NIFI-15523: Fix EventIterator of factory not returning items

An EventIterator created using the `of` factory would return
Optional.empty() when there are actually items in the internal iterator.

Flipping the ternary result expressions fixes this issue.

Added some basic tests to exercise the two factories and the default
filter implementation.

Author: ohnoitsyou

nifi-framework-bundle/nifi-framework-extensions/nifi-provenance-repository-bundle/nifi-persistent-provenance-repository/src/main/java/org/apache/nifi/provenance/store/iterator/EventIterator.java

@@ -33,7 +33,7 @@ public interface EventIterator extends Closeable {
 
     EventIterator EMPTY = new EventIterator() {
         @Override
-        public void close() throws IOException {
+        public void close() {
         }
 
         @Override
@@ -46,17 +46,16 @@ static EventIterator of(final ProvenanceEventRecord... events) {
         final Iterator<ProvenanceEventRecord> itr = Arrays.asList(events).iterator();
         return new EventIterator() {
             @Override
-            public void close() throws IOException {
+            public void close() {
             }
 
             @Override
             public Optional<ProvenanceEventRecord> nextEvent() {
-                return itr.hasNext() ? Optional.empty() : Optional.of(itr.next());
+                return itr.hasNext() ? Optional.of(itr.next()) : Optional.empty();
             }
         };
     }
 
-
     default EventIterator filter(Predicate<ProvenanceEventRecord> predicate) {
         final EventIterator self = this;
 
@@ -70,7 +69,7 @@ public void close() throws IOException {
             public Optional<ProvenanceEventRecord> nextEvent() throws IOException {
                 while (true) {
                     Optional<ProvenanceEventRecord> next = self.nextEvent();
-                    if (!next.isPresent()) {
+                    if (next.isEmpty()) {
                         return next;
                     }
 

nifi-framework-bundle/nifi-framework-extensions/nifi-provenance-repository-bundle/nifi-persistent-provenance-repository/src/test/java/org/apache/nifi/provenance/store/iterator/EventIteratorTest.java

@@ -0,0 +1,64 @@
+/*
+ * 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.nifi.provenance.store.iterator;
+
+import org.apache.nifi.provenance.TestUtil;
+import org.junit.jupiter.api.Test;
+
+import java.io.IOException;
+import java.util.Objects;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class EventIteratorTest {
+
+    @Test
+    void testCanCreateAndRetrieveUsingOfFactory() throws IOException{
+        var event = TestUtil.createEvent();
+        var eventIterator = EventIterator.of(event);
+        var foundEvent = eventIterator.nextEvent();
+        assertTrue(foundEvent.isPresent());
+        assertEquals(foundEvent.get().getAttribute("uuid"), event.getAttribute("uuid"));
+    }
+
+    @Test
+    void testEmptyReturnedWhenExhausted() throws IOException {
+        var eventIterator = EventIterator.of(TestUtil.createEvent());
+        assertTrue(eventIterator.nextEvent().isPresent());
+        assertTrue(eventIterator.nextEvent().isEmpty());
+    }
+
+    @Test
+    void testCanFilterEvents() throws IOException{
+        var eventOne = TestUtil.createEvent();
+        var eventTwo = TestUtil.createEvent();
+
+        var eventIterator = EventIterator.of(eventOne, eventTwo);
+        // Filter out the first event
+        var filteredIterator  = eventIterator.filter((e) -> !Objects.equals(e.getAttribute("uuid"), eventOne.getAttribute("uuid")));
+
+        var foundEvent = filteredIterator.nextEvent().orElseThrow();
+        assertEquals(foundEvent.getAttribute("uuid"), eventTwo.getAttribute("uuid"));
+    }
+
+    @Test
+    void testEmptyFactoryIsEmpty() throws IOException {
+        var eventIterator = EventIterator.EMPTY;
+        assertTrue(eventIterator.nextEvent().isEmpty());
+    }
+}