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https://issues.apache.org/jira/browse/HDFS-14285?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16774642#comment-16774642
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Wei-Chiu Chuang edited comment on HDFS-14285 at 2/22/19 1:09 AM:
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Patch makes sense to me. This is exactly what hdfsPread() does.
was (Author: jojochuang):
Patch makes sense to me.
> libhdfs hdfsRead copies entire array even if its only partially filled
> ----------------------------------------------------------------------
>
> Key: HDFS-14285
> URL: https://issues.apache.org/jira/browse/HDFS-14285
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: hdfs-client, libhdfs, native
> Reporter: Sahil Takiar
> Assignee: Sahil Takiar
> Priority: Major
> Attachments: HDFS-14285.001.patch, HDFS-14285.002.patch
>
>
> There is a bug in libhdfs {{hdfsRead}}
> {code:java}
> jthr = invokeMethod(env, &jVal, INSTANCE, jInputStream, HADOOP_ISTRM,
> "read", "([B)I", jbRarray);
> if (jthr) {
> destroyLocalReference(env, jbRarray);
> errno = printExceptionAndFree(env, jthr, PRINT_EXC_ALL,
> "hdfsRead: FSDataInputStream#read");
> return -1;
> }
> if (jVal.i < 0) {
> // EOF
> destroyLocalReference(env, jbRarray);
> return 0;
> } else if (jVal.i == 0) {
> destroyLocalReference(env, jbRarray);
> errno = EINTR;
> return -1;
> }
> (*env)->GetByteArrayRegion(env, jbRarray, 0, noReadBytes, buffer);
> {code}
> The method makes a call to {{FSInputStream#read(byte[])}} to fill in the Java
> byte array, however, {{#read(byte[])}} is not guaranteed to fill up the
> entire array, instead it returns the number of bytes written to the array
> (which could be less than the size of the array). Yet `{{GetByteArrayRegion}}
> decides to copy the entire contents of the {{jbArray}} into the buffer
> ({{noReadBytes}} is initialized to the length of the buffer and is never
> updated). So if {{FSInputStream#read(byte[])}} decides to read less data than
> the size of the byte array, the call to {{GetByteArrayRegion}} will
> essentially copy more bytes than necessary.
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