fix reportSet inheritance in Maven 4 model building

compat/maven-model-builder/src/test/resources/poms/inheritance/plugin-configuration-child.xml

@@ -58,4 +58,20 @@ under the License.
       </plugin>
     </plugins>
   </build>
+
+  <reporting>
+    <plugins>
+      <plugin>
+        <artifactId>MNG-5115-2</artifactId>
+        <reportSets>
+          <reportSet>
+            <id>inherited-append</id>
+            <reports>
+              <report>from-child</report>
+            </reports>
+          </reportSet>
+        </reportSets>
+      </plugin>
+    </plugins>
+  </reporting>
 </project>

compat/maven-model-builder/src/test/resources/poms/inheritance/plugin-configuration-expected.xml

@@ -82,6 +82,18 @@ under the License.
           <!-- reportSet with inherited=false is not here -->
         </reportSets>
       </plugin>
+      <plugin>
+        <artifactId>MNG-5115-2</artifactId>
+        <reportSets>
+          <reportSet>
+            <id>inherited-append</id>
+            <reports>
+              <report>from-child</report>
+              <report>to-be-inherited</report>
+            </reports>
+          </reportSet>
+        </reportSets>
+      </plugin>
     </plugins>
   </reporting>
 </project>

compat/maven-model-builder/src/test/resources/poms/inheritance/plugin-configuration-parent.xml

@@ -84,6 +84,17 @@ under the License.
           </reportSet>
         </reportSets>
       </plugin>
+      <plugin>
+        <artifactId>MNG-5115-2</artifactId>
+        <reportSets>
+          <reportSet>
+            <id>inherited-append</id>
+            <reports>
+              <report>to-be-inherited</report>
+            </reports>
+          </reportSet>
+        </reportSets>
+      </plugin>
     </plugins>
   </reporting>
 </project>

impl/maven-impl/pom.xml

@@ -180,6 +180,11 @@ under the License.
       <artifactId>jmh-generator-annprocess</artifactId>
       <scope>test</scope>
     </dependency>
+    <dependency>
+      <groupId>org.xmlunit</groupId>
+      <artifactId>xmlunit-core</artifactId>
+      <scope>test</scope>
+    </dependency>
   </dependencies>
 
   <build>

impl/maven-impl/src/main/java/org/apache/maven/impl/model/MavenModelMerger.java

@@ -599,7 +599,7 @@ protected void mergeReportPlugin_ReportSets(
                 Object key = getReportSetKey().apply(element);
                 ReportSet existing = merged.get(key);
                 if (existing != null) {
-                    mergeReportSet(element, existing, sourceDominant, context);
+                    element = mergeReportSet(element, existing, sourceDominant, context);
                 }
                 merged.put(key, element);
             }

impl/maven-impl/src/test/java/org/apache/maven/impl/model/DefaultInheritanceAssemblerTest.java

@@ -0,0 +1,98 @@
+/*
+ * 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.maven.impl.model;
+
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+
+import org.apache.maven.api.model.Model;
+import org.apache.maven.api.services.xml.XmlReaderRequest;
+import org.apache.maven.api.services.xml.XmlWriterRequest;
+import org.apache.maven.impl.DefaultModelXmlFactory;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.xmlunit.builder.DiffBuilder;
+import org.xmlunit.diff.Diff;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+
+class DefaultInheritanceAssemblerTest {
+
+    private DefaultModelXmlFactory xmlFactory;
+
+    private DefaultInheritanceAssembler assembler;
+
+    @BeforeEach
+    void setUp() {
+        xmlFactory = new DefaultModelXmlFactory();
+        assembler = new DefaultInheritanceAssembler();
+    }
+
+    private Path getPom(String name) {
+        return Paths.get("../../compat/maven-model-builder/src/test/resources/poms/inheritance/" + name + ".xml");
+    }
+
+    private Model getModel(String name) throws Exception {
+        return xmlFactory.read(XmlReaderRequest.builder().path(getPom(name)).build());
+    }
+
+    @Test
+    void testPluginConfiguration() throws Exception {
+        testInheritance("plugin-configuration");
+    }
+
+    public void testInheritance(String baseName) throws Exception {
+        testInheritance(baseName, false);
+        testInheritance(baseName, true);
+    }
+
+    public void testInheritance(String baseName, boolean fromRepo) throws Exception {
+        Model parent = getModel(baseName + "-parent");
+        Model child = getModel(baseName + "-child");
+
+        if (!fromRepo) {
+            // when model is built from disk, pomFile is set
+            // (has consequences in inheritance algorithm since getProjectDirectory() returns non-null)
+            parent = parent.withPomFile(getPom(baseName + "-parent").toAbsolutePath());
+            child = child.withPomFile(getPom(baseName + "-child").toAbsolutePath());
+        }
+
+        Model assembled = assembler.assembleModelInheritance(child, parent, null, null);
+
+        // write baseName + "-actual"
+        Path actual = Paths.get(
+                "target/test-classes/poms/inheritance/" + baseName + (fromRepo ? "-build" : "-repo") + "-actual.xml");
+        Files.createDirectories(actual.getParent());
+        xmlFactory.write(XmlWriterRequest.<Model>builder()
+                .content(assembled)
+                .path(actual)
+                .build());
+
+        // check with getPom( baseName + "-expected" )
+        Path expected = getPom(baseName + "-expected");
+
+        Diff diff = DiffBuilder.compare(expected.toFile())
+                .withTest(actual.toFile())
+                .ignoreComments()
+                .ignoreWhitespace()
+                .build();
+        assertFalse(diff.hasDifferences(), "XML files should be identical: " + diff.toString());
+    }
+}