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https://issues.apache.org/jira/browse/HDFS-4403?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13554396#comment-13554396
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Suresh Srinivas commented on HDFS-4403:
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Prior to this change, the client always got default crcType in
OpBlockChecksumResponseProto. The old client code just called getCrcType()
without checking hasCrcType(). The old clients talking to new server with this
change, get null pointer exception, if the new server does not set the crcType.
> DFSClient can infer checksum type when not provided by reading first byte
> -------------------------------------------------------------------------
>
> Key: HDFS-4403
> URL: https://issues.apache.org/jira/browse/HDFS-4403
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: hdfs-client
> Affects Versions: 2.0.2-alpha
> Reporter: Todd Lipcon
> Assignee: Todd Lipcon
> Priority: Minor
> Attachments: hdfs-4403.txt
>
>
> HDFS-3177 added the checksum type to OpBlockChecksumResponseProto, but the
> new protobuf field is optional, with a default of CRC32. This means that this
> API, when used against an older cluster (like earlier 0.23 releases) will
> falsely return CRC32 even if that cluster has written files with CRC32C. This
> can cause issues for distcp, for example.
> Instead of defaulting the protobuf field to CRC32, we can leave it with no
> default, and if the OpBlockChecksumResponseProto has no checksum type set,
> the client can send OP_READ_BLOCK to read the first byte of the block, then
> grab the checksum type out of that response (which has always been present)
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