Prevent accidental group deletion if path rename fails

If an update to the group path fails validation, the unsaved attributes
would still be presented because the controller would fall through and
call `render` on the unsaved group object. There was an attempt to reset
the path to the previous value, but this did not work because the Rails
form helper called `Group#to_param`, which loads the full route.

While it may have been intentional to retain the failed values, it's
better to be safe and reset all the values back to their original state.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/241110

app/controllers/groups_controller.rb

@@ -123,7 +123,7 @@ class GroupsController < Groups::ApplicationController
     if Groups::UpdateService.new(@group, current_user, group_params).execute
       redirect_to edit_group_path(@group, anchor: params[:update_section]), notice: "Group '#{@group.name}' was successfully updated."
     else
-      @group.path = @group.path_before_last_save || @group.path_was
+      @group.reset
       render action: "edit"
     end
   end

changelogs/unreleased/security-group-rename-deletion.yml

+---
+title: Prevent accidental group deletion if path rename fails
+merge_request: 40353
+author:
+type: fixed

spec/controllers/groups_controller_spec.rb

@@ -555,6 +555,21 @@ RSpec.describe GroupsController do
       end
     end
 
+    context 'when there is a conflicting group path' do
+      render_views
+
+      let!(:conflict_group) { create(:group, path: SecureRandom.hex(12) ) }
+      let!(:old_name) { group.name }
+
+      it 'does not render references to the conflicting group' do
+        put :update, params: { id: group.to_param, group: { path: conflict_group.path } }
+
+        expect(response).to have_gitlab_http_status(:ok)
+        expect(group.reload.name).to eq(old_name)
+        expect(response.body).not_to include(conflict_group.path)
+      end
+    end
+
     context 'when a project inside the group has container repositories' do
       before do
         stub_container_registry_config(enabled: true)