NPE when applying AvailableSpaceBlockPlacementPolicy

As HDFS-8131 introduced an AvailableSpaceBlockPlacementPolicy, but In some cases, it caused NPE.

Here are my namenode daemon logs :

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.blockmanagement.BlockPlacementPolicyDefault.chooseLocalRack(BlockPlacementPolicyDefault.java:572)
at org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseTargetInOrder(BlockPlacementPolicyDefault.java:457)
at org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseTarget(BlockPlacementPolicyDefault.java:367)
at org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseTarget(BlockPlacementPolicyDefault.java:242)
at org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseTarget(BlockPlacementPolicyDefault.java:114)
at org.apache.hadoop.hdfs.server.blockmanagement.BlockPlacementPolicyDefault.chooseTarget(BlockPlacementPolicyDefault.java:130)
at org.apache.hadoop.hdfs.server.blockmanagement.BlockManager.chooseTarget4NewBlock(BlockManager.java:1606)
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.getAdditionalBlock(FSNamesystem.java:3315)
at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.addBlock(NameNodeRpcServer.java:679)
at org.apache.hadoop.hdfs.server.namenode.AuthorizationProviderProxyClientProtocol.addBlock(AuthorizationProviderProxyClientProtocol.java:214)
at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolServerSideTranslatorPB.addBlock(ClientNamenodeProtocolServerSideTranslatorPB.java:489)
at org.apache.hadoop.hdfs.protocol.proto.ClientNamenodeProtocolProtos$ClientNamenodeProtocol$2.callBlockingMethod(ClientNamenodeProtocolProtos.java)
at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:617)
at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:1073)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2086)
at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:2082)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:422)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1693)
at org.apache.hadoop.ipc.Server$Handler.run(Server.java:2080)

I reviewed the source code, and found the bug in method chooseDataNode. clusterMap.chooseRandom may return null, which cannot compare using equals a.equals(b) method.

Though this exception can be caught, and then retry another call. I think this bug should be fixed.