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https://issues.apache.org/jira/browse/HDFS-4701?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13632406#comment-13632406
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Chris Nauroth commented on HDFS-4701:
-------------------------------------
The problem occurs in the code below in {{DataNode}}. The last line is at risk
for a {{NullPointerException}}, because the block pool ID may have already been
removed from the {{BlockPoolManager}}. In this case, the call to
{{BlockPoolManager#get}} would return null.
{code}
/** Recover a block */
private void recoverBlock(RecoveringBlock rBlock) throws IOException {
ExtendedBlock block = rBlock.getBlock();
String blookPoolId = block.getBlockPoolId();
DatanodeID[] datanodeids = rBlock.getLocations();
List<BlockRecord> syncList = new ArrayList<BlockRecord>(datanodeids.length);
int errorCount = 0;
//check generation stamps
for(DatanodeID id : datanodeids) {
try {
BPOfferService bpos = blockPoolManager.get(blookPoolId);
DatanodeRegistration bpReg = bpos.bpRegistration;
{code}
> race condition between block recovery and removal of block pool can cause
> block recovery thread to exit with NullPointerException
> ---------------------------------------------------------------------------------------------------------------------------------
>
> Key: HDFS-4701
> URL: https://issues.apache.org/jira/browse/HDFS-4701
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: datanode
> Affects Versions: 3.0.0
> Reporter: Chris Nauroth
>
> I saw this happen once while shutting down a datanode: a NullPointerException
> thrown from a block recovery thread.
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