apacheGH-37829: [Java] Avoid resizing data buffer twice when appendin…

…g variable length vectors (apache#37844)

### Rationale for this change
This change prevents avoidable `OversizedAllocationException`s when appending a variable-length vector with many small elements to a variable-length vector with a few large elements. 

When appending variable-length vectors, `VectorAppender` iteratively doubles the offset and validity buffers until they can accommodate the combined elements. In the previous implementation, each iteration would also double the data buffer's capacity. This behavior is appropriate for vectors of fixed-size types but can result in an oversized data buffers when appending many small elements to a variable length vector with a large data buffer. 

### What changes are included in this PR?
The new behavior only resizes the offset and validity buffers when resizing the target vector's buffers to ensure they can hold the total number of combined elements.

The data buffer is resized based on the total required data size of the combined elements.

### Are these changes tested?
Yes. I added a unit test that results in an `OversizedAllocationException` when run against the previous version of the code. 

### Are there any user-facing changes?
No.

* Closes: apache#37829

Authored-by: hrishisd <[email protected]>
Signed-off-by: David Li <[email protected]>

java/vector/src/main/java/org/apache/arrow/vector/util/VectorAppender.java

@@ -116,7 +116,7 @@ public ValueVector visit(BaseVariableWidthVector deltaVector, Void value) {
 
     // make sure there is enough capacity
     while (targetVector.getValueCapacity() < newValueCount) {
-      targetVector.reAlloc();
+      ((BaseVariableWidthVector) targetVector).reallocValidityAndOffsetBuffers();
     }
     while (targetVector.getDataBuffer().capacity() < newValueCapacity) {
       ((BaseVariableWidthVector) targetVector).reallocDataBuffer();
@@ -170,7 +170,7 @@ public ValueVector visit(BaseLargeVariableWidthVector deltaVector, Void value) {
 
     // make sure there is enough capacity
     while (targetVector.getValueCapacity() < newValueCount) {
-      targetVector.reAlloc();
+      ((BaseLargeVariableWidthVector) targetVector).reallocValidityAndOffsetBuffers();
     }
     while (targetVector.getDataBuffer().capacity() < newValueCapacity) {
       ((BaseLargeVariableWidthVector) targetVector).reallocDataBuffer();

java/vector/src/test/java/org/apache/arrow/vector/util/TestVectorAppender.java

@@ -21,11 +21,14 @@
 import static junit.framework.TestCase.assertTrue;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
+import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 
 import org.apache.arrow.memory.BufferAllocator;
 import org.apache.arrow.memory.RootAllocator;
+import org.apache.arrow.vector.BaseValueVector;
 import org.apache.arrow.vector.BigIntVector;
 import org.apache.arrow.vector.BitVector;
 import org.apache.arrow.vector.Float4Vector;
@@ -63,7 +66,8 @@ public class TestVectorAppender {
 
   @Before
   public void prepare() {
-    allocator = new RootAllocator(1024 * 1024);
+    // Permit allocating 4 vectors of max size.
+    allocator = new RootAllocator(4 * BaseValueVector.MAX_ALLOCATION_SIZE);
   }
 
   @After
@@ -185,6 +189,27 @@ public void testAppendEmptyVariableWidthVector() {
     }
   }
 
+  @Test
+  public void testAppendLargeAndSmallVariableVectorsWithinLimit() {
+    int sixteenthOfMaxAllocation = Math.toIntExact(BaseValueVector.MAX_ALLOCATION_SIZE / 16);
+    try (VarCharVector target = makeVarCharVec(1, sixteenthOfMaxAllocation);
+        VarCharVector delta = makeVarCharVec(sixteenthOfMaxAllocation, 1)) {
+      new VectorAppender(delta).visit(target, null);
+      new VectorAppender(target).visit(delta, null);
+    }
+  }
+
+  private VarCharVector makeVarCharVec(int numElements, int bytesPerElement) {
+    VarCharVector v = new VarCharVector("text", allocator);
+    v.allocateNew((long) numElements * bytesPerElement, numElements);
+    for (int i = 0; i < numElements; i++) {
+      String s = String.join("", Collections.nCopies(bytesPerElement, "a"));
+      v.setSafe(i, s.getBytes(StandardCharsets.US_ASCII));
+    }
+    v.setValueCount(numElements);
+    return v;
+  }
+
   @Test
   public void testAppendLargeVariableWidthVector() {
     final int length1 = 5;

java/vector/src/test/java/org/apache/arrow/vector/util/TestVectorSchemaRootAppender.java

@@ -50,7 +50,7 @@ public void shutdown() {
   }
 
   @Test
-  public void testVectorScehmaRootAppend() {
+  public void testVectorSchemaRootAppend() {
     final int length1 = 5;
     final int length2 = 3;
     final int length3 = 2;