- Client want to write, but it gives some exclude DN.
- When NN is handing the request, DN shutdown and remove the DN from cluster.
- So NN get null value, but it still add null value to a set(code#3).
- Following code will use the null value and exception happens.
- Actually, due to the exclude node is defined by client, we can give the wrong host name(e.g. the host is not in cluster), and this bug can also be triggered. Our UT trigger this bug in such way.