A few lines later we set `upd->next = cbt->ins->upd`, and that's checked against *upd_entry in the serialization function; this one is a little odd because I replaced `old_upd` with `upd->next` in the serialization function, so it's two changes in one -- I think that
`if (upd->next != *upd_entry)` in the serialization function should be sufficient, but I might be wrong...