FIX: Invalidate site settings cache in all instances (#16155)

Previous cache implementation did not support multisite instances and
the cache was invalidated only in the instance where the change took
place.

app/models/site_setting.rb

@@ -101,23 +101,33 @@ class SiteSetting < ActiveRecord::Base
     :markdown_typographer_quotation_marks
   ]
 
-  def self.reset_cached_settings!
-    @blocked_attachment_content_types_regex = nil
-    @blocked_attachment_filenames_regex = nil
-    @allowed_unicode_username_regex = nil
-  end
-
   def self.blocked_attachment_content_types_regex
-    @blocked_attachment_content_types_regex ||= Regexp.union(SiteSetting.blocked_attachment_content_types.split("|"))
+    current_db = RailsMultisite::ConnectionManagement.current_db
+
+    @blocked_attachment_content_types_regex ||= {}
+    @blocked_attachment_content_types_regex[current_db] ||= begin
+      Regexp.union(SiteSetting.blocked_attachment_content_types.split("|"))
+    end
   end
 
   def self.blocked_attachment_filenames_regex
-    @blocked_attachment_filenames_regex ||= Regexp.union(SiteSetting.blocked_attachment_filenames.split("|"))
+    current_db = RailsMultisite::ConnectionManagement.current_db
+
+    @blocked_attachment_filenames_regex ||= {}
+    @blocked_attachment_filenames_regex[current_db] ||= begin
+      Regexp.union(SiteSetting.blocked_attachment_filenames.split("|"))
+    end
   end
 
   def self.allowed_unicode_username_characters_regex
-    @allowed_unicode_username_regex ||= SiteSetting.allowed_unicode_username_characters.present? \
-      ? Regexp.new(SiteSetting.allowed_unicode_username_characters) : nil
+    current_db = RailsMultisite::ConnectionManagement.current_db
+
+    @allowed_unicode_username_regex ||= {}
+    @allowed_unicode_username_regex[current_db] ||= begin
+      if SiteSetting.allowed_unicode_username_characters.present?
+        Regexp.new(SiteSetting.allowed_unicode_username_characters)
+      end
+    end
   end
 
   # helpers for getting s3 settings that fallback to global
@@ -246,6 +256,16 @@ class SiteSetting < ActiveRecord::Base
       send("#{new_method}=", args)
     end
   end
+
+  protected
+
+  def self.clear_cache!
+    super
+
+    @blocked_attachment_content_types_regex = nil
+    @blocked_attachment_filenames_regex = nil
+    @allowed_unicode_username_regex = nil
+  end
 end
 
 # == Schema Information

config/initializers/014-track-setting-changes.rb

@@ -45,10 +45,6 @@ DiscourseEvent.on(:site_setting_changed) do |name, old_value, new_value|
     SiteIconManager.ensure_optimized!
   end
 
-  if SiteSetting::WATCHED_SETTINGS.include?(name)
-    SiteSetting.reset_cached_settings!
-  end
-
   # Make sure medium and high priority thresholds were calculated.
   if name == :reviewable_low_priority_threshold && Reviewable.min_score_for_priority(:medium) > 0
     Reviewable.set_priorities(low: new_value)