Fail the aggregate goal for non-POM packaging (#320)

* Warn when aggregate runs on non-POM packaging

AggregatorSourceJarMojo.doExecute() packages sources only for pom
packaging and otherwise returns without a word, so a project that reaches
the goal with some other packaging gets no source archive and no
indication of why.

Warn instead, naming the project and its actual packaging, in the style
of the classifier warning packageSources already emits.

* Log the non-POM aggregate skip at info instead of warn

aggregate is often declared in a parent POM and inherited by jar
modules, where the skip is expected, so warn would flag every one of
them. info matches the plugin's other expected-skip messages ("Skipping
source per configuration", "NOT adding java-sources ...") and stays
visible at the default log level, unlike debug.

* Fail the aggregate goal for non-POM packaging

There is nothing to aggregate in a project without POM packaging, so
stop the build with a message naming the project and its packaging
instead of skipping.

Author: kratos0718

src/main/java/org/apache/maven/plugins/source/AggregatorSourceJarMojo.java

@@ -36,8 +36,11 @@ public class AggregatorSourceJarMojo extends SourceJarMojo {
      */
     @Override
     protected void doExecute() throws MojoException {
-        if (Type.POM.equals(getProject().getPackaging().type().id())) {
-            packageSources(reactorProjects);
+        String type = getProject().getPackaging().type().id();
+        if (!Type.POM.equals(type)) {
+            throw new MojoException("The aggregate goal requires a project with [" + Type.POM
+                    + "] packaging. Current project [" + getProject().getId() + "] has a [" + type + "] packaging.");
         }
+        packageSources(reactorProjects);
     }
 }

src/test/java/org/apache/maven/plugins/source/AggregatorSourceJarMojoTest.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.plugins.source;
+
+import java.util.Collections;
+import java.util.List;
+
+import org.apache.maven.api.Packaging;
+import org.apache.maven.api.Project;
+import org.apache.maven.api.Type;
+import org.apache.maven.api.plugin.Log;
+import org.apache.maven.api.plugin.MojoException;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verifyNoInteractions;
+import static org.mockito.Mockito.when;
+
+class AggregatorSourceJarMojoTest {
+
+    /**
+     * Records the projects handed to {@code packageSources} instead of building an archive, so the test can tell
+     * whether the goal did its work or skipped.
+     */
+    private static class RecordingAggregatorSourceJarMojo extends AggregatorSourceJarMojo {
+        private List<Project> packagedProjects;
+
+        @Override
+        protected void packageSources(List<Project> theProjects) {
+            packagedProjects = theProjects;
+        }
+    }
+
+    @Test
+    void testPomPackagingPackagesTheReactorProjects() {
+        Log log = mock(Log.class);
+        RecordingAggregatorSourceJarMojo mojo = mojoWithPackaging(Type.POM, log);
+        List<Project> reactorProjects = Collections.singletonList(mock(Project.class));
+        mojo.reactorProjects = reactorProjects;
+
+        mojo.doExecute();
+
+        assertEquals(reactorProjects, mojo.packagedProjects);
+        verifyNoInteractions(log);
+    }
+
+    /**
+     * The goal cannot aggregate anything for non-POM packaging, so it fails rather than leaving the user with no
+     * source archive and no reason why.
+     */
+    @Test
+    void testNonPomPackagingFails() {
+        RecordingAggregatorSourceJarMojo mojo = mojoWithPackaging("jar", mock(Log.class));
+
+        MojoException e = assertThrows(MojoException.class, mojo::doExecute);
+
+        assertNull(mojo.packagedProjects, "Sources should not be packaged for non-POM packaging");
+        assertTrue(e.getMessage().contains("jar"), "Message should name the actual packaging: " + e.getMessage());
+        assertTrue(e.getMessage().contains("aggregate:1.0:pom"), "Message should name the project: " + e.getMessage());
+    }
+
+    private static RecordingAggregatorSourceJarMojo mojoWithPackaging(String type, Log log) {
+        Type packagingType = mock(Type.class);
+        when(packagingType.id()).thenReturn(type);
+
+        Packaging packaging = mock(Packaging.class);
+        when(packaging.type()).thenReturn(packagingType);
+
+        Project project = mock(Project.class);
+        when(project.getPackaging()).thenReturn(packaging);
+        when(project.getId()).thenReturn("aggregate:1.0:pom");
+
+        RecordingAggregatorSourceJarMojo mojo = new RecordingAggregatorSourceJarMojo();
+        mojo.project = project;
+        mojo.log = log;
+        return mojo;
+    }
+}