DEV: improve design of site setting default provider

This refactors it so "Defaults provider" is only responsible for "defaults"

Locale handling and management of locale settings is moved back into
SiteSettingExtension

This eliminates complex state management using DistributedCache and makes
it way easier to test SiteSettingExtension

lib/site_settings/defaults_provider.rb

@@ -1,31 +1,23 @@
+# frozen_string_literal: true
+
 module SiteSettings; end
 
 # A cache for providing default value based on site locale
 class SiteSettings::DefaultsProvider
-  include Enumerable
-
-  CONSUMED_OPTS = %i[default locale_default].freeze
-  DEFAULT_LOCALE_KEY = :default_locale
-  DEFAULT_LOCALE = 'en'.freeze
-  DEFAULT_CATEGORY = 'required'.freeze
-
-  @@site_locales ||= DistributedCache.new('site_locales')
+  DEFAULT_LOCALE = 'en'
 
   def initialize(site_setting)
     @site_setting = site_setting
-    @site_setting.refresh_settings << DEFAULT_LOCALE_KEY
     @defaults = {}
     @defaults[DEFAULT_LOCALE.to_sym] = {}
-
-    refresh_site_locale!
   end
 
-  def load_setting(name_arg, value, opts = {})
+  def load_setting(name_arg, value, locale_defaults)
     name = name_arg.to_sym
     @defaults[DEFAULT_LOCALE.to_sym][name] = value
 
-    if (locale_default = opts[:locale_default])
-      locale_default.each do |locale, v|
+    if (locale_defaults)
+      locale_defaults.each do |locale, v|
         locale = locale.to_sym
         @defaults[locale] ||= {}
         @defaults[locale][name] = v
@@ -34,15 +26,19 @@ class SiteSettings::DefaultsProvider
   end
 
   def db_all
-    @site_setting.provider.all.delete_if { |s| s.name.to_sym == DEFAULT_LOCALE_KEY }
+    @site_setting.provider.all
   end
 
-  def all
-    @defaults[DEFAULT_LOCALE.to_sym].merge(@defaults[self.site_locale.to_sym] || {})
+  def all(locale = nil)
+    if locale
+      @defaults[DEFAULT_LOCALE.to_sym].merge(@defaults[locale.to_sym] || {})
+    else
+      @defaults[DEFAULT_LOCALE.to_sym].dup
+    end
   end
 
-  def get(name)
-    @defaults.dig(self.site_locale.to_sym, name.to_sym) ||
+  def get(name, locale = DEFAULT_LOCALE)
+    @defaults.dig(locale.to_sym, name.to_sym) ||
       @defaults.dig(DEFAULT_LOCALE.to_sym, name.to_sym)
   end
   alias [] get
@@ -50,81 +46,25 @@ class SiteSettings::DefaultsProvider
   # Used to override site settings in dev/test env
   def set_regardless_of_locale(name, value)
     name = name.to_sym
-    if @site_setting.has_setting?(name)
+    if name == :default_locale || @site_setting.has_setting?(name)
       @defaults.each { |_, hash| hash.delete(name) }
       @defaults[DEFAULT_LOCALE.to_sym][name] = value
       value, type = @site_setting.type_supervisor.to_db_value(name, value)
-      @defaults[self.site_locale.to_sym] ||= {}
-      @defaults[self.site_locale.to_sym][name] = @site_setting.type_supervisor.to_rb_value(name, value, type)
+      @defaults[SiteSetting.default_locale.to_sym] ||= {}
+      @defaults[SiteSetting.default_locale.to_sym][name] = @site_setting.type_supervisor.to_rb_value(name, value, type)
     else
       raise ArgumentError.new("No setting named '#{name}' exists")
     end
   end
 
-  def site_locale
-    @@site_locales[current_db]
-  end
-
-  def site_locale=(val)
-    val = val.to_s
-    raise Discourse::InvalidParameters.new(:value) unless LocaleSiteSetting.valid_value?(val)
-
-    if val != @@site_locales[current_db]
-      @site_setting.provider.save(DEFAULT_LOCALE_KEY, val, SiteSetting.types[:string])
-      refresh_site_locale!
-      @site_setting.refresh!
-      Discourse.request_refresh!
-    end
-
-    @@site_locales[current_db]
-  end
-
-  def each(&block)
-    self.all.each do |key, value|
-      block.call(key.to_sym, value)
-    end
-  end
-
-  def locale_setting_hash
-    {
-      setting: DEFAULT_LOCALE_KEY,
-      default: DEFAULT_LOCALE,
-      category: DEFAULT_CATEGORY,
-      description: @site_setting.description(DEFAULT_LOCALE_KEY),
-      type: SiteSetting.types[SiteSetting.types[:enum]],
-      preview: nil,
-      value: @@site_locales[current_db],
-      valid_values: LocaleSiteSetting.values,
-      translate_names: LocaleSiteSetting.translate_names?
-    }
-  end
-
-  def refresh_site_locale!
-    RailsMultisite::ConnectionManagement.each_connection do |db|
-      @@site_locales[db] =
-        if GlobalSetting.respond_to?(DEFAULT_LOCALE_KEY) &&
-            (global_val = GlobalSetting.send(DEFAULT_LOCALE_KEY)) &&
-            !global_val.blank?
-          global_val
-        elsif (db_val = @site_setting.provider.find(DEFAULT_LOCALE_KEY))
-          db_val.value.to_s
-        else
-          DEFAULT_LOCALE
-        end
-
-      @@site_locales[db]
-    end
-  end
-
   def has_setting?(name)
-    has_key?(name.to_sym) || has_key?("#{name.to_s}?".to_sym)
+    has_key?(name.to_sym) || has_key?("#{name.to_s}?".to_sym) || name.to_sym == :default_locale
   end
 
   private
 
   def has_key?(name)
-    @defaults[self.site_locale.to_sym]&.key?(name) ||
-      @defaults[DEFAULT_LOCALE.to_sym].key?(name) || name == DEFAULT_LOCALE_KEY
+    @defaults[DEFAULT_LOCALE.to_sym].key?(name)
   end
 
   def current_db