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From "Wei-Chiu Chuang (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (HDFS-14285) libhdfs hdfsRead copies entire array even if its only partially filled
Date Fri, 22 Feb 2019 01:10:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-14285?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16774642#comment-16774642
] 

Wei-Chiu Chuang edited comment on HDFS-14285 at 2/22/19 1:09 AM:
-----------------------------------------------------------------

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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