[MNG-8414] Warn when not able to downgrade consumer POM unless preser…

…veModelVersion is forced to true (#1981)

impl/maven-core/src/main/java/org/apache/maven/internal/transformation/impl/DefaultConsumerPomBuilder.java

@@ -44,13 +44,17 @@
 import org.apache.maven.model.v4.MavenModelVersion;
 import org.apache.maven.project.MavenProject;
 import org.eclipse.aether.RepositorySystemSession;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 
 @Named
 class DefaultConsumerPomBuilder implements ConsumerPomBuilder {
     private static final String BOM_PACKAGING = "bom";
 
     public static final String POM_PACKAGING = "pom";
 
+    private static final Logger LOGGER = LoggerFactory.getLogger(DefaultConsumerPomBuilder.class);
+
     private final LifecycleBindingsInjector lifecycleBindingsInjector;
 
     @Inject
@@ -103,14 +107,15 @@ private ModelBuilderResult buildModel(RepositorySystemSession session, MavenProj
 
     static Model transform(Model model, MavenProject project) {
         String packaging = model.getPackaging();
+        boolean preserveModelVersion = model.isPreserveModelVersion();
         if (POM_PACKAGING.equals(packaging)) {
             // raw to consumer transform
             model = model.withRoot(false).withModules(null).withSubprojects(null);
             if (model.getParent() != null) {
                 model = model.withParent(model.getParent().withRelativePath(null));
             }
 
-            if (!model.isPreserveModelVersion()) {
+            if (!preserveModelVersion) {
                 model = model.withPreserveModelVersion(false);
                 String modelVersion = new MavenModelVersion().getModelVersion(model);
                 model = model.withModelVersion(modelVersion);
@@ -137,6 +142,9 @@ static Model transform(Model model, MavenProject project) {
 
             model = builder.build();
             String modelVersion = new MavenModelVersion().getModelVersion(model);
+            if (!ModelBuilder.MODEL_VERSION_4_0_0.equals(modelVersion) && !preserveModelVersion) {
+                warnNotDowngraded(project);
+            }
             model = model.withModelVersion(modelVersion);
         } else {
             Model.Builder builder = prune(
@@ -150,11 +158,22 @@ static Model transform(Model model, MavenProject project) {
 
             model = builder.build();
             String modelVersion = new MavenModelVersion().getModelVersion(model);
+            if (!ModelBuilder.MODEL_VERSION_4_0_0.equals(modelVersion) && !preserveModelVersion) {
+                warnNotDowngraded(project);
+            }
             model = model.withModelVersion(modelVersion);
         }
         return model;
     }
 
+    static void warnNotDowngraded(MavenProject project) {
+        LOGGER.warn("The consumer POM for " + project.getId() + " cannot be downgraded to 4.0.0. "
+                + "If you intent your build to be consumed with Maven 3 projects, you need to remove "
+                + "the features that request a newer model version.  If you're fine with having the "
+                + "consumer POM not consumable with Maven 3, add the `preserve.model.version='true'` "
+                + "attribute on the <project> element of your POM.");
+    }
+
     private static List<Profile> prune(List<Profile> profiles) {
         return profiles.stream()
                 .map(p -> {

its/core-it-suite/src/test/java/org/apache/maven/it/MavenITmng8414ConsumerPomWithNewFeaturesTest.java

