Write an empty relativePath on dependency-reduced-pom.xml

src/main/java/org/apache/maven/plugins/shade/mojo/ShadeMojo.java

@@ -1230,8 +1230,17 @@ private void rewriteDependencyReducedPomIfWeHaveReduction(
 
                     parentFile = parentFile.getCanonicalFile();
 
-                    String relPath = RelativizePath.convertToRelativePath(parentFile, f);
-                    model.getParent().setRelativePath(relPath);
+                    File actualParentFile =
+                            project.getParent() != null ? project.getParent().getFile() : null;
+                    if (parentFileIsTheConfiguredRelativePath(parentFile, actualParentFile)) {
+                        String relPath = RelativizePath.convertToRelativePath(parentFile, f);
+                        model.getParent().setRelativePath(relPath);
+                    } else {
+                        // ../pom.xml is a different GAV (or missing). An empty
+                        // relativePath stops Maven 4 from walking into that POM
+                        // and reporting a false parent cycle.
+                        model.getParent().setRelativePath("");
+                    }
                 }
 
                 try {
@@ -1260,6 +1269,23 @@ private void rewriteDependencyReducedPomIfWeHaveReduction(
         }
     }
 
+    /**
+     * Keep a local relativePath only when it really points at the parent POM.
+     * Otherwise Maven 4 treats the default {@code ..} as the parent and reports
+     * a cycle against a sibling POM that happens to share the same parent GAV.
+     */
+    static boolean parentFileIsTheConfiguredRelativePath(File configured, File actual) {
+        try {
+            return actual != null
+                    && configured != null
+                    && actual.isFile()
+                    && configured.isFile()
+                    && actual.getCanonicalFile().equals(configured.getCanonicalFile());
+        } catch (IOException e) {
+            return false;
+        }
+    }
+
     private void removeSystemScopedDependencies(Set<String> artifactsToRemove, List<Dependency> originalDependencies) {
         for (Dependency dependency : originalDependencies) {
             if (dependency.getScope() != null && dependency.getScope().equalsIgnoreCase("system")) {

src/main/java/org/apache/maven/plugins/shade/pom/MavenJDOMWriter.java

@@ -1525,7 +1525,13 @@ protected void updateParent(Parent value, String xmlTag, Counter counter, Elemen
             findAndReplaceSimpleElement(innerCount, root, "artifactId", value.getArtifactId(), null);
             findAndReplaceSimpleElement(innerCount, root, "groupId", value.getGroupId(), null);
             findAndReplaceSimpleElement(innerCount, root, "version", value.getVersion(), null);
-            findAndReplaceSimpleElement(innerCount, root, "relativePath", value.getRelativePath(), "../pom.xml");
+            String relativePath = value.getRelativePath();
+            if (relativePath != null && relativePath.isEmpty()) {
+                Element rel = updateElement(innerCount, root, "relativePath", true);
+                rel.setText("");
+            } else {
+                findAndReplaceSimpleElement(innerCount, root, "relativePath", relativePath, "../pom.xml");
+            }
         }
     }
 

src/test/java/org/apache/maven/plugins/shade/mojo/ShadeMojoParentPathTest.java

@@ -0,0 +1,56 @@
+/*
+ * 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.shade.mojo;
+
+import java.io.File;
+import java.nio.file.Files;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class ShadeMojoParentPathTest {
+    @TempDir
+    File temporaryFolder;
+
+    @Test
+    public void matchingParentFilesKeepTheRelativePath() throws Exception {
+        File pom = new File(temporaryFolder, "pom.xml");
+        Files.write(pom.toPath(), "<project/>".getBytes("UTF-8"));
+        assertTrue(ShadeMojo.parentFileIsTheConfiguredRelativePath(pom, pom));
+    }
+
+    @Test
+    public void aSiblingPomIsNotTheParent() throws Exception {
+        File parentPom = new File(temporaryFolder, "real-parent.xml");
+        File siblingPom = new File(temporaryFolder, "pom.xml");
+        Files.write(parentPom.toPath(), "<project/>".getBytes("UTF-8"));
+        Files.write(siblingPom.toPath(), "<project/>".getBytes("UTF-8"));
+        assertFalse(ShadeMojo.parentFileIsTheConfiguredRelativePath(siblingPom, parentPom));
+    }
+
+    @Test
+    public void missingActualParentDropsTheRelativePath() throws Exception {
+        File siblingPom = new File(temporaryFolder, "pom.xml");
+        Files.write(siblingPom.toPath(), "<project/>".getBytes("UTF-8"));
+        assertFalse(ShadeMojo.parentFileIsