public synchronized void position(TopicPartition tp, FetchPosition position) {
        assignedState(tp).position(position);
    }
@Test
    public void cantChangePositionForNonAssignedPartition() {
        assertThrows(IllegalStateException.class, () -> state.position(tp0,
            new SubscriptionState.FetchPosition(1, Optional.empty(), leaderAndEpoch)));
    }