@@ -0,0 +1,103 @@
+/*
+ * 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.it;
+
+import java.io.Reader;
+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.model.v4.MavenStaxReader;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * This is a test set for <a href="https://issues.apache.org/jira/browse/MNG-8414">MNG-8414</a>.
+ */
+class MavenITmng8414ConsumerPomWithNewFeaturesTest extends AbstractMavenIntegrationTestCase {
+
+    MavenITmng8414ConsumerPomWithNewFeaturesTest() {
+        super("[4.0.0-rc-2,)");
+    }
+
+    /**
+     *  Verify behavior of the consumer POM when using a feature that require a newer model.
+     */
+    @Test
+    void testNotPreserving() throws Exception {
+        Path basedir =
+                extractResources("/mng-8414-consumer-pom-with-new-features").toPath();
+
+        Verifier verifier = newVerifier(basedir.toString(), null);
+        verifier.addCliArguments("package");
+        verifier.execute();
+        verifier.verifyErrorFreeLog();
+
+        verifier.verifyTextInLog(
+                "The consumer POM for org.apache.maven.its:mng-8414:jar:1.0.0-SNAPSHOT cannot be downgraded to 4.0.0.");
+
+        Path consumerPom = basedir.resolve(Paths.get(
+                "target",
+                "project-local-repo",
+                "org.apache.maven.its",
+                "mng-8414",
+                "1.0.0-SNAPSHOT",
+                "mng-8414-1.0.0-SNAPSHOT-consumer.pom"));
+        assertTrue(Files.exists(consumerPom));
+        Model consumerPomModel;
+        try (Reader r = Files.newBufferedReader(consumerPom)) {
+            consumerPomModel = new MavenStaxReader().read(r);
+        }
+        assertEquals("4.1.0", consumerPomModel.getModelVersion());
+    }
+
+    /**
+     *  Verify behavior of the consumer POM when using a feature that require a newer model.
+     */
+    @Test
+    void testPreserving() throws Exception {
+        Path basedir =
+                extractResources("/mng-8414-consumer-pom-with-new-features").toPath();
+
+        Verifier verifier = newVerifier(basedir.toString(), null);
+        verifier.setLogFileName("log-preserving.txt");
+        verifier.addCliArguments("-f", "pom-preserving.xml", "package");
+        verifier.execute();
+        verifier.verifyErrorFreeLog();
+
+        verifier.verifyTextNotInLog("cannot be downgraded to 4.0.0.");
+
+        Path consumerPom = basedir.resolve(Paths.get(
+                "target",
+                "project-local-repo",
+                "org.apache.maven.its",
+                "mng-8414-preserving",
+                "1.0.0-SNAPSHOT",
+                "mng-8414-preserving-1.0.0-SNAPSHOT-consumer.pom"));
+        assertTrue(Files.exists(consumerPom));
+        Model consumerPomModel;
+        try (Reader r = Files.newBufferedReader(consumerPom)) {
+            consumerPomModel = new MavenStaxReader().read(r);
+        }
+        assertEquals("4.1.0", consumerPomModel.getModelVersion());
+    }
+}

its/core-it-suite/src/test/java/org/apache/maven/it/TestSuiteOrdering.java

@@ -100,6 +100,7 @@ public TestSuiteOrdering() {
          * the tests are to finishing. Newer tests are also more likely to fail, so this is
          * a fail fast technique as well.
          */
+        suite.addTestSuite(MavenITmng8414ConsumerPomWithNewFeaturesTest.class);
         suite.addTestSuite(MavenITmng8245BeforePhaseCliTest.class);
         suite.addTestSuite(MavenITmng8244PhaseAllTest.class);
         suite.addTestSuite(MavenITmng8421MavenEncryptionTest.class);

its/core-it-suite/src/test/resources/mng-8414-consumer-pom-with-new-features/pom-preserving.xml

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.1.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.1.0 https://maven.apache.org/xsd/maven-4.1.0.xsd"
+         root="true" preserve.model.version="true">
+
+  <groupId>org.apache.maven.its</groupId>
+  <artifactId>mng-8414-preserving</artifactId>
+  <version>1.0.0-SNAPSHOT</version>
+
+  <profiles>
+    <profile>
+      <id>my-profile</id>
+      <activation>
+        <condition>true</condition>
+      </activation>
+    </profile>
+  </profiles>
+</project>

its/core-it-suite/src/test/resources/mng-8414-consumer-pom-with-new-features/pom.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.1.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" root="true" xsi:schemaLocation="http://maven.apache.org/POM/4.1.0 https://maven.apache.org/xsd/maven-4.1.0.xsd">
+
+  <groupId>org.apache.maven.its</groupId>
+  <artifactId>mng-8414</artifactId>
+  <version>1.0.0-SNAPSHOT</version>
+
+  <profiles>
+    <profile>
+      <id>my-profile</id>
+      <activation>
+        <condition>true</condition>
+      </activation>
+    </profile>
+  </profiles>
+</project>