The introduction of rollback-enabled write operations has increased the number of DBDirectClient calls that can be interrupted (previously, DBDirectClient interruption was disabled if the caller was holding a write lock and the operation had already performed a write, see SERVER-15999), and thus increased the possibility of a bug manifesting from any users of DBDirectClient that incorrectly assumed that their DBDirectClient operations would complete successfully. As a precaution, we should disallow interruption of DBDirectClient operations entirely.