ARROW-320: ComplexCopier.copy(FieldReader, FieldWriter) should not st…

…art a list if reader is not set

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

@@ -47,23 +47,25 @@ private static void writeValue(FieldReader reader, FieldWriter writer) {
       switch (mt) {
 
       case LIST:
-        writer.startList();
-        while (reader.next()) {
-          writeValue(reader.reader(), getListWriterForReader(reader.reader(), writer));
+        if (reader.isSet()) {
+          writer.startList();
+          while (reader.next()) {
+            writeValue(reader.reader(), getListWriterForReader(reader.reader(), writer));
+          }
+          writer.endList();
         }
-        writer.endList();
         break;
       case MAP:
-        writer.start();
         if (reader.isSet()) {
+          writer.start();
           for(String name : reader){
             FieldReader childReader = reader.reader(name);
             if(childReader.isSet()){
               writeValue(childReader, getMapWriterForReader(childReader, writer, name));
             }
           }
+          writer.end();
         }
-        writer.end();
         break;
   <#list vv.types as type><#list type.minor as minor><#assign name = minor.class?cap_first />
   <#assign fields = minor.fields!type.fields />

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

@@ -0,0 +1,87 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.arrow.vector;
+
+import org.apache.arrow.memory.BufferAllocator;
+import org.apache.arrow.vector.complex.ListVector;
+import org.apache.arrow.vector.complex.impl.ComplexCopier;
+import org.apache.arrow.vector.complex.impl.UnionListReader;
+import org.apache.arrow.vector.complex.impl.UnionListWriter;
+import org.apache.arrow.vector.complex.reader.FieldReader;
+import org.apache.arrow.vector.types.pojo.Field;
+import org.junit.After;
+import org.junit.Assert;
+import org.junit.Before;
+import org.junit.Test;
+
+public class TestListVector {
+  private final static String EMPTY_SCHEMA_PATH = "";
+
+  private BufferAllocator allocator;
+
+  @Before
+  public void init() {
+    allocator = new DirtyRootAllocator(Long.MAX_VALUE, (byte) 100);
+  }
+
+  @After
+  public void terminate() throws Exception {
+    allocator.close();
+  }
+
+  @Test
+  public void testCopyFrom() throws Exception {
+    try (ListVector inVector = new ListVector("input", allocator, null);
+         ListVector outVector = new ListVector("output", allocator, null)) {
+      UnionListWriter writer = inVector.getWriter();
+      writer.allocate();
+
+      // populate input vector with the following records
+      // [1, 2, 3]
+      // null
+      // []
+      writer.setPosition(0); // optional
+      writer.startList();
+      writer.bigInt().writeBigInt(1);
+      writer.bigInt().writeBigInt(2);
+      writer.bigInt().writeBigInt(3);
+      writer.endList();
+
+      writer.setPosition(2);
+      writer.startList();
+      writer.endList();
+
+      writer.setValueCount(3);
+
+      // copy values from input to output
+      outVector.allocateNew();
+      for (int i = 0; i < 3; i++) {
+        outVector.copyFrom(i, i, inVector);
+      }
+      outVector.getMutator().setValueCount(3);
+
+      // assert the output vector is correct
+      FieldReader reader = outVector.getReader();
+      Assert.assertTrue("shouldn't be null", reader.isSet());
+      reader.setPosition(1);
+      Assert.assertFalse("should be null", reader.isSet());
+      reader.setPosition(2);
+      Assert.assertTrue("shouldn't be null", reader.isSet());
+    }
+  }
+}