "127.0.0.1" "/run/postgresql" ];
+    hasLocalStrings = lib.any (lib.flip lib.hasInfix url) localStrings;
+  in config.services.postgresql.enable && lib.hasPrefix "postgresql://" url && hasLocalStrings;
+in
+{
+  options = {
+    services.atticd = {
+      enable = lib.mkOption {
+        description = ''
+          Whether to enable the atticd, the Nix Binary Cache server.
+        '';
+        type = types.bool;
+        default = false;
+      };
+      package = lib.mkOption {
+        description = ''
+          The package to use.
+        '';
+        type = types.package;
+        default = pkgs.attic-server;
+      };
+      credentialsFile = lib.mkOption {
+        description = ''
+          Path to an EnvironmentFile containing required environment
+          variables:
+
+          - ATTIC_SERVER_TOKEN_HS256_SECRET_BASE64: The Base64-encoded version of the
+            HS256 JWT secret.
+        '';
+        type = types.path;
+      };
+      settings = lib.mkOption {
+        description = ''
+          Structured configurations of atticd.
+        '';
+        type = format.type;
+        default = {}; # setting defaults here does not compose well
+      };
+      configFile = lib.mkOption {
+        description = ''
+          Path to an existing atticd configuration file.
+
+          By default, it's generated from `services.atticd.settings`.
+        '';
+        type = types.path;
+        default = format.generate "server.toml" cfg.settings;
+        defaultText = "generated from `services.atticd.settings`";
+      };
+
+      # Internal flags
+      useFlakeCompatOverlay = lib.mkOption {
+        description = ''
+          Whether to insert the overlay with flake-compat.
+        '';
+        type = types.bool;
+        internal = true;
+        default = true;
+      };
+    };
+  };
+  config = lib.mkIf (cfg.enable) (lib.mkMerge [
+    {
+      assertions = [
+        {
+          assertion = !lib.isStorePath cfg.credentialsFile;
+          message = ''
+            <option>services.atticd.credentialsFile</option> points to a path in the Nix store. The Nix store is globally readable.
+
+            You should use a quoted absolute path to prevent this.
+          '';
+        }
+      ];
+
+      services.atticd.settings = {
+        database.url = lib.mkDefault "sqlite:///var/lib/atticd/server.db?mode=rwc";
+
+        # "storage" is internally tagged
+        # if the user sets something the whole thing must be replaced
+        storage = lib.mkDefault {
+          type = "local";
+          path = "/var/lib/atticd/storage";
+        };
+      };
+
+      systemd.services.atticd = {
+        wantedBy = [ "multi-user.target" ];
+        after = [ "network.target" ] ++ lib.optional hasLocalPostgresDB "postgresql.service";
+        serviceConfig = {
+          ExecStart = "${cfg.package}/bin/atticd -f ${checkedConfigFile}";
+          EnvironmentFile = cfg.credentialsFile;
+          StateDirectory = "atticd"; # for usage with local storage and sqlite
+          DynamicUser = true;
+          ProtectHome = true;
+          ProtectHostname = true;
+          ProtectKernelLogs = true;
+          ProtectKernelModules = true;
+          ProtectKernelTunables = true;
+          ProtectProc = "invisible";
+          ProtectSystem = "strict";
+          RestrictAddressFamilies = [ "AF_INET" "AF_INET6" "AF_UNIX" ];
+          RestrictNamespaces = true;
+          RestrictRealtime = true;
+          RestrictSUIDSGID = true;
+        };
+      };
+
+      environment.systemPackages = [ cfg.package ];
+    }
+    (lib.mkIf cfg.useFlakeCompatOverlay {
+      nixpkgs.overlays = [ overlay ];
+    })
+  ]);
+}