nix-serve: Add nixos module

This allows sharing the Nix store of the machine as a binary cache
simply by setting 'services.nix-serve.enable = true'.

nixos/modules/module-list.nix

@@ -289,6 +289,7 @@
   ./services/networking/nat.nix
   ./services/networking/networkmanager.nix
   ./services/networking/ngircd.nix
+  ./services/networking/nix-serve.nix
   ./services/networking/notbit.nix
   ./services/networking/nsd.nix
   ./services/networking/ntopng.nix

nixos/modules/services/networking/nix-serve.nix

@@ -0,0 +1,56 @@
+{ config, pkgs, lib, ... }:
+
+with lib;
+
+let
+  cfg = config.services.nix-serve;
+in
+{
+  options = {
+    services.nix-serve = {
+      enable = mkEnableOption "nix-serve, the standalone Nix binary cache server";
+
+      port = mkOption {
+        type = types.int;
+        default = 5000;
+        description = ''
+          Port number where nix-serve will listen on.
+        '';
+      };
+
+      bindAddress = mkOption {
+        type = types.string;
+        default = "0.0.0.0";
+        description = ''
+          IP address where nix-serve will bind its listening socket.
+        '';
+      };
+
+      extraParams = mkOption {
+        type = types.string;
+        default = "";
+        description = ''
+          Extra command line parameters for nix-serve.
+        '';
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    systemd.services.nix-serve = {
+      description = "nix-serve binary cache server";
+      after = [ "network.target" ];
+      wantedBy = [ "multi-user.target" ];
+
+      path = [ config.nix.package pkgs.bzip2 ];
+      environment.NIX_REMOTE = "daemon";
+
+      serviceConfig = {
+        ExecStart = "${pkgs.nix-serve}/bin/nix-serve " +
+          "--port ${cfg.bindAddress}:${toString cfg.port} ${cfg.extraParams}";
+        User = "nobody";
+        Group = "nogroup";
+      };
+    };
+  };
+}