[jira] [Created] (HDFS-9705) Refine the behaviour of getFileChecksum when length = 0

["Kai Zheng (JIRA)" <ji...@apache.org>]

Kai Zheng created HDFS-9705:
-------------------------------

             Summary: Refine the behaviour of getFileChecksum when length = 0
                 Key: HDFS-9705
                 URL: https://issues.apache.org/jira/browse/HDFS-9705
             Project: Hadoop HDFS
          Issue Type: Improvement
            Reporter: Kai Zheng
            Assignee: Kai Zheng
            Priority: Minor


{{FileSystem#getFileChecksum}} may accept {{length}} parameter and 0 is a valid value. Currently it will return {{null}} when length is 0, in the following code block:
{code}
    //compute file MD5
    final MD5Hash fileMD5 = MD5Hash.digest(md5out.getData());
    switch (crcType) {
    case CRC32:
      return new MD5MD5CRC32GzipFileChecksum(bytesPerCRC,
          crcPerBlock, fileMD5);
    case CRC32C:
      return new MD5MD5CRC32CastagnoliFileChecksum(bytesPerCRC,
          crcPerBlock, fileMD5);
    default:
      // If there is no block allocated for the file,
      // return one with the magic entry that matches what previous
      // hdfs versions return.
      if (locatedblocks.size() == 0) {
        return new MD5MD5CRC32GzipFileChecksum(0, 0, fileMD5);
      }

      // we should never get here since the validity was checked
      // when getCrcType() was called above.
      return null;
    }
{code}
The comment says "we should never get here since the validity was checked" but it does. As we're using the MD5-MD5-X approach, and {{EMPTY--CONTENT}} actually is a valid case in which the MD5 value is {{d41d8cd98f00b204e9800998ecf8427e}}, so suggest we return a reasonable value other than null. At least some useful information in the returned value can be seen, like values from block checksum header.



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