DEV: allow the plugin to register valid site setting areas (#29432)

In this PR, we defined the ability to group site settings by area - https://github.com/discourse/discourse/pull/28570

Plugins should be able to register in their own areas.

app/controllers/admin/config/site_settings_controller.rb

@@ -12,7 +12,7 @@ class Admin::Config::SiteSettingsController < Admin::AdminController
   # UI itself uses the Admin::SiteSettingsController#index endpoint,
   # which also supports a `category` and `plugin` filter.
   def index
-    if params[:filter_names].blank? && SiteSetting::VALID_AREAS.exclude?(params[:filter_area])
+    if params[:filter_names].blank? && SiteSetting.valid_areas.exclude?(params[:filter_area])
       raise Discourse::InvalidParameters
     end
 

lib/discourse_plugin_registry.rb

@@ -72,6 +72,7 @@ class DiscoursePluginRegistry
   define_register :demon_processes, Set
   define_register :groups_callback_for_users_search_controller_action, Hash
   define_register :mail_pollers, Set
+  define_register :site_setting_areas, Set
 
   define_filtered_register :staff_user_custom_fields
   define_filtered_register :public_user_custom_fields

lib/plugin/instance.rb

@@ -849,6 +849,14 @@ class Plugin::Instance
     end
   end
 
+  # Site setting areas are a way to group site settings below
+  # the setting category level. This is useful for creating focused
+  # config areas that update a small selection of settings, and otherwise
+  # grouping related settings in the UI.
+  def register_site_setting_area(area)
+    DiscoursePluginRegistry.site_setting_areas << area
+  end
+
   def javascript_includes
     assets
       .map do |asset, opts|

lib/site_setting_extension.rb

@@ -523,6 +523,10 @@ module SiteSettingExtension
     end
   end
 
+  def valid_areas
+    Set.new(SiteSetting::VALID_AREAS | DiscoursePluginRegistry.site_setting_areas.to_a)
+  end
+
   protected
 
   def clear_cache!
@@ -695,9 +699,9 @@ module SiteSettingExtension
 
       if opts[:area]
         split_areas = opts[:area].split("|")
-        if split_areas.any? { |area| !SiteSetting::VALID_AREAS.include?(area) }
+        if split_areas.any? { |area| !SiteSetting.valid_areas.include?(area) }
           raise Discourse::InvalidParameters.new(
-                  "Area is incorrect. Valid areas: #{SiteSetting::VALID_AREAS.join(", ")}",
+                  "Area is invalid, valid areas are: #{SiteSetting.valid_areas.join(", ")}",
                 )
         end
         areas[name] = split_areas

spec/lib/site_setting_extension_spec.rb

@@ -510,6 +510,17 @@ RSpec.describe SiteSettingExtension do
         settings.refresh!
       }.to raise_error(Discourse::InvalidParameters)
     end
+
+    it "allows plugin to register valid areas" do
+      plugin = Plugin::Instance.new nil, "/tmp/test.rb"
+      plugin.register_site_setting_area("plugin_area")
+      settings.setting(:test_plugin_setting, 88, area: "plugin_area")
+      expect(
+        settings
+          .all_settings(filter_area: "plugin_area", include_locale_setting: false)
+          .map { |s| s[:setting].to_sym },
+      ).to eq(%i[test_plugin_setting])
+    end
   end
 
   describe "setting with a validator" do