nixos/cross-seed: add 'useGenConfigDefaults'

This to me is the cleanest way of setting the default value and track
changes to the values from `cross-seed gen-config` with low churn.

nixos/modules/services/torrent/cross-seed.nix

@@ -14,6 +14,15 @@ let
     types
     ;
   settingsFormat = pkgs.formats.json { };
+
+  generatedConfig =
+    pkgs.runCommand "cross-seed-gen-config" { nativeBuildInputs = [ pkgs.cross-seed ]; }
+      ''
+        export HOME=$(mktemp -d)
+        cross-seed gen-config
+        mkdir $out
+        cp -r $HOME/.cross-seed/config.js $out/
+      '';
 in
 {
   options.services.cross-seed = {
@@ -40,6 +49,22 @@ in
       description = "Cross-seed config directory";
     };
 
+    useGenConfigDefaults = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to use the option defaults from the configuration generated by
+        {command}`cross-seed gen-config`.
+
+        Those are the settings recommended by the project, and can be inspected
+        from their [template file](https://github.com/cross-seed/cross-seed/blob/master/src/config.template.cjs).
+
+        Settings set in {option}`services.cross-seed.settings` and
+        {option}`services.cross-seed.settingsFile` will override the ones from
+        this option.
+      '';
+    };
+
     settings = mkOption {
       default = { };
       type = types.submodule {
@@ -120,6 +145,13 @@ in
     let
       jsonSettingsFile = settingsFormat.generate "settings.json" cfg.settings;
 
+      genConfigSegment =
+        lib.optionalString cfg.useGenConfigDefaults # js
+          ''
+            const gen_config_js = "${generatedConfig}/config.js";
+            Object.assign(loaded_settings, require(gen_config_js));
+          '';
+
       # Since cross-seed uses a javascript config file, we can use node's
       # ability to parse JSON directly to avoid having to do any conversion.
       # This also means we don't need to use any external programs to merge the
@@ -138,7 +170,9 @@ in
             "use strict";
             const fs = require("fs");
             const settings_json = "${jsonSettingsFile}";
-            let loaded_settings = JSON.parse(fs.readFileSync(settings_json, "utf8"));
+            let loaded_settings = {};
+            ${genConfigSegment}
+            Object.assign(loaded_settings, JSON.parse(fs.readFileSync(settings_json, "utf8")));
             ${secretSettingsSegment}
             module.exports = loaded_settings;
           '';