Fix application settings cache not expiring after changes

cache_key is an instance method that relies on updated_at. When changes
were made, the time-dependent key was being used instead of X.application_setting.last.

Closes #3609

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.3.0 (unreleased)
+  - Fix application settings cache not expiring after changes (Stan Hu)
   - Fix: Assignee selector is empty when 'Unassigned' is selected (Jose Corcuera)
   - Fix 500 error when update group member permission
   - Trim leading and trailing whitespace of milestone and issueable titles (Jose Corcuera)

app/models/application_setting.rb

@@ -30,6 +30,8 @@
 #
 
 class ApplicationSetting < ActiveRecord::Base
+  CACHE_KEY = 'application_setting.last'
+
   serialize :restricted_visibility_levels
   serialize :import_sources
   serialize :restricted_signup_domains, Array
@@ -73,21 +75,17 @@ class ApplicationSetting < ActiveRecord::Base
   end
 
   after_commit do
-    Rails.cache.write(cache_key, self)
+    Rails.cache.write(CACHE_KEY, self)
   end
 
   def self.current
-    Rails.cache.fetch(cache_key) do
+    Rails.cache.fetch(CACHE_KEY) do
       ApplicationSetting.last
     end
   end
 
   def self.expire
-    Rails.cache.delete(cache_key)
-  end
-
-  def self.cache_key
-    'application_setting.last'
+    Rails.cache.delete(CACHE_KEY)
   end
 
   def self.create_from_defaults

app/models/ci/application_setting.rb

@@ -12,17 +12,18 @@
 module Ci
   class ApplicationSetting < ActiveRecord::Base
     extend Ci::Model
+    CACHE_KEY = 'ci_application_setting.last'
 
     after_commit do
-      Rails.cache.write(cache_key, self)
+      Rails.cache.write(CACHE_KEY, self)
     end
 
     def self.expire
-      Rails.cache.delete(cache_key)
+      Rails.cache.delete(CACHE_KEY)
     end
 
     def self.current
-      Rails.cache.fetch(cache_key) do
+      Rails.cache.fetch(CACHE_KEY) do
         Ci::ApplicationSetting.last
       end
     end
@@ -33,9 +34,5 @@ module Ci
         add_pusher: Settings.gitlab_ci['add_pusher'],
       )
     end
-
-    def self.cache_key
-      'ci_application_setting.last'
-    end
   end
 end