zhouyingchao created HDFS-10182:
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Summary: Hedged read might overwrite user's buf
Key: HDFS-10182
URL: https://issues.apache.org/jira/browse/HDFS-10182
Project: Hadoop HDFS
Issue Type: Bug
Reporter: zhouyingchao
Assignee: zhouyingchao
In DFSInputStream::hedgedFetchBlockByteRange, the passed-in buf from the caller
is passed to another thread to fill in the first attempt. If the first attempt
is timed out, the second attempt would be issued with another ByteBuffer. Now
suppose the second attempt wins and the first attempt is blocked somewhere in
the IO path. The second attempt's result would be copied to the buf provided by
the caller and then caller would think the pread is all set. Later the caller
might use the buf to do something else (for e.g. read another chunk of data),
however, the first attempt in earlier hedgedFetchBlockByteRange might get some
data and fill into the buf ...
If this happens, the caller's buf would then be corrupted.
To fix the issue, we should allocate a temp buf for the first attempt too.
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