[jira] [Created] (HDFS-14551) NN throws NPE if downgrade it during rolling upgrade from 3.x to 2.x

["Yuxuan Wang (JIRA)" <ji...@apache.org>]

Yuxuan Wang created HDFS-14551:
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             Summary: NN throws NPE if downgrade it during rolling upgrade from 3.x to 2.x
                 Key: HDFS-14551
                 URL: https://issues.apache.org/jira/browse/HDFS-14551
             Project: Hadoop HDFS
          Issue Type: Bug
            Reporter: Yuxuan Wang


We can downgrade NN during roling upgrade (runned "hdfs dfsadmin -rollingUpgrade prepare) with HDFS-8432 involved. But with HDFS-14172 if the image has any unrecogized section, it will throw IOException at
{code:title=FSImageFormatProtobuf.java|borderStyle=solid}
private void loadInternal(......) {
    ......
    String n = s.getName();
    SectionName sectionName = SectionName.fromString(n);
    if (sectionName == null) {
      throw new IOException("Unrecognized section " + n);
    }
    ......
}
{code}
and throw NPE on Hadoop 2.x
{code:title=FSImageFormatProtobuf.java|borderStyle=solid}
private void loadInternal(......) {
    ......
    String n = s.getName();
    switch (sectionName)
    ......
}
{code}
When we downgrade NN from 3.x to 2.x, NN may load the image saved by 3.x NN. Then the lack of {{SectionName.ERASURE_CODING}} can break 2.x NN.
We should just skip the unrecogized section instead of throwing exception.



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