KAFKA-4650: Add unit tests for GraphNode class (#18951)

Unit tests added for the class GraphNode.

Change applied to GraphNode parentNodes() function to return a copy of
the collection, which is consistent with how the children() collection
is returned.

Reviewers: Bill Bejeck <bbejeck@apache.org>

---------

Co-authored-by: Lorcan <lorcanjames1@gmail.com>

streams/src/main/java/org/apache/kafka/streams/kstream/internals/graph/GraphNode.java

@@ -48,7 +48,7 @@ public abstract class GraphNode {
     }
 
     public Collection<GraphNode> parentNodes() {
-        return parentNodes;
+        return new LinkedHashSet<>(parentNodes);
     }
 
     String[] parentNodeNames() {

streams/src/test/java/org/apache/kafka/streams/kstream/internals/graph/GraphNodeTest.java

@@ -0,0 +1,127 @@
+/*
+ * 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.kafka.streams.kstream.internals.graph;
+
+import org.apache.kafka.streams.processor.internals.InternalTopologyBuilder;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Collection;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class GraphNodeTest {
+
+    @Test
+    public void testAddChild() {
+        final GraphNode parentNode = new ExtendedGraphNode("");
+        final GraphNode childNode = new ExtendedGraphNode("");
+        parentNode.addChild(childNode);
+
+        assertTrue(parentNode.children().contains(childNode));
+        assertEquals(1, parentNode.children().size());
+
+        assertTrue(childNode.parentNodes().contains(parentNode));
+        assertEquals(1, childNode.parentNodes().size());
+    }
+
+    @Test
+    public void testRemoveValidChild() {
+        final GraphNode parentNode = new ExtendedGraphNode("");
+        final GraphNode childNode = new ExtendedGraphNode("");
+        parentNode.addChild(childNode);
+
+        parentNode.removeChild(childNode);
+        assertFalse(parentNode.children().contains(childNode));
+        assertEquals(0, parentNode.children().size());
+
+        assertFalse(childNode.parentNodes().contains(parentNode));
+        assertEquals(0, childNode.parentNodes().size());
+    }
+
+    @Test
+    public void testClearChildren() {
+        final GraphNode parentNode = new ExtendedGraphNode("");
+        final GraphNode firstChildNode = new ExtendedGraphNode("");
+        final GraphNode secondChildNode = new ExtendedGraphNode("");
+        final GraphNode thirdChildNode = new ExtendedGraphNode("");
+
+        parentNode.addChild(firstChildNode);
+        parentNode.addChild(secondChildNode);
+        parentNode.addChild(thirdChildNode);
+
+        parentNode.clearChildren();
+
+        assertEquals(0, parentNode.children().size());
+        assertEquals(0, firstChildNode.parentNodes().size());
+        assertEquals(0, secondChildNode.parentNodes().size());
+        assertEquals(0, thirdChildNode.parentNodes().size());
+    }
+
+    @Test
+    public void testParentsWrittenToTopology() {
+        final GraphNode firstParentNode = new ExtendedGraphNode("");
+        final GraphNode secondParentNode = new ExtendedGraphNode("");
+        final GraphNode childNode = new ExtendedGraphNode("");
+
+        firstParentNode.addChild(childNode);
+        secondParentNode.addChild(childNode);
+        assertFalse(childNode.allParentsWrittenToTopology());
+
+        firstParentNode.setHasWrittenToTopology(true);
+        assertFalse(childNode.allParentsWrittenToTopology());
+
+        secondParentNode.setHasWrittenToTopology(true);
+        assertTrue(childNode.allParentsWrittenToTopology());
+    }
+
+    @Test
+    public void testToString() {
+        final GraphNode parentNode = new ExtendedGraphNode("Test");
+        final GraphNode childNode = new ExtendedGraphNode("");
+
+        parentNode.addChild(childNode);
+        final String[] parentNodeNames = childNode.parentNodeNames();
+
+        assertEquals(1, parentNodeNames.length);
+        assertEquals("Test", parentNodeNames[0]);
+    }
+
+    @Test
+    public void testCopyParentsCollection() {
+        final GraphNode parentNode = new ExtendedGraphNode("");
+        final GraphNode childNode = new ExtendedGraphNode("");
+        parentNode.addChild(childNode);
+
+        final Collection<GraphNode> childParentNodes = childNode.parentNodes();
+        childParentNodes.remove(parentNode);
+
+        assertEquals(1, childNode.parentNodes().size());
+        assertTrue(childNode.parentNodes().contains(parentNode));
+    }
+
+    private static class ExtendedGraphNode extends GraphNode {
+        ExtendedGraphNode(final String nodeName) {
+            super(nodeName);
+        }
+
+        @Override
+        public void writeToTopology(final InternalTopologyBuilder topologyBuilder) {}
+    }
+}