ARROW-895: Fix lastSet in fillEmpties() and copyFrom()

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

@@ -393,6 +393,7 @@ public void copyFrom(int fromIndex, int thisIndex, ${className} from){
     if (!fromAccessor.isNull(fromIndex)) {
       mutator.set(thisIndex, fromAccessor.get(fromIndex));
     }
+    <#if type.major == "VarLen">mutator.lastSet = thisIndex;</#if>
   }
 
   public void copyFromSafe(int fromIndex, int thisIndex, ${valuesName} from){
@@ -401,6 +402,7 @@ public void copyFromSafe(int fromIndex, int thisIndex, ${valuesName} from){
     </#if>
     values.copyFromSafe(fromIndex, thisIndex, from);
     bits.getMutator().setSafeToOne(thisIndex);
+    <#if type.major == "VarLen">mutator.lastSet = thisIndex;</#if>
   }
 
   public void copyFromSafe(int fromIndex, int thisIndex, ${className} from){
@@ -409,6 +411,7 @@ public void copyFromSafe(int fromIndex, int thisIndex, ${className} from){
     </#if>
     bits.copyFromSafe(fromIndex, thisIndex, from.bits);
     values.copyFromSafe(fromIndex, thisIndex, from.values);
+    <#if type.major == "VarLen">mutator.lastSet = thisIndex;</#if>
   }
 
   public final class Accessor extends BaseDataValueVector.BaseAccessor <#if type.major = "VarLen">implements VariableWidthVector.VariableWidthAccessor</#if> {
@@ -532,7 +535,7 @@ private void fillEmpties(int index){
       while(index > bits.getValueCapacity()) {
         bits.reAlloc();
       }
-      lastSet = index;
+      lastSet = index - 1;
     }
 
     @Override

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

@@ -21,6 +21,7 @@
 import static org.apache.arrow.vector.TestUtils.newVector;
 import static org.junit.Assert.assertArrayEquals;
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
 import static org.junit.Assert.assertTrue;
 
 import java.nio.charset.Charset;
@@ -473,9 +474,46 @@ public void testFillEmptiesNotOverfill() {
 
       vector.getMutator().setSafe(4094, "hello".getBytes(), 0, 5);
       vector.getMutator().setValueCount(4095);
+
       assertEquals(4096 * 4, vector.getFieldBuffers().get(1).capacity());
     }
   }
 
+  @Test
+  public void testCopyFromWithNulls() {
+    try (final NullableVarCharVector vector = newVector(NullableVarCharVector.class, EMPTY_SCHEMA_PATH, MinorType.VARCHAR, allocator);
+          final NullableVarCharVector vector2 = newVector(NullableVarCharVector.class, EMPTY_SCHEMA_PATH, MinorType.VARCHAR, allocator)) {
+      vector.allocateNew();
+
+      for (int i = 0; i < 4095; i++) {
+        if (i % 3 == 0) {
+          continue;
+        }
+        byte[] b = Integer.toString(i).getBytes();
+        vector.getMutator().setSafe(i, b, 0, b.length);
+      }
+
+      vector.getMutator().setValueCount(4095);
+
+      vector2.allocateNew();
+
+      for (int i = 0; i < 4095; i++) {
+        vector2.copyFromSafe(i, i, vector);
+      }
+
+      vector2.getMutator().setValueCount(4095);
+
+      for (int i = 0; i < 4095; i++) {
+        if (i % 3 == 0) {
+          assertNull(vector2.getAccessor().getObject(i));
+        } else {
+          assertEquals(Integer.toString(i), vector2.getAccessor().getObject(i).toString());
+        }
+
+      }
+
+
+    }
+  }
 
 }