Check if maintenance_mode setting is available

lib/gitlab.rb

@@ -123,6 +123,16 @@ module Gitlab
   def self.maintenance_mode?
     return false unless ::Gitlab::CurrentSettings.current_application_settings?
 
+    # `maintenance_mode` column was added to the `current_settings` table in 13.2
+    # When upgrading from < 13.2 to >=13.8 `maintenance_mode` will not be
+    # found in settings.
+    # `Gitlab::CurrentSettings#uncached_application_settings` in
+    # lib/gitlab/current_settings.rb is expected to handle such cases, and use
+    # the default value for the setting instead, but in this case, it doesn't,
+    # see https://gitlab.com/gitlab-org/gitlab/-/issues/321836
+    # As a work around, we check if the setting method is available
+    return false unless ::Gitlab::CurrentSettings.respond_to?(:maintenance_mode)
+
     ::Gitlab::CurrentSettings.maintenance_mode
   end
 end

spec/lib/gitlab_spec.rb

@@ -363,8 +363,13 @@ RSpec.describe Gitlab do
       expect(described_class.maintenance_mode?).to eq(false)
     end
 
-    it 'returns false when maintenance mode feature flag is disabled' do
-      stub_feature_flags(maintenance_mode: false)
+    it 'returns false when maintenance mode column is not present' do
+      stub_maintenance_mode_setting(true)
+
+      allow(::Gitlab::CurrentSettings.current_application_settings)
+        .to receive(:respond_to?)
+        .with(:maintenance_mode, false)
+        .and_return(false)
 
       expect(described_class.maintenance_mode?).to eq(false)
     end