mojohaus#858 Fix child module resolution for multi-level projects

Child modules were processed in a parallel stream which added elements to
an unsynchonized LinkedHashMap.

This may resulted an incomplete set of child modules.

Note that as the stream items are not processed heavily there is no or minor
advantage of parallel processing.

versions-common/src/main/java/org/codehaus/mojo/versions/api/PomHelper.java

@@ -1356,7 +1356,7 @@ private static Map<File, Model> getChildModels(Model model, Log logger) throws I
 
         File baseDir = model.getPomFile().getParentFile();
 
-        getAllChildModules(model, logger).parallelStream()
+        getAllChildModules(model, logger).stream()
                 .map(moduleName -> new File(baseDir, moduleName))
                 .map(file -> file.isFile() ? file : new File(file, "pom.xml"))
                 .filter(File::exists)

versions-common/src/test/java/org/codehaus/mojo/versions/api/PomHelperTest.java

@@ -25,8 +25,16 @@
 import java.io.File;
 import java.io.StringReader;
 import java.net.URL;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Collections;
+import java.util.LinkedHashMap;
+import java.util.Map;
 
+import org.apache.commons.io.FileUtils;
 import org.apache.maven.model.Model;
+import org.apache.maven.model.io.DefaultModelWriter;
+import org.apache.maven.model.io.ModelWriter;
 import org.apache.maven.model.io.xpp3.MavenXpp3Reader;
 import org.apache.maven.plugin.logging.SystemStreamLog;
 import org.apache.maven.plugin.testing.AbstractMojoTestCase;
@@ -49,6 +57,8 @@
  * Tests the methods of {@link PomHelper}.
  */
 public class PomHelperTest extends AbstractMojoTestCase {
+    private static final int NUMBER_OF_CHILD_PROJECTS = 30;
+
     @Rule
     public MojoRule mojoRule = new MojoRule(this);
 
@@ -219,4 +229,50 @@ public void testIssue505ChildModules() throws Exception {
                 mojoRule.readMavenProject(new File("src/test/resources/org/codehaus/mojo/versions/api/issue-505"));
         assertThat(PomHelper.getChildModels(project, new SystemStreamLog()).entrySet(), hasSize(3));
     }
+
+    @Test
+    public void testChildModelsForMultiLevelProject() throws Exception {
+        Path tempDirectory = Files.createTempDirectory("testChildModelsForLargeNumberOfModules");
+        ModelWriter modelWriter = new DefaultModelWriter();
+        Map<Path, Model> createdModels = new LinkedHashMap<>();
+
+        try {
+            Model rootProject = createSimpleModel("root");
+            createdModels.put(tempDirectory, rootProject);
+            for (int levelOne = 0; levelOne < NUMBER_OF_CHILD_PROJECTS; levelOne++) {
+                Model levelOneProject = createSimpleModel("child-" + levelOne);
+                Path levelOneProjectDirectory = tempDirectory.resolve(levelOneProject.getArtifactId());
+                rootProject.addModule(levelOneProject.getArtifactId());
+                createdModels.put(levelOneProjectDirectory, levelOneProject);
+
+                for (int levelTwo = 0; levelTwo < NUMBER_OF_CHILD_PROJECTS; levelTwo++) {
+                    Model levelTwoProject = createSimpleModel("child-" + levelOne + "-" + levelTwo);
+                    Path levelTwoProjectDirectory = levelOneProjectDirectory.resolve(levelTwoProject.getArtifactId());
+                    levelOneProject.addModule(levelTwoProject.getArtifactId());
+                    createdModels.put(levelTwoProjectDirectory, levelTwoProject);
+                }
+            }
+
+            for (Map.Entry<Path, Model> entry : createdModels.entrySet()) {
+                modelWriter.write(entry.getKey().resolve("pom.xml").toFile(), Collections.emptyMap(), entry.getValue());
+            }
+
+            MavenProject project = mojoRule.readMavenProject(tempDirectory.toFile());
+
+            assertThat(
+                    PomHelper.getChildModels(project, new SystemStreamLog()).entrySet(), hasSize(createdModels.size()));
+        } finally {
+            FileUtils.deleteDirectory(tempDirectory.toFile());
+        }
+    }
+
+    private Model createSimpleModel(String artifactId) {
+        Model module = new Model();
+        module.setGroupId("child.test");
+        module.setArtifactId(artifactId);
+        module.setVersion("1.0.0-SNAPSHOT");
+        module.setPackaging("pom");
+        module.setModelVersion("4.0.0");
+        return module;
+    }
 }