Stable timestamp can be unset with an oldest timestamp set

    /*

     * The oldest and stable timestamps must always satisfy the condition that oldest <= stable.

     */

    if ((has_oldest || has_stable) && (has_oldest || txn_global->has_oldest_timestamp) &&

      (has_stable || txn_global->has_stable_timestamp) && oldest_ts > stable_ts) {

        __wt_readunlock(session, &txn_global->rwlock);

        WT_RET_MSG(session, EINVAL,

          "set_timestamp: oldest timestamp %s must not be later than "

          "stable timestamp %s",

          __wt_timestamp_to_string(oldest_ts, ts_string[0]),

          __wt_timestamp_to_string(stable_ts, ts_string[1]));

    }

(gdb) p (*(WT_CONNECTION_IMPL*)session->iface.connection)->txn_global

$17 = {current = 68200, last_running = 68200, oldest_id = 68200, durable_timestamp = 112793, last_ckpt_timestamp = 0, meta_ckpt_timestamp = 0,

  oldest_timestamp = 93921, pinned_timestamp = 93921, recovery_timestamp = 0, stable_timestamp = 0, has_durable_timestamp = true,