Refactor `TokenAuthenticatable` to improve reusability

This adds a ability to use multiple different authentication token
fields in other models. From now on it is necessary to add
authentication token field manually in each class that implements this
mixin.

app/models/application_setting.rb

@@ -31,6 +31,9 @@
 #
 
 class ApplicationSetting < ActiveRecord::Base
+  include TokenAuthenticatable
+  add_authentication_token_field :runners_registration_token
+
   CACHE_KEY = 'application_setting.last'
 
   serialize :restricted_visibility_levels

app/models/concerns/token_authenticatable.rb

 module TokenAuthenticatable
   extend ActiveSupport::Concern
 
-  module ClassMethods
-    def find_by_authentication_token(authentication_token = nil)
-      if authentication_token
-        where(authentication_token: authentication_token).first
-      end
+  class_methods do
+    def authentication_token_fields
+      @token_fields || []
     end
-  end
 
-  def ensure_authentication_token
-    if authentication_token.blank?
-      self.authentication_token = generate_authentication_token
-    end
-  end
+    private
+
+    def add_authentication_token_field(token_field)
+      @token_fields = [] unless @token_fields
+      @token_fields << token_field
 
-  def reset_authentication_token!
-    self.authentication_token = generate_authentication_token
-    save
+      define_singleton_method("find_by_#{token_field}") do |token|
+        where(token_field => token).first if token
+      end
+
+      define_method("ensure_#{token_field}") do
+        write_attribute(token_field, generate_token_for(token_field)) if
+          read_attribute(token_field).blank?
+      end
+
+      define_method("reset_#{token_field}!") do
+        write_attribute(token_field, generate_token_for(token_field))
+        save
+      end
+    end
   end
 
   private
 
-  def generate_authentication_token
+  def generate_token_for(token_field)
     loop do
       token = Devise.friendly_token
-      break token unless self.class.unscoped.where(authentication_token: token).first
+      break token unless self.class.unscoped.where(token_field => token).first
     end
   end
 end

app/models/user.rb

@@ -69,8 +69,10 @@ class User < ActiveRecord::Base
   include Gitlab::CurrentSettings
   include Referable
   include Sortable
-  include TokenAuthenticatable
   include CaseSensitivity
+  include TokenAuthenticatable
+
+  add_authentication_token_field :authentication_token
 
   default_value_for :admin, false
   default_value_for :can_create_group, gitlab_config.default_can_create_group