Keeping the backward compatibility when empty array added (#6084)

* Keeping the backward compatibility when empty array added to a key using AddEntryProcessor.

Signed-off-by: Santhosh Gandhe <1909520+san81@users.noreply.github.com>

Author: san81

data-prepper-plugins/mutate-event-processors/src/main/java/org/opensearch/dataprepper/plugins/processor/mutateevent/AddEntryProcessor.java

@@ -362,19 +362,14 @@ private void mergeValueToMap(final Map<String, Object> item, final String key, f
 
     private void mergeValue(final Object value, Supplier<Object> getter, Consumer<Object> setter) {
         final Object currentValue = getter.get();
-        final List<Object> mergedValue;
+        final List<Object> mergedValue = new ArrayList<>();
         if (currentValue instanceof List) {
-            mergedValue = new ArrayList<>((List<Object>)currentValue);
+            mergedValue.addAll((List<Object>) currentValue);
         } else {
-            mergedValue = new ArrayList<>(2);
             mergedValue.add(currentValue);
         }
-        
-        if (value instanceof List) {
-            mergedValue.addAll((List<Object>)value);
-        } else {
-            mergedValue.add(value);
-        }
+
+        mergedValue.add(value);
         setter.accept(mergedValue);
     }
 }

data-prepper-plugins/mutate-event-processors/src/test/java/org/opensearch/dataprepper/plugins/processor/mutateevent/AddEntryProcessorTests.java

@@ -31,6 +31,7 @@
 import static org.hamcrest.CoreMatchers.equalTo;
 import static org.hamcrest.CoreMatchers.is;
 import static org.hamcrest.MatcherAssert.assertThat;
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.eq;
@@ -64,6 +65,39 @@ void invalid_add_when_throws_InvalidPluginConfigurationException() {
         assertThrows(InvalidPluginConfigurationException.class, this::createObjectUnderTest);
     }
 
+    @Test
+    public void test_add_empty_array_followed_by_list_of_map_to_the_same_key() {
+        when(mockConfig.getEntries()).thenReturn(createListOfEntries(
+                createEntry("actor/user/groups", null, new ArrayList<>(), null, null, false, false, null, null, true, null),
+                createEntry("actor/user/groups", null, List.of(Map.of("name", "")), null, null, false, true, null, null, true, null)));
+
+        final AddEntryProcessor processor = createObjectUnderTest();
+        final List<Map<String, Object>> mapList = List.of(Map.of("testKey", "testValue"));
+        final Map<String, Object> data = Map.of("message", mapList);
+        final Record<Event> record = getEvent(mapList);
+
+        final List<Record<Event>> editedRecords = (List<Record<Event>>) processor.doExecute(Collections.singletonList(record));
+        assertEquals("{\"message\":[{\"testKey\":\"testValue\"}],\"actor\":{\"user\":{\"groups\":[[{\"name\":\"\"}]]}}}",
+                editedRecords.get(0).getData().toJsonString());
+    }
+
+    @Test
+    public void test_add_list_of_map_followed_by_empty_array_to_the_same_key() {
+        when(mockConfig.getEntries()).thenReturn(createListOfEntries(
+                createEntry("actor/user/groups", null, List.of(Map.of("name", "")), null, null, false, true, null, null, true, null),
+                createEntry("actor/user/groups", null, new ArrayList<>(), null, null, false, false, null, null, true, null),
+                createEntry("actor/user/groups", null, List.of(Map.of("second", "slistval")), null, null, false, true, null, null, true, null)));
+
+        final AddEntryProcessor processor = createObjectUnderTest();
+        final List<Map<String, Object>> mapList = List.of(Map.of("testKey", "testValue"));
+        final Map<String, Object> data = Map.of("message", mapList);
+        final Record<Event> record = getEvent(mapList);
+
+        final List<Record<Event>> editedRecords = (List<Record<Event>>) processor.doExecute(Collections.singletonList(record));
+        assertEquals("{\"message\":[{\"testKey\":\"testValue\"}],\"actor\":{\"user\":{\"groups\":[{\"name\":\"\"},[{\"second\":\"slistval\"}]]}}}",
+                editedRecords.get(0).getData().toJsonString());
+    }
+
     @Test
     public void testSingleAddProcessorTests() {
         when(mockConfig.getEntries()).thenReturn(createListOfEntries(createEntry("newMessage", null, 3, null, null, false, false,null, null, true, null)));
@@ -359,7 +393,7 @@ public void testListMergeOptimization() {
         final Record<Event> record = getEvent(currentList);
         final List<Record<Event>> editedRecords = (List<Record<Event>>) processor.doExecute(Collections.singletonList(record));
 
-        assertThat(editedRecords.get(0).getData().get("message", List.class), equalTo(Arrays.asList(1, 2, 3, 4, 5)));
+        assertThat(editedRecords.get(0).getData().get("message", List.class), equalTo(Arrays.asList(1, 2, 3, Arrays.asList(4, 5))));
     }
 
     @Test