Support postStart scripts in service units

modules/system/boot/systemd-unit-options.nix

@@ -122,6 +122,15 @@ with pkgs.lib;
       '';
     };
 
+    postStart = mkOption {
+      type = types.string;
+      default = "";
+      description = ''
+        Shell commands executed after the service's main process
+        is started.
+      '';
+    };
+
     restartIfChanged = mkOption {
       type = types.bool;
       default = true;

modules/system/boot/systemd.nix

@@ -229,6 +229,13 @@ let
             ''}
           ''}
 
+          ${optionalString (def.postStart != "") ''
+            ExecStartPost=${makeJobScript "${name}-poststart.sh" ''
+              #! ${pkgs.stdenv.shell} -e
+              ${def.postStart}
+            ''}
+          ''}
+
           ${attrsToSection def.serviceConfig}
         '';
     };