[jira] [Created] (HBASE-20211) ReadOnlyBufferException In UnsafeAccess

["BELUGA BEHR (JIRA)" <ji...@apache.org>]

BELUGA BEHR created HBASE-20211:
-----------------------------------

             Summary: ReadOnlyBufferException In UnsafeAccess
                 Key: HBASE-20211
                 URL: https://issues.apache.org/jira/browse/HBASE-20211
             Project: HBase
          Issue Type: Bug
          Components: hbase
    Affects Versions: 2.0.0
            Reporter: BELUGA BEHR


If you trace the BBUtils API, what you see is this code:
{code:java}
  public static void copyFromBufferToArray(byte[] out, ByteBuffer in, int sourceOffset,
      int destinationOffset, int length) {
    if (in.hasArray()) {
      System.arraycopy(in.array(), sourceOffset + in.arrayOffset(), out, destinationOffset, length);
    } else if (UNSAFE_AVAIL) {
      UnsafeAccess.copy(in, sourceOffset, out, destinationOffset, length);
    } else {
      ByteBuffer inDup = in.duplicate();
      inDup.position(sourceOffset);
      inDup.get(out, destinationOffset, length);
    }
  }
{code}

A ByteBuffer is being used here, which is not read-only, so it actually hits on the first condition and executes this code:
{quote}System.arraycopy(in.array(), sourceOffset + in.arrayOffset(), out, destinationOffset, length);
{quote}
Which is almost exactly what the {{ByteBuffer}} relative bulk get method does anyway, so there is no savings here, just overheard and complexity.

In regards to the second condition... there is a bug there that I just noticed.
{code:java|title=org.apache.hadoop.hbase.util.UnsafeAccess}
  public static void copy(ByteBuffer src, int srcOffset, byte[] dest, int destOffset,
      int length) {
    long srcAddress = srcOffset;
    Object srcBase = null;
    if (src.isDirect()) {
      srcAddress = srcAddress + ((DirectBuffer) src).address();
    } else {
      srcAddress = srcAddress + BYTE_ARRAY_BASE_OFFSET + src.arrayOffset();
      srcBase = src.array();
    }
    long destAddress = destOffset + BYTE_ARRAY_BASE_OFFSET;
    unsafeCopy(srcBase, srcAddress, dest, destAddress, length);
  }
{code}
This issue here is the [arrayOffset()|https://docs.oracle.com/javase/8/docs/api/java/nio/ByteBuffer.html#arrayOffset--] call. The JavaDocs here say:
{quote}Invoke the hasArray method before invoking this method in order to ensure that this buffer has an accessible backing array.
{quote}
However, as we saw in the previous method, if _hasArray_ returns true, we do _System.arraycopy,_ so the only reason we would be in this _copy_ code is if there was no access to the backing array, yet here it is, depending on it having such access. That could cause problems with Read-Only ByteBuffers that does not affect the _relative bulk get method_.
{code:java}
public class Test {
  public static void main(String[] args) throws IOException {
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    ByteBufferWriterOutputStream bbwos = new ByteBufferWriterOutputStream(baos);
    ByteBuffer bbSmall = ByteBuffer.wrap(new byte[512]).asReadOnlyBuffer();
    bbwos.write(bbSmall, 0, 512);
    bbwos.close();
  }
}

Exception in thread "main" java.nio.ReadOnlyBufferException
	at java.nio.ByteBuffer.arrayOffset(ByteBuffer.java:1024)
	at org.apache.hadoop.hbase.util.UnsafeAccess.copy(UnsafeAccess.java:398)
	at org.apache.hadoop.hbase.util.ByteBufferUtils.copyFromBufferToArray(ByteBufferUtils.java:54)
	at org.apache.hadoop.hbase.io.ByteBufferWriterOutputStream.write(ByteBufferWriterOutputStream.java:59)
	at org.apache.hadoop.hbase.io.Test.main(Test.java:14)
{code}



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