GH-812: Fix appending to an empty UnionVector

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

@@ -601,8 +601,8 @@ public ValueVector visit(UnionVector deltaVector, Void value) {
     UnionVector targetUnionVector = (UnionVector) targetVector;
     int newValueCount = targetVector.getValueCount() + deltaVector.getValueCount();
 
-    // make sure there is enough capacity
-    while (targetUnionVector.getValueCapacity() < newValueCount) {
+    // Child vectors are created and expanded below; an empty union has zero value capacity.
+    while (targetUnionVector.getTypeBuffer().capacity() / UnionVector.TYPE_WIDTH < newValueCount) {
       targetUnionVector.reAlloc();
     }
 

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

@@ -804,6 +804,26 @@ public void testAppendEmptyStructVector() {
     }
   }
 
+  @Test
+  public void testAppendToEmptyUnionVector() {
+    try (BufferAllocator limitedAllocator = allocator.newChildAllocator("empty union", 0, 1048576);
+        UnionVector target = UnionVector.empty("target", limitedAllocator);
+        UnionVector delta = UnionVector.empty("delta", limitedAllocator)) {
+      delta.setType(0, Types.MinorType.FLOAT4);
+      delta.setType(1, Types.MinorType.FLOAT4);
+      Float4Vector values = delta.getFloat4Vector();
+      values.allocateNew();
+      ValueVectorDataPopulator.setVector(values, 1f, 2f);
+      delta.setValueCount(2);
+
+      delta.accept(new VectorAppender(target), null);
+
+      assertEquals(2, target.getValueCount());
+      assertEquals(1f, target.getObject(0));
+      assertEquals(2f, target.getObject(1));
+    }
+  }
+
   @Test
   public void testAppendUnionVector() {
     final int length1 = 10;