DX-85876: Failure in UnionReader.read after DecimalVector promotion t…

…o UnionVector (dremio#61)

When a DecimalVector is promoted to a UnionVector via a
PromotableWriter, the UnionVector will have the decimal vector in it's
internal struct vector, but the decimalVector field will not be set.
If UnionReader.read is then used to read from the UnionVector, it will
fail when it tries to read one of the promoted decimal values, due
to decimalVector being null, and the exact decimal type not being
provided.  This failure is unnecessary though as we have a pre-existing
decimal vector, the caller just does not know the exact type - and
it shouldn't be required to.

The change here is to check for a pre-existing decimal vector in the
internal struct when getDecimalVector() is called.  If one exists,
set the decimalVector field and return.  Otherwise, if none exists,
throw the exception.

java/vector/src/main/codegen/templates/UnionVector.java

@@ -23,6 +23,7 @@
 import org.apache.arrow.util.Preconditions;
 import org.apache.arrow.vector.BaseValueVector;
 import org.apache.arrow.vector.BitVectorHelper;
+import org.apache.arrow.vector.DecimalVector;
 import org.apache.arrow.vector.FieldVector;
 import org.apache.arrow.vector.ValueVector;
 import org.apache.arrow.vector.complex.AbstractStructVector;
@@ -278,7 +279,10 @@ public StructVector getStruct() {
   <#if minor.class?starts_with("Decimal") || is_timestamp_tz(minor.class) || minor.class == "Duration" || minor.class == "FixedSizeBinary">
   public ${name}Vector get${name}Vector() {
     if (${uncappedName}Vector == null) {
-      throw new IllegalArgumentException("No ${name} present. Provide ArrowType argument to create a new vector");
+      ${uncappedName}Vector = internalStruct.getChild(fieldName(MinorType.${name?upper_case}), ${name}Vector.class);
+      if (${uncappedName}Vector == null) {
+        throw new IllegalArgumentException("No ${name} present. Provide ArrowType argument to create a new vector");
+      }
     }
     return ${uncappedName}Vector;
   }

java/vector/src/test/java/org/apache/arrow/vector/complex/impl/TestPromotableWriter.java

@@ -22,11 +22,13 @@
 import static org.junit.Assert.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
+import java.math.BigDecimal;
 import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
 
 import org.apache.arrow.memory.ArrowBuf;
 import org.apache.arrow.memory.BufferAllocator;
+import org.apache.arrow.vector.DecimalVector;
 import org.apache.arrow.vector.DirtyRootAllocator;
 import org.apache.arrow.vector.complex.ListVector;
 import org.apache.arrow.vector.complex.NonNullableStructVector;
@@ -35,14 +37,18 @@
 import org.apache.arrow.vector.complex.writer.BaseWriter.StructWriter;
 import org.apache.arrow.vector.holders.DurationHolder;
 import org.apache.arrow.vector.holders.FixedSizeBinaryHolder;
+import org.apache.arrow.vector.holders.NullableDecimalHolder;
+import org.apache.arrow.vector.holders.NullableIntHolder;
 import org.apache.arrow.vector.holders.NullableTimeStampMilliTZHolder;
 import org.apache.arrow.vector.holders.TimeStampMilliTZHolder;
+import org.apache.arrow.vector.holders.UnionHolder;
 import org.apache.arrow.vector.types.TimeUnit;
 import org.apache.arrow.vector.types.Types;
 import org.apache.arrow.vector.types.pojo.ArrowType;
 import org.apache.arrow.vector.types.pojo.ArrowType.ArrowTypeID;
 import org.apache.arrow.vector.types.pojo.Field;
 import org.apache.arrow.vector.types.pojo.FieldType;
+import org.apache.arrow.vector.util.DecimalUtility;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
@@ -392,4 +398,41 @@ public void testNoPromoteFixedSizeBinaryToUnionWithNull() throws Exception {
       buf.close();
     }
   }
+
+  @Test
+  public void testPromoteToUnionFromDecimal() throws Exception {
+    try (final NonNullableStructVector container = NonNullableStructVector.empty(EMPTY_SCHEMA_PATH, allocator);
+         final DecimalVector v = container.addOrGet("dec",
+             FieldType.nullable(new ArrowType.Decimal(38, 1, 128)), DecimalVector.class);
+         final PromotableWriter writer = new PromotableWriter(v, container)) {
+
+      container.allocateNew();
+      container.setValueCount(1);
+
+      writer.setPosition(0);
+      writer.writeDecimal(new BigDecimal("0.1"));
+      writer.setPosition(1);
+      writer.writeInt(1);
+
+      container.setValueCount(3);
+
+      UnionVector unionVector = (UnionVector) container.getChild("dec");
+      UnionHolder holder = new UnionHolder();
+
+      unionVector.get(0, holder);
+      NullableDecimalHolder decimalHolder = new NullableDecimalHolder();
+      holder.reader.read(decimalHolder);
+
+      assertEquals(1, decimalHolder.isSet);
+      assertEquals(new BigDecimal("0.1"),
+          DecimalUtility.getBigDecimalFromArrowBuf(decimalHolder.buffer, 0, decimalHolder.scale, 128));
+
+      unionVector.get(1, holder);
+      NullableIntHolder intHolder = new NullableIntHolder();
+      holder.reader.read(intHolder);
+
+      assertEquals(1, intHolder.isSet);
+      assertEquals(1, intHolder.value);
+    }
+  }
 }