GH-692: Preserve nullability information while transfering DecimalVector and Decimal256Vector (#693)

## What's Changed

This PR proposes to use the "from" `ValueVector`'s field while
transferring `DecimalVector` and `Decimal256Vector` to preserve
nullability information. This is to have similar behavior with all the
other primitive value vectors. Note that the `FieldType` of the value
vector has nullability information.

Closes #692 .

Author: bodduv

vector/src/main/java/org/apache/arrow/vector/Decimal256Vector.java

@@ -567,8 +567,11 @@ private class TransferImpl implements TransferPair {
 
     public TransferImpl(String ref, BufferAllocator allocator) {
       to =
-          new Decimal256Vector(
-              ref, allocator, Decimal256Vector.this.precision, Decimal256Vector.this.scale);
+          (Decimal256Vector.this.field != null
+                  && Decimal256Vector.this.field.getFieldType() != null)
+              ? new Decimal256Vector(ref, Decimal256Vector.this.field.getFieldType(), allocator)
+              : new Decimal256Vector(
+                  ref, allocator, Decimal256Vector.this.precision, Decimal256Vector.this.scale);
     }
 
     public TransferImpl(Field field, BufferAllocator allocator) {

vector/src/main/java/org/apache/arrow/vector/DecimalVector.java

@@ -565,7 +565,10 @@ private class TransferImpl implements TransferPair {
 
     public TransferImpl(String ref, BufferAllocator allocator) {
       to =
-          new DecimalVector(ref, allocator, DecimalVector.this.precision, DecimalVector.this.scale);
+          (DecimalVector.this.field != null && DecimalVector.this.field.getFieldType() != null)
+              ? new DecimalVector(ref, DecimalVector.this.field.getFieldType(), allocator)
+              : new DecimalVector(
+                  ref, allocator, DecimalVector.this.precision, DecimalVector.this.scale);
     }
 
     public TransferImpl(Field field, BufferAllocator allocator) {

vector/src/test/java/org/apache/arrow/vector/TestDecimal256Vector.java

@@ -26,6 +26,7 @@
 import org.apache.arrow.memory.ArrowBuf;
 import org.apache.arrow.memory.BufferAllocator;
 import org.apache.arrow.vector.types.pojo.ArrowType;
+import org.apache.arrow.vector.types.pojo.FieldType;
 import org.apache.arrow.vector.util.TransferPair;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
@@ -375,6 +376,33 @@ public void testGetTransferPairWithField() {
     assertSame(fromVector.getField(), toVector.getField());
   }
 
+  @Test
+  public void testGetTransferPairWithoutField() {
+    final Decimal256Vector fromVector = new Decimal256Vector("decimal", allocator, 10, scale);
+    final TransferPair transferPair =
+        fromVector.getTransferPair(fromVector.getField().getName(), allocator);
+    final Decimal256Vector toVector = (Decimal256Vector) transferPair.getTo();
+    // A new Field created inside a new vector should reuse the field type (should be the same in
+    // memory as the original Field's field type).
+    assertSame(fromVector.getField().getFieldType(), toVector.getField().getFieldType());
+  }
+
+  @Test
+  public void testGetTransferPairWithoutFieldNonNullable() {
+    final FieldType decimal256NonNullableType =
+        new FieldType(
+            false, new ArrowType.Decimal(10, scale, Decimal256Vector.TYPE_WIDTH * 8), null);
+    final Decimal256Vector fromVector =
+        new Decimal256Vector("decimal", decimal256NonNullableType, allocator);
+    final TransferPair transferPair =
+        fromVector.getTransferPair(fromVector.getField().getName(), allocator);
+    final Decimal256Vector toVector = (Decimal256Vector) transferPair.getTo();
+    // A new Field created inside a new vector should reuse the field type (should be the same in
+    // memory as the original Field's field type).
+    assertSame(fromVector.getField().getFieldType(), toVector.getField().getFieldType());
+    assertSame(decimal256NonNullableType, toVector.getField().getFieldType());
+  }
+
   private void verifyWritingArrowBufWithBigEndianBytes(
       Decimal256Vector decimalVector, ArrowBuf buf, BigDecimal[] expectedValues, int length) {
     decimalVector.allocateNew();

vector/src/test/java/org/apache/arrow/vector/TestDecimalVector.java

@@ -26,6 +26,7 @@
 import org.apache.arrow.memory.ArrowBuf;
 import org.apache.arrow.memory.BufferAllocator;
 import org.apache.arrow.vector.types.pojo.ArrowType;
+import org.apache.arrow.vector.types.pojo.FieldType;
 import org.apache.arrow.vector.util.TransferPair;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
@@ -371,6 +372,31 @@ public void testGetTransferPairWithField() {
     assertSame(fromVector.getField(), toVector.getField());
   }
 
+  @Test
+  public void testGetTransferPairWithoutField() {
+    final DecimalVector fromVector = new DecimalVector("decimal", allocator, 10, scale);
+    final TransferPair transferPair =
+        fromVector.getTransferPair(fromVector.getField().getName(), allocator);
+    final DecimalVector toVector = (DecimalVector) transferPair.getTo();
+    // A new Field created inside a new vector should reuse the field type (should be the same in
+    // memory as the original Field's field type).
+    assertSame(fromVector.getField().getFieldType(), toVector.getField().getFieldType());
+  }
+
+  @Test
+  public void testGetTransferPairWithoutFieldNonNullable() {
+    final FieldType decimalNonNullableType =
+        new FieldType(false, new ArrowType.Decimal(10, scale), null);
+    final DecimalVector fromVector =
+        new DecimalVector("decimal", decimalNonNullableType, allocator);
+    final TransferPair transferPair =
+        fromVector.getTransferPair(fromVector.getField().getName(), allocator);
+    final DecimalVector toVector = (DecimalVector) transferPair.getTo();
+    // A new Field created inside a new vector should reuse the field type (should be the same in
+    // memory as the original Field's field type).
+    assertSame(fromVector.getField().getFieldType(), toVector.getField().getFieldType());
+  }
+
   private void verifyWritingArrowBufWithBigEndianBytes(
       DecimalVector decimalVector, ArrowBuf buf, BigDecimal[] expectedValues, int length) {
     decimalVector.allocateNew();