HDFS不能上传文件，但可以上传目录问题

1.现象

HDFS datanode节点掉线，重启namenode后出现可以从HDFS上下载文件，可以把0 byte大小的文件或者目录上传目录至HDFS集群，不能put文件到hdfs上。

2.报错输出

2016-08-02 13:05:03,271 WARN org.apache.hadoop.ipc.Server: IPC Server handler 13 on 8020, call org.apache.hadoop.hdfs.protocol.ClientProtocol.addBlock from 10.132.89.79:14001 Call#56 Retry#0
java.lang.NullPointerException
at org.apache.hadoop.hdfs.server.blockmanagement.AvailableSpaceBlockPlacementPolicy.compareDataNode(AvailableSpaceBlockPlacementPolicy.java:95)
at org.apache.hadoop.hdfs.server.blockmanagement.AvailableSpaceBlockPlacementPolicy.chooseDataNode(AvailableSpaceBlockPlacementPolicy.java:80)
at org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseRandom(BlockPlacementPolicyDefault.java:691)
at org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseRandom(BlockPlacementPolicyDefault.java:665)
at org.apache.hadoop.hdfs.server.blockmana