[MNG-8544] Conflicting extensions went unnoticed

impl/maven-cli/src/main/java/org/apache/maven/cling/invoker/BaseParser.java

@@ -295,17 +295,45 @@ protected Map<String, String> populateUserProperties(LocalContext context) throw
 
     protected List<CoreExtension> readCoreExtensionsDescriptor(LocalContext context)
             throws ParserException, IOException {
-        ArrayList<CoreExtension> extensions = new ArrayList<>();
         String installationExtensionsFile = context.userProperties.get(Constants.MAVEN_INSTALLATION_EXTENSIONS);
-        extensions.addAll(readCoreExtensionsDescriptorFromFile(
+        ArrayList<CoreExtension> installationExtensions = new ArrayList<>(readCoreExtensionsDescriptorFromFile(
                 context.installationDirectory.resolve(installationExtensionsFile)));
 
+        String userExtensionsFile = context.userProperties.get(Constants.MAVEN_USER_EXTENSIONS);
+        ArrayList<CoreExtension> userExtensions = new ArrayList<>(
+                readCoreExtensionsDescriptorFromFile(context.userHomeDirectory.resolve(userExtensionsFile)));
+
         String projectExtensionsFile = context.userProperties.get(Constants.MAVEN_PROJECT_EXTENSIONS);
-        extensions.addAll(readCoreExtensionsDescriptorFromFile(context.cwd.resolve(projectExtensionsFile)));
+        ArrayList<CoreExtension> projectExtensions =
+                new ArrayList<>(readCoreExtensionsDescriptorFromFile(context.cwd.resolve(projectExtensionsFile)));
 
-        String userExtensionsFile = context.userProperties.get(Constants.MAVEN_USER_EXTENSIONS);
-        extensions.addAll(readCoreExtensionsDescriptorFromFile(context.userHomeDirectory.resolve(userExtensionsFile)));
+        // merge these 3 but check for GA uniqueness; we don't want to load up same extension w/ different Vs
+        HashMap<String, String> gas = new HashMap<>();
+        ArrayList<String> conflicts = new ArrayList<>();
+
+        ArrayList<CoreExtension> coreExtensions =
+                new ArrayList<>(installationExtensions.size() + userExtensions.size() + projectExtensions.size());
+        coreExtensions.addAll(mergeExtensions(installationExtensions, installationExtensionsFile, gas, conflicts));
+        coreExtensions.addAll(mergeExtensions(userExtensions, userExtensionsFile, gas, conflicts));
+        coreExtensions.addAll(mergeExtensions(projectExtensions, projectExtensionsFile, gas, conflicts));
+
+        if (!conflicts.isEmpty()) {
+            throw new ParserException("Extension conflicts: " + String.join("; ", conflicts));
+        }
 
+        return coreExtensions;
+    }
+
+    private List<CoreExtension> mergeExtensions(
+            List<CoreExtension> extensions, String extensionsSource, Map<String, String> gas, List<String> conflicts) {
+        for (CoreExtension extension : extensions) {
+            String ga = extension.getGroupId() + ":" + extension.getArtifactId();
+            if (gas.containsKey(ga)) {
+                conflicts.add(ga + " from " + extensionsSource + " already specified in " + gas.get(ga));
+            } else {
+                gas.put(ga, extensionsSource);
+            }
+        }
         return extensions;
     }
 

impl/maven-cli/src/test/java/org/apache/maven/cling/invoker/mvn/MavenInvokerTest.java

@@ -19,13 +19,15 @@
 package org.apache.maven.cling.invoker.mvn;
 
 import java.nio.file.FileSystem;
+import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.Arrays;
 
 import com.google.common.jimfs.Configuration;
 import com.google.common.jimfs.Jimfs;
 import org.apache.maven.api.cli.Invoker;
 import org.apache.maven.api.cli.Parser;
+import org.apache.maven.api.cli.ParserException;
 import org.apache.maven.cling.invoker.ProtoLookup;
 import org.codehaus.plexus.classworlds.ClassWorld;
 import org.junit.jupiter.api.Disabled;
@@ -34,6 +36,8 @@
 import org.junit.jupiter.api.io.CleanupMode;
 import org.junit.jupiter.api.io.TempDir;
 
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
 /**
  * Local UT.
  */
@@ -59,6 +63,35 @@ void defaultFs(
         invoke(cwd, userHome, Arrays.asList("clean", "verify"));
     }
 
+    @Test
+    void conflictingExtensions(
+            @TempDir(cleanup = CleanupMode.ON_SUCCESS) Path cwd,
+            @TempDir(cleanup = CleanupMode.ON_SUCCESS) Path userHome)
+            throws Exception {
+        String extensionsXml =
+                """
+                <?xml version="1.0" encoding="UTF-8"?>
+                <extensions>
+                    <extension>
+                        <groupId>eu.maveniverse.maven.mimir</groupId>
+                        <artifactId>extension3</artifactId>
+                        <version>0.3.4</version>
+                    </extension>
+                </extensions>
+                """;
+        Path dotMvn = cwd.resolve(".mvn");
+        Files.createDirectories(dotMvn);
+        Path projectExtensions = dotMvn.resolve("extensions.xml");
+        Files.writeString(projectExtensions, extensionsXml);
+
+        Path userConf = userHome.resolve(".m2");
+        Files.createDirectories(userConf);
+        Path userExtensions = userConf.resolve("extensions.xml");
+        Files.writeString(userExtensions, extensionsXml);
+
+        assertThrows(ParserException.class, () -> invoke(cwd, userHome, Arrays.asList("clean", "verify")));
+    }
+
     @Disabled("Until we move off fully from File")
     @Test
     void jimFs() throws Exception {

impl/maven-cli/src/test/java/org/apache/maven/cling/invoker/mvn/MavenInvokerTestSupport.java

@@ -88,7 +88,7 @@ protected void invoke(Path cwd, Path userHome, Collection<String> goals) throws
                         .resolve("maven.properties")),
                 "${maven.home}/conf/maven.properties must be a file");
 
-        Files.createDirectory(cwd.resolve(".mvn"));
+        Files.createDirectories(cwd.resolve(".mvn"));
         Path pom = cwd.resolve("pom.xml").toAbsolutePath();
         Files.writeString(pom, POM_STRING);
         Path appJava = cwd.resolve("src/main/java/org/apache/maven/samples/sample/App.java");