sane service: add saned support

nixos/modules/services/hardware/sane.nix

@@ -7,9 +7,26 @@ let
   pkg = if config.hardware.sane.snapshot
     then pkgs.sane-backends-git
     else pkgs.sane-backends;
-  backends = [ pkg ] ++ config.hardware.sane.extraBackends;
+
+  sanedConf = pkgs.writeTextFile {
+    name = "saned.conf";
+    destination = "/etc/sane.d/saned.conf";
+    text = ''
+      localhost
+      ${config.services.saned.extraConfig}
+    '';
+  };
+
+  env = {
+    SANE_CONFIG_DIR = config.hardware.sane.configDir;
+    LD_LIBRARY_PATH = [ "${saneConfig}/lib/sane" ];
+  };
+
+  backends = [ pkg ] ++ optional config.services.saned.enable sanedConf ++ config.hardware.sane.extraBackends;
   saneConfig = pkgs.mkSaneConfig { paths = backends; };
 
+  enabled = config.hardware.sane.enable || config.services.saned.enable;
+
 in
 
 {
@@ -51,27 +68,77 @@ in
 
     hardware.sane.configDir = mkOption {
       type = types.string;
+      internal = true;
       description = "The value of SANE_CONFIG_DIR.";
     };
 
+    services.saned.enable = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Enable saned network daemon for remote connection to scanners.
+
+        saned would be runned from <literal>scanner</literal> user; to allow
+        access to hardware that doesn't have <literal>scanner</literal> group
+        you should add needed groups to this user.
+      '';
+    };
+
+    services.saned.extraConfig = mkOption {
+      type = types.lines;
+      default = "";
+      example = "192.168.0.0/24";
+      description = ''
+        Extra saned configuration lines.
+      '';
+    };
+
   };
 
 
   ###### implementation
 
-  config = mkIf config.hardware.sane.enable {
+  config = mkMerge [
+    (mkIf enabled {
+      hardware.sane.configDir = mkDefault "${saneConfig}/etc/sane.d";
 
-    hardware.sane.configDir = mkDefault "${saneConfig}/etc/sane.d";
+      environment.systemPackages = backends;
+      environment.sessionVariables = env;
+      services.udev.packages = backends;
 
-    environment.systemPackages = backends;
-    environment.sessionVariables = {
-      SANE_CONFIG_DIR = config.hardware.sane.configDir;
-      LD_LIBRARY_PATH = [ "${saneConfig}/lib/sane" ];
-    };
-    services.udev.packages = backends;
+      users.extraGroups."scanner".gid = config.ids.gids.scanner;
+    })
 
-    users.extraGroups."scanner".gid = config.ids.gids.scanner;
+    (mkIf config.services.saned.enable {
+      networking.firewall.connectionTrackingModules = [ "sane" ];
 
-  };
+      systemd.services."saned@" = {
+        description = "Scanner Service";
+        environment = mapAttrs (name: val: toString val) env;
+        serviceConfig = {
+          User = "scanner";
+          Group = "scanner";
+          ExecStart = "${pkg}/bin/saned";
+        };
+      };
+
+      systemd.sockets.saned = {
+        description = "saned incoming socket";
+        wantedBy = [ "sockets.target" ];
+        listenStreams = [ "0.0.0.0:6566" "[::]:6566" ];
+        socketConfig = {
+          # saned needs to distinguish between IPv4 and IPv6 to open matching data sockets.
+          BindIPv6Only = "ipv6-only";
+          Accept = true;
+          MaxConnections = 1;
+        };
+      };
+
+      users.extraUsers."scanner" = {
+        uid = config.ids.uids.scanner;
+        group = "scanner";
+      };
+    })
+  ];
 
 }