Extract EE-specific lines from Gitlab::Email::Handler

ee/lib/ee/gitlab/email/handler.rb

+require 'gitlab/email/handler/ee/service_desk_handler'
+
+module EE
+  module Gitlab
+    module Email
+      module Handler
+        extend ActiveSupport::Concern
+
+        class_methods do
+          extend ::Gitlab::Utils::Override
+
+          override :load_handlers
+          def load_handlers
+            [
+              ::Gitlab::Email::Handler::EE::ServiceDeskHandler,
+              *super
+            ]
+          end
+        end
+      end
+    end
+  end
+end

lib/gitlab/email/handler.rb

@@ -3,21 +3,28 @@ require 'gitlab/email/handler/create_note_handler'
 require 'gitlab/email/handler/create_issue_handler'
 require 'gitlab/email/handler/unsubscribe_handler'
 
-require 'gitlab/email/handler/ee/service_desk_handler'
+require 'ee/gitlab/email/handler'
 
 module Gitlab
   module Email
     module Handler
-      HANDLERS = [
-        EE::ServiceDeskHandler,
+      prepend ::EE::Gitlab::Email::Handler
+
+      def self.handlers
+        @handlers ||= load_handlers
+      end
+
+      def self.load_handlers
+        [
           UnsubscribeHandler,
           CreateNoteHandler,
           CreateMergeRequestHandler,
           CreateIssueHandler
-      ].freeze
+        ]
+      end
 
       def self.for(mail, mail_key)
-        HANDLERS.find do |klass|
+        handlers.find do |klass|
           handler = klass.new(mail, mail_key)
           break handler if handler.can_handle?
         end

spec/lib/gitlab/email/handler_spec.rb

@@ -40,7 +40,7 @@ describe Gitlab::Email::Handler do
   end
 
   def ce_handlers
-    @ce_handlers ||= Gitlab::Email::Handler::HANDLERS.reject do |handler|
+    @ce_handlers ||= Gitlab::Email::Handler.handlers.reject do |handler|
       handler.name.start_with?('Gitlab::Email::Handler::EE::')
     end
   end