HBASE-18921 Fix Result.current ArrayIndexOutOfBoundsException

Patch ArrayIndexOutOfBoundsException when current() is called after
advance() has already returned false

Signed-off-by: Chia-Ping Tsai <chia7712@gmail.com>

hbase-client/src/main/java/org/apache/hadoop/hbase/client/Result.java

@@ -30,6 +30,7 @@ import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
 import java.util.NavigableMap;
+import java.util.NoSuchElementException;
 import java.util.TreeMap;
 
 import org.apache.hadoop.hbase.Cell;
@@ -949,14 +950,23 @@ public class Result implements CellScannable, CellScanner {
 
   @Override
   public Cell current() {
-    if (cells == null) return null;
-    return (cellScannerIndex < 0)? null: this.cells[cellScannerIndex];
+    if (cells == null
+            || cellScannerIndex == INITIAL_CELLSCANNER_INDEX
+            || cellScannerIndex >= cells.length)
+      return null;
+    return this.cells[cellScannerIndex];
   }
 
   @Override
   public boolean advance() {
     if (cells == null) return false;
-    return ++cellScannerIndex < this.cells.length;
+    cellScannerIndex++;
+    if (cellScannerIndex < this.cells.length) {
+      return true;
+    } else if (cellScannerIndex == this.cells.length) {
+      return false;
+    }
+    throw new NoSuchElementException("Cannot advance beyond the last cell");
   }
 
   public Boolean getExists() {

hbase-server/src/test/java/org/apache/hadoop/hbase/client/TestResult.java

@@ -25,6 +25,7 @@ import java.io.IOException;
 import java.nio.ByteBuffer;
 import java.util.Arrays;
 import java.util.List;
+import java.util.NoSuchElementException;
 
 import junit.framework.TestCase;
 
@@ -105,6 +106,23 @@ public class TestResult extends TestCase {
     }
   }
 
+  public void testCurrentOnEmptyCell() throws IOException {
+    Result r = Result.create(new Cell[0]);
+    assertFalse(r.advance());
+    assertNull(r.current());
+  }
+
+  public void testAdvanceTwiceOnEmptyCell() throws IOException {
+    Result r = Result.create(new Cell[0]);
+    assertFalse(r.advance());
+    try {
+      r.advance();
+      fail("NoSuchElementException should have been thrown!");
+    } catch (NoSuchElementException ex) {
+      LOG.debug("As expected: " + ex.getMessage());
+    }
+  }
+
   public void testMultiVersionGetColumn() throws Exception {
     KeyValue [] kvs1 = genKVs(row, family, value, 1, 100);
     KeyValue [] kvs2 = genKVs(row, family, value, 200, 100);