lib/services: add some generalization options (#535695)

lib/services/service.nix

@@ -6,6 +6,7 @@
 # The module
 {
   lib,
+  config,
   ...
 }:
 let
@@ -49,6 +50,53 @@ in
           a shell script or `importas` from `pkgs.execline`.
         '';
       };
+
+      reloadSignal = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        example = "HUP";
+        description = ''
+          Configures the reload signal to send to the service manager.
+        '';
+      };
+
+      reloadCommand = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        example = lib.literalExpression ''"''${pkgs.coreutils}/bin/kill -HUP $MAINPID"'';
+
+        description = ''
+          Command used for reloading in the underlying service manager to reload.
+        '';
+      };
+    };
+
+    notificationProtocol = mkOption {
+      type = types.listOf (
+        types.enum [
+          "systemd"
+          "s6"
+        ]
+      );
+
+      default = [ ];
+      apply = v: lib.unique v;
+      description = ''
+        Notification protocol that this service supports with the underlying service manager.
+      '';
     };
   };
+
+  config = {
+    assertions = [
+      {
+        assertion = config.process.reloadSignal != null && config.process.reloadCommand != null;
+        message = "reloadSignal conflicts with reloadCommand. Please either use reloadSignal or reloadCommand.";
+      }
+    ];
+
+    process.reloadCommand = (lib.mkIf config.process.reloadSignal != null) (
+      lib.mkForce "${pkgs.coreutils}/bin/kill -${config.process.reloadSignal} $MAINPID"
+    );
+  };
 }

nixos/modules/system/service/systemd/service.nix

@@ -113,6 +113,48 @@ in
       defaultText = lib.literalExpression "config.systemd.lib.escapeSystemdExecArgs config.process.argv";
     };
 
+    systemd.mainExecReload = mkOption {
+      description = ''
+        Main command line for systemd's ExecReload with systemd's specifier and
+        environment variable substitution enabled.
+
+        This option sets the primary ExecRestart entry. Additional ExecReload entries
+        can be added via `systemd.service.serviceConfig.ExecReload` with `lib.mkBefore`
+        or `lib.mkAfter`.
+
+        This option allows you to use systemd specifiers like `%n` (unit name),
+        `%i` (instance), `%t` (runtime directory), and environment variables using
+        `''${VAR}` syntax in your command line.
+
+        By default, it is either set to null or this is set to the escaped version of {option}`process.reloadCommand`
+        when specified to prevent systemd substitution. Set this option explicitly to enable
+        systemd's substitution features.
+
+        To extend {option}`process.reloadCommand` with systemd specifiers, you can append
+        to the escaped arguments:
+
+        ```nix
+        systemd.mainExecReload =
+          config.systemd.lib.escapeSystemdExecArgs config.process.reload + " --systemd-unit %n";
+        ```
+
+        This pattern allows you to pass the unit name (or other systemd specifiers)
+        as additional arguments while keeping the base command from {option}`process.argv`
+        properly escaped.
+
+        See {manpage}`systemd.service(5)` (section "COMMAND LINES") for details on
+        variable substitution and {manpage}`systemd.unit(5)` (section "SPECIFIERS")
+        for available specifiers like `%n`, `%i`, `%t`.
+      '';
+      type = types.nullOr types.str;
+      default =
+        if config.process.reloadCommand then
+          config.systemd.lib.escapeSystemdExecArgs config.process.reloadCommand
+        else
+          "";
+      defaultText = lib.literalExpression "config.systemd.lib.escapeSystemdExecArgs config.process.reloadCommand";
+    };
+
     systemd.services = mkOption {
       description = ''
         This module configures systemd services, with the notable difference that their unit names will be prefixed with the abstract service name.
@@ -170,7 +212,10 @@ in
       # TODO description;
       wantedBy = lib.mkDefault [ "multi-user.target" ];
       serviceConfig = {
-        Type = lib.mkDefault "simple";
+        ExecReload = config.systemd.mainExecReload;
+        Type = lib.mkDefault (
+          if (config.serviceManager.notificationProtocol == "systemd") then "notify" else "simple"
+        );
         Restart = lib.mkDefault "always";
         RestartSec = lib.mkDefault "5";
         ExecStart = [