Display read-only mode message when in maintenance

app/assets/javascripts/maintenance_mode_settings/components/app.vue

@@ -34,7 +34,9 @@ export default {
       <gl-form-textarea
         id="maintenanceBannerMessage"
         v-model="bannerMessage"
-        :placeholder="__(`GitLab is undergoing maintenance and is operating in a read-only mode.`)"
+        :placeholder="
+          __('This GitLab instance is undergoing maintenance and is operating in read-only mode.')
+        "
       />
     </gl-form-group>
     <div class="mt-4">

ee/app/helpers/ee/application_helper.rb

@@ -11,10 +11,26 @@ module EE
 
     override :read_only_message
     def read_only_message
-      return _('You are on a read-only GitLab instance.') if maintenance_mode?
+      message = ::Gitlab::Geo.secondary? ? geo_secondary_read_only_message : super
 
-      return super unless ::Gitlab::Geo.secondary?
+      return message unless maintenance_mode?
+      return maintenance_mode_message.concat(message) if message
 
+      maintenance_mode_message
+    end
+
+    def maintenance_mode_message
+      html = tag.div do
+        tag.p(class: 'gl-mb-3') do
+          concat(sprite_icon('information-o', css_class: 'gl-icon gl-mr-3'))
+          concat(custom_maintenance_mode_message)
+        end
+      end
+
+      html
+    end
+
+    def geo_secondary_read_only_message
       message = @limited_actions_message ? s_('Geo|You may be able to make a limited amount of changes or perform a limited amount of actions on this page.') : s_('Geo|If you want to make changes, you must visit the primary site.')
 
       message = "#{message} #{lag_message}".html_safe if lag_message
@@ -114,6 +130,11 @@ module EE
 
     private
 
+    def custom_maintenance_mode_message
+      ::Gitlab::CurrentSettings.maintenance_mode_message&.html_safe ||
+        s_('This GitLab instance is undergoing maintenance and is operating in read-only mode.')
+    end
+
     def appearance
       ::Appearance.current
     end

ee/spec/features/read_only_spec.rb

@@ -26,6 +26,6 @@ RSpec.describe 'Geo read-only message', :geo do
       stub_application_setting(maintenance_mode: true)
     end
 
-    it_behaves_like 'Read-only instance', /You are on a read\-only GitLab instance./
+    it_behaves_like 'Read-only instance', /This GitLab instance is undergoing maintenance and is operating in read\-only mode./
   end
 end

ee/spec/helpers/application_helper_spec.rb

@@ -6,6 +6,8 @@ RSpec.describe ApplicationHelper do
   include EE::GeoHelpers
 
   describe '#read_only_message', :geo do
+    let(:default_maintenance_mode_message) { 'This GitLab instance is undergoing maintenance and is operating in read-only mode.' }
+
     context 'when not in a Geo secondary' do
       it 'returns a fallback message if database is readonly' do
         expect(Gitlab::Database).to receive(:read_only?) { true }
@@ -16,6 +18,54 @@ RSpec.describe ApplicationHelper do
       it 'returns nil when database is not read_only' do
         expect(helper.read_only_message).to be_nil
       end
+
+      context 'maintenance mode' do
+        context 'enabled' do
+          before do
+            stub_application_setting(maintenance_mode: true)
+          end
+
+          it 'returns default message' do
+            expect(helper.read_only_message).to match(default_maintenance_mode_message)
+          end
+
+          it 'returns user set custom maintenance mode message' do
+            custom_message = 'Maintenance window ends at 00:00.'
+            stub_application_setting(maintenance_mode_message: custom_message)
+
+            expect(helper.read_only_message).to match(/#{custom_message}/)
+          end
+
+          context 'when database is read-only' do
+            it 'stacks read-only and maintenance mode messages' do
+              expect(Gitlab::Database).to receive(:read_only?).twice { true }
+
+              expect(helper.read_only_message).to match('You are on a read-only GitLab instance')
+              expect(helper.read_only_message).to match(/#{default_maintenance_mode_message}/)
+            end
+          end
+        end
+
+        context 'disabled' do
+          it 'returns nil' do
+            stub_application_setting(maintenance_mode: false)
+
+            expect(helper.read_only_message).to be_nil
+          end
+        end
+      end
+    end
+
+    context 'on a geo secondary' do
+      context 'maintenance mode on' do
+        it 'returns messages for both' do
+          expect(Gitlab::Geo).to receive(:secondary?).twice { true }
+          stub_application_setting(maintenance_mode: true)
+
+          expect(helper.read_only_message).to match(/you must visit the primary site/)
+          expect(helper.read_only_message).to match(/#{default_maintenance_mode_message}/)
+        end
+      end
     end
 
     context 'when in a Geo Secondary' do

locale/gitlab.pot

@@ -12802,9 +12802,6 @@ msgstr ""
 msgid "GitLab is obtaining a Let's Encrypt SSL certificate for this domain. This process can take some time. Please try again later."
 msgstr ""
 
-msgid "GitLab is undergoing maintenance and is operating in a read-only mode."
-msgstr ""
-
 msgid "GitLab member or Email address"
 msgstr ""
 
@@ -27710,6 +27707,9 @@ msgstr ""
 msgid "This GitLab instance is licensed at the %{insufficient_license} tier. Geo is only available for users who have at least a Premium license."
 msgstr ""
 
+msgid "This GitLab instance is undergoing maintenance and is operating in read-only mode."
+msgstr ""
+
 msgid "This Project is currently archived and read-only. Please unarchive the project first if you want to resume Pull mirroring"
 msgstr ""