[jira] [Comment Edited] (HBASE-15064) Possbile Bug in MultiByteBuffer

["Anoop Sam John (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-15064?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15082455#comment-15082455 ] 

Anoop Sam John edited comment on HBASE-15064 at 1/5/16 6:07 AM:
----------------------------------------------------------------

In BufferedEncodedSeeker#next() we can see
{code}
public boolean next() {
      if (!currentBuffer.hasRemaining()) {
        return false;
      }
      decodeNext();
	  ...
}
{code}
So when we are the end of a block, this hasRemaining is supposed to return false and so no more calls to decodeNext. In normal case of MBB and SBB this works fine.
When the MBB is created over the last bucket, we are returning the last dummy BB of 0 size also.
{code}
} else {
    buffers[i] = ByteBuffer.allocate(0);
}
{code}
Because of that even when we have finished all cells from a buffer, hasRemaining() will say true for such case and for decodeNext() we will move to the last BB which is of limit 0.
{code}
@Override
  public final boolean hasRemaining() {
    return this.curItem.hasRemaining() || this.curItemIndex < this.items.length - 1;
  }
{code}


We have to solve this by fixing ByteBufferArray#asSubBuffer by not including the last dummy buffer of 0 size from returning back to user.



was (Author: anoop.hbase):
In BufferedEncodedSeeker#next() we can see
{code}
public boolean next() {
      if (!currentBuffer.hasRemaining()) {
        return false;
      }
      decodeNext();
	  ...
}
{code}
So when we are the end of a block, this hasRemaining is supposed to return false and so no more calls to decodeNext. In normal case of MBB and SBB this works fine.
When the MBB is created over the last bucket, we are returning the last dummy BB of 0 size also.
{code}
} else {
    buffers[i] = ByteBuffer.allocate(0);
}
{code}
Because of that even when we have finished all cells from a buffer, hasRemaining() will say true for such case and for decodeNext() we will move to the last BB which is of limit 0.

We have to solve this by fixing ByteBufferArray#asSubBuffer by not including the last dummy buffer of 0 size from returning back to user.


> Possbile Bug in MultiByteBuffer
> -------------------------------
>
>                 Key: HBASE-15064
>                 URL: https://issues.apache.org/jira/browse/HBASE-15064
>             Project: HBase
>          Issue Type: Bug
>          Components: io
>    Affects Versions: 2.0.0
>            Reporter: deepankar
>            Assignee: Anoop Sam John
>            Priority: Critical
>
> While running the newer patches on our production system, I saw this error come couple of times 
> {noformat}
> ipc.RpcServer: Unexpected throwable object 
> 2016-01-01 16:42:56,090 ERROR [B.defaultRpcServer.handler=20,queue=20,port=60020] ipc.RpcServer: Unexpected throwable object 
> java.nio.BufferUnderflowException
> at java.nio.Buffer.nextGetIndex(Buffer.java:500)
> at java.nio.DirectByteBuffer.get(DirectByteBuffer.java:249)
> at org.apache.hadoop.hbase.nio.MultiByteBuff.get(MultiByteBuff.java:494)
> at org.apache.hadoop.hbase.io.encoding.FastDiffDeltaEncoder$1.decode(FastDiffDeltaEncoder.java:402) 
> at org.apache.hadoop.hbase.io.encoding.FastDiffDeltaEncoder$1.decodeNext(FastDiffDeltaEncoder.java:517) 
> at org.apache.hadoop.hbase.io.encoding.BufferedDataBlockEncoder$BufferedEncodedSeeker.next(BufferedDataBlockEncoder.java:815)
> at org.apache.hadoop.hbase.regionserver.StoreFileScanner.next(StoreFileScanner.java:138)
> {noformat}
> Looking at the get code 
> {code}
> if (this.curItem.remaining() == 0) {
>       if (items.length - 1 == this.curItemIndex) {
>         // means cur item is the last one and we wont be able to read a long. Throw exception
>         throw new BufferUnderflowException();
>       }
>       this.curItemIndex++;
>       this.curItem = this.items[this.curItemIndex];
>     }
> return this.curItem.get();
> {code}
> Can the new currentItem have zero elements (position == limit), does it make sense to change the {{if}} to {{while}} ? {{while (this.curItem.remaining() == 0)}}. This logic is repeated may make sense abstract to a new function if we plan to change to  {{if}} to {{while}}



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