tl;dr Since HdfsClientConfigKeys holds client side config keys, we need to add this class to TestHdfsConfigFields#configurationClasses.
Now the TestHdfsConfigFields unit test passes because DFSConfigKeys still contains all the client side config keys, though marked @deprecated. As we add new client config keys (e.g.
HDFS-9259), the unit test will fail with the following error:
hdfs-default.xml has 1 properties missing in class org.apache.hadoop.hdfs.DFSConfigKeys
If the logic is to make the DFSConfigKeys and HdfsClientConfigKeys together cover all config keys in hdfs-default.xml, we need to put both of them in TestHdfsConfigFields#configurationClasses.