Fail on duplicate modules on the module path

When resolving the module path, no module id is allowed to exist
multiple times.

The location manager used to drop duplicates silently which led to
hard to understand error messages when a plugin using plexus-java
dropped a dependency without any notice.

This change enforces uniqueness of module names on the module path
and will add an exception to the PathResolveResult in that case.

plexus-java/src/main/java/org/codehaus/plexus/languages/java/jpms/LocationManager.java

@@ -262,6 +262,12 @@ public String extract(Path path) throws IOException {
                             .put(
                                     entry.getKey(),
                                     moduleNameSources.get(entry.getValue().name()));
+                } else {
+                    result.getPathExceptions()
+                            .put(
+                                    entry.getKey(),
+                                    new IllegalStateException(
+                                            "Module '" + entry.getValue().name() + "' is already on the module path!"));
                 }
             } else {
                 result.getClasspathElements().add(entry.getKey());

plexus-java/src/test/java/org/codehaus/plexus/languages/java/jpms/LocationManagerTest.java

@@ -161,6 +161,36 @@ void testIdenticalModuleNames() throws Exception {
         ResolvePathsRequest<Path> request =
                 ResolvePathsRequest.ofPaths(Arrays.asList(pj1, pj2)).setMainModuleDescriptor(mockModuleInfoJava);
 
+        when(asmParser.getModuleDescriptor(pj1))
+                .thenReturn(JavaModuleDescriptor.newModule("plexus.java").build());
+        when(asmParser.getModuleDescriptor(pj2))
+                .thenReturn(JavaModuleDescriptor.newModule("plexus.java").build());
+
+        ResolvePathsResult<Path> result = locationManager.resolvePaths(request);
+
+        assertThat(result.getMainModuleDescriptor()).isEqualTo(descriptor);
+        assertThat(result.getPathElements()).hasSize(2);
+        assertThat(result.getModulepathElements()).hasSize(1);
+        assertThat(result.getModulepathElements()).containsKey(pj1);
+        assertThat(result.getModulepathElements()).doesNotContainKey(pj2);
+        assertThat(result.getClasspathElements()).isEmpty();
+        // duplicate is flagged as an error
+        assertThat(result.getPathExceptions()).containsOnlyKeys(pj2);
+        assertThat(result.getPathExceptions().get(pj2))
+                .isInstanceOf(IllegalStateException.class)
+                .hasMessageContaining("Module 'plexus.java' is already on the module path!");
+    }
+
+    @Test
+    public void testIdenticalAutomaticModuleNames() throws Exception {
+        Path pj1 = Paths.get("src/test/resources/jar.empty/plexus-java-1.0.0-SNAPSHOT.jar");
+        Path pj2 = Paths.get("src/test/resources/jar.empty.2/plexus-java-2.0.0-SNAPSHOT.jar");
+        JavaModuleDescriptor descriptor =
+                JavaModuleDescriptor.newModule("base").requires("plexus.java").build();
+        when(qdoxParser.fromSourcePath(any(Path.class))).thenReturn(descriptor);
+        ResolvePathsRequest<Path> request =
+                ResolvePathsRequest.ofPaths(Arrays.asList(pj1, pj2)).setMainModuleDescriptor(mockModuleInfoJava);
+
         when(asmParser.getModuleDescriptor(pj1))
                 .thenReturn(
                         JavaModuleDescriptor.newAutomaticModule("plexus.java").build());
@@ -173,8 +203,42 @@ void testIdenticalModuleNames() throws Exception {
         assertThat(result.getPathElements()).hasSize(2);
         assertThat(result.getModulepathElements()).containsOnlyKeys(pj1);
         assertThat(result.getModulepathElements()).doesNotContainKey(pj2);
-        assertThat(result.getClasspathElements()).hasSize(0);
-        assertThat(result.getPathExceptions()).hasSize(0);
+        assertThat(result.getClasspathElements()).isEmpty();
+        // duplicate is flagged as an error
+        assertThat(result.getPathExceptions()).containsOnlyKeys(pj2);
+        assertThat(result.getPathExceptions().get(pj2))
+                .isInstanceOf(IllegalStateException.class)
+                .hasMessageContaining("Module 'plexus.java' is already on the module path!");
+    }
+
+    @Test
+    public void testMainJarModuleAndTestJarAutomatic() throws Exception {
+        Path pj1 = Paths.get("src/test/resources/jar.tests/plexus-java-1.0.0-SNAPSHOT.jar");
+        Path pj2 = Paths.get("src/test/resources/jar.tests/plexus-java-1.0.0-SNAPSHOT-tests.jar");
+        JavaModuleDescriptor descriptor =
+                JavaModuleDescriptor.newModule("base").requires("plexus.java").build();
+        when(qdoxParser.fromSourcePath(any(Path.class))).thenReturn(descriptor);
+        ResolvePathsRequest<Path> request =
+                ResolvePathsRequest.ofPaths(Arrays.asList(pj1, pj2)).setMainModuleDescriptor(mockModuleInfoJava);
+
+        when(asmParser.getModuleDescriptor(pj1))
+                .thenReturn(JavaModuleDescriptor.newModule("plexus.java").build());
+        when(asmParser.getModuleDescriptor(pj2)).thenReturn(null);
+
+        ResolvePathsResult<Path> result = locationManager.resolvePaths(request);
+
+        assertThat(result.getMainModuleDescriptor()).isEqualTo(descriptor);
+        assertThat(result.getPathElements()).hasSize(2);
+        assertThat(result.getModulepathElements()).hasSize(1);
+        assertThat(result.getModulepathElements()).containsKey(pj1);
+        assertThat(result.getModulepathElements()).doesNotContainKey(pj2);
+        assertThat(result.getClasspathElements()).isEmpty();
+
+        // duplicate is flagged as an error
+        assertThat(result.getPathExceptions()).containsOnlyKeys(pj2);
+        assertThat(result.getPathExceptions().get(pj2))
+                .isInstanceOf(IllegalStateException.class)
+                .hasMessageContaining("Module 'plexus.java' is already on the module path!");
     }
 
     @Test
@@ -473,9 +537,13 @@ void testDuplicateModule() throws Exception {
         ResolvePathsResult<Path> result = locationManager.resolvePaths(request);
         assertThat(result.getPathElements()).hasSize(2);
         assertThat(result.getModulepathElements()).containsOnlyKeys(moduleB);
-        // with current default the duplicate will be ignored
         assertThat(result.getClasspathElements()).hasSize(0);
-        assertThat(result.getPathExceptions()).hasSize(0);
+        assertThat(result.getPathExceptions()).hasSize(1);
+        // duplicate (module B / module C) is flagged as an error
+        assertThat(result.getPathExceptions()).containsOnlyKeys(moduleC);
+        assertThat(result.getPathExceptions().get(moduleC))
+                .isInstanceOf(IllegalStateException.class)
+                .hasMessageContaining("Module 'anonymous' is already on the module path!");
     }
 
     @Test