HBASE-26688 Threads shared EMPTY_RESULT may lead to unexpected client job down (#4073)

Signed-off-by: Duo Zhang <zhangduo@apache.org>

Author: YutSean

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

@@ -913,7 +913,7 @@ public CellScanner cellScanner() {
 
   @Override
   public Cell current() {
-    if (cells == null
+    if (isEmpty()
             || cellScannerIndex == INITIAL_CELLSCANNER_INDEX
             || cellScannerIndex >= cells.length)
       return null;
@@ -922,7 +922,9 @@ public Cell current() {
 
   @Override
   public boolean advance() {
-    if (cells == null) return false;
+    if (isEmpty()) {
+      return false;
+    }
     cellScannerIndex++;
     if (cellScannerIndex < this.cells.length) {
       return true;

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

@@ -125,14 +125,12 @@ public void testCurrentOnEmptyCell() throws IOException {
     assertNull(r.current());
   }
 
-  public void testAdvanceTwiceOnEmptyCell() throws IOException {
+  public void testAdvanceMultipleOnEmptyCell() 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());
+    // After HBASE-26688, advance of result with empty cell list will always return false.
+    // Here 10 is an arbitrary number to test the logic.
+    for (int i = 0; i < 10; i++) {
+      assertFalse(r.advance());
     }
   }