Refactor Member#add_user for GitLab EE

GitLab EE extends Member#add_user by adding some LDAP specific flags.
Prior to these changes, the only way this could be done was by modifying
Member#add_user in place. This could lead to merge conflicts whenever a
developer wants to change this method.

To resolve this issue, the logic that EE extends has been moved into a
separate method with the appropriate arguments. This allows EE to extend
the logic by prepending the method using an EE specific module.

app/models/member.rb

@@ -9,7 +9,6 @@ class Member < ActiveRecord::Base
   include Presentable
 
   attr_accessor :raw_invite_token
-  attr_accessor :skip_notification
 
   belongs_to :created_by, class_name: "User"
   belongs_to :user
@@ -153,18 +152,13 @@ class Member < ActiveRecord::Base
 
       return member unless can_update_member?(current_user, member)
 
-      member.attributes = {
-        created_by: member.created_by || current_user,
-        access_level: access_level,
-        expires_at: expires_at
-      }
-
-      ## EE-only START
-      member.attributes = {
-        skip_notification: ldap,
+      set_member_attributes(
+        member,
+        access_level,
+        current_user: current_user,
+        expires_at: expires_at,
         ldap: ldap
-      }
-      ## EE-only END
+      )
 
       if member.request?
         ::Members::ApproveAccessRequestService.new(
@@ -183,6 +177,18 @@ class Member < ActiveRecord::Base
       # rubocop: enable CodeReuse/ServiceClass
     end
 
+    # Populates the attributes of a member.
+    #
+    # This logic resides in a separate method so that EE can extend this logic,
+    # without having to patch the `add_user` method directly.
+    def set_member_attributes(member, access_level, current_user: nil, expires_at: nil, ldap: false)
+      member.attributes = {
+        created_by: member.created_by || current_user,
+        access_level: access_level,
+        expires_at: expires_at
+      }
+    end
+
     def add_users(source, users, access_level, current_user: nil, expires_at: nil)
       return [] unless users.present?
 
@@ -425,3 +431,5 @@ class Member < ActiveRecord::Base
     {}
   end
 end
+
+Member.prepend(EE::Member)

ee/app/models/ee/member.rb

+# frozen_string_literal: true
+
+module EE
+  module Member
+    extend ActiveSupport::Concern
+
+    attr_accessor :skip_notification
+
+    class_methods do
+      extend ::Gitlab::Utils::Override
+
+      override :set_member_attributes
+      def set_member_attributes(member, access_level, current_user: nil, expires_at: nil, ldap: false)
+        super
+
+        member.attributes = {
+          skip_notification: ldap,
+          ldap: ldap
+        }
+      end
+    end
+  end
+end