Allow user sign-in in maintenance mode

ee/lib/ee/gitlab/middleware/read_only/controller.rb

@@ -24,6 +24,10 @@ module EE
             'repositories/lfs_locks_api' => %w{verify create unlock}
           }.freeze
 
+          ALLOWLISTED_SIGN_IN_ROUTES = {
+            'sessions' => %w{create}
+          }.freeze
+
           private
 
           # In addition to routes allowed in FOSS, allow geo node update route
@@ -36,7 +40,7 @@ module EE
             allowed = super || geo_node_update_route? || geo_api_route? || admin_settings_update?
 
             return true if allowed
-            return false if ::Gitlab.maintenance_mode?
+            return sign_in_route? if ::Gitlab.maintenance_mode?
             return false unless ::Gitlab::Geo.secondary?
 
             git_write_routes
@@ -82,6 +86,12 @@ module EE
             end
           end
 
+          def sign_in_route?
+            return unless request.post? && request.path.start_with?('/users/sign_in')
+
+            ALLOWLISTED_SIGN_IN_ROUTES[route_hash[:controller]]&.include?(route_hash[:action])
+          end
+
           def lfs_locks_route?
             # Calling route_hash may be expensive. Only do it if we think there's a possible match
             unless request.path.end_with?('/info/lfs/locks', '/info/lfs/locks/verify') ||

ee/spec/support/shared_examples/lib/gitlab/middleware/maintenance_mode_gitlab_ee_instance_shared_examples.rb

@@ -77,6 +77,13 @@ RSpec.shared_examples 'write access for a read-only GitLab (EE) instance in main
           expect(subject).to disallow_request
         end
       end
+
+      it "expects a POST to /users/sign_in URL to be allowed" do
+        response = request.post('/users/sign_in')
+
+        expect(response).not_to be_redirect
+        expect(subject).not_to disallow_request
+      end
     end
   end
 end

spec/support/shared_examples/lib/gitlab/middleware/read_only_gitlab_instance_shared_examples.rb

@@ -125,6 +125,9 @@ RSpec.shared_examples 'write access for a read-only GitLab instance' do
       where(:description, :path) do
         'LFS request to batch'        | '/root/rouge.git/info/lfs/objects/batch'
         'request to git-upload-pack'  | '/root/rouge.git/git-upload-pack'
+        'user sign out'               | '/users/sign_out'
+        'admin session'               | '/admin/session'
+        'admin session destroy'       | '/admin/session/destroy'
       end
 
       with_them do