expose service_desk_enabled and refresh the key

... automatically when it goes from false to true

app/controllers/projects_controller.rb

@@ -349,6 +349,7 @@ class ProjectsController < Projects::ApplicationController
       mirror_user_id
       repository_size_limit
       reset_approvals_on_push
+      service_desk_enabled
     ]
   end
 

app/models/ee/project.rb

@@ -14,6 +14,8 @@ module EE
 
       delegate :actual_shared_runners_minutes_limit,
         :shared_runners_minutes_used?, to: :namespace
+
+      before_validation :refresh_service_desk_key
     end
 
     def shared_runners_available?
@@ -23,5 +25,15 @@ module EE
     def shared_runners_minutes_limit_enabled?
       !public? && shared_runners_enabled? && namespace.shared_runners_minutes_limit_enabled?
     end
+
+    private
+    def refresh_service_desk_key
+      return unless ::Gitlab::EE::ServiceDesk.enabled?
+      return unless self.service_desk_enabled?
+
+      if service_desk_mail_key.blank? || service_desk_enabled_changed?
+        self.service_desk_mail_key = Devise.friendly_token(16)
+      end
+    end
   end
 end

spec/models/ee/project_spec.rb

@@ -115,4 +115,33 @@ describe Project, models: true do
       it { is_expected.to be_falsey }
     end
   end
+
+  describe '#regenerate_service_desk_key' do
+    subject { create(:project) }
+
+    it 'leaves it blank by default' do
+      expect(subject.service_desk_mail_key).to be_blank
+    end
+
+    it 'updates when enabled' do
+      subject.service_desk_enabled = true
+      subject.validate
+      expect(subject.service_desk_mail_key).not_to be_blank
+    end
+
+    it 'changes when enabled' do
+      subject.update!(service_desk_mail_key: '12345')
+      subject.service_desk_enabled = true
+      expect { subject.validate }.to change { subject.service_desk_mail_key }
+    end
+
+    it 'ensures mail key is never nil when enabled' do
+      subject.update!(service_desk_enabled: true)
+
+      expect { subject.update!(service_desk_mail_key: nil) }
+        .to change { subject.service_desk_mail_key }
+
+      expect(subject.service_desk_mail_key).not_to be_blank
+    end
+  end
 end