[#384] Do not modify the project model in effective-pom

src/main/java/org/apache/maven/plugins/help/EffectivePomMojo.java

@@ -178,7 +178,7 @@ private boolean shouldWriteAllEffectivePOMsInReactor() {
      * @throws MojoExecutionException if any
      */
     private void writeEffectivePom(MavenProject project, XMLWriter writer) throws MojoExecutionException {
-        Model pom = project.getModel();
+        Model pom = project.getModel().clone();
         cleanModel(pom);
 
         StringWriter sWriter = new StringWriter();

src/test/java/org/apache/maven/plugins/help/EffectivePomMojoTest.java

@@ -0,0 +1,112 @@
+/*
+ * 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.plugins.help;
+
+import javax.inject.Inject;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Collections;
+import java.util.Properties;
+
+import org.apache.maven.api.plugin.testing.InjectMojo;
+import org.apache.maven.api.plugin.testing.MojoParameter;
+import org.apache.maven.api.plugin.testing.MojoTest;
+import org.apache.maven.execution.MavenSession;
+import org.apache.maven.model.Model;
+import org.apache.maven.plugin.MojoExecution;
+import org.apache.maven.project.MavenProject;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.junit.jupiter.api.io.TempDir;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import static org.apache.maven.api.plugin.testing.MojoExtension.setVariableValueToObject;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertSame;
+import static org.mockito.Mockito.when;
+
+/**
+ * Test class for the effective-pom mojo of the Help Plugin.
+ */
+@ExtendWith(MockitoExtension.class)
+@MojoTest
+class EffectivePomMojoTest {
+
+    @Inject
+    private MavenProject project;
+
+    @Inject
+    private MavenSession mavenSession;
+
+    @Mock
+    private MojoExecution mojoExecution;
+
+    @TempDir
+    private Path tempDir;
+
+    private final Model model = new Model();
+
+    private final Properties originalProperties = new Properties();
+
+    @BeforeEach
+    void setup() throws IOException {
+        originalProperties.setProperty("b.property", "b-value");
+        originalProperties.setProperty("a.property", "a-value");
+
+        model.setProperties(originalProperties);
+
+        when(project.getModel()).thenReturn(model);
+
+        Path outputPath = Files.createTempFile(tempDir, "maven-help-plugin-test-", ".xml");
+        mavenSession.getUserProperties().setProperty("outputPath", outputPath.toString());
+    }
+
+    /**
+     * The effective-pom goal only displays the model, so it must not modify the project it reads from.
+     *
+     * @throws Exception in case of errors.
+     */
+    @Test
+    @InjectMojo(goal = "effective-pom")
+    @MojoParameter(name = "output", value = "${outputPath}")
+    void testExecuteDoesNotModifyProjectModel(EffectivePomMojo mojo) throws Exception {
+        // snapshot of the contents before the mojo runs, to detect in-place modification
+        Properties expectedProperties = new Properties();
+        expectedProperties.putAll(originalProperties);
+
+        setVariableValueToObject(mojo, "projects", Collections.singletonList(project));
+        setVariableValueToObject(mojo, "mojoExecution", mojoExecution);
+
+        mojo.execute();
+
+        assertSame(
+                originalProperties,
+                model.getProperties(),
+                "effective-pom must not replace the properties of the project model");
+
+        assertEquals(
+                expectedProperties,
+                model.getProperties(),
+                "effective-pom must not modify the properties of the project model");
+    }
+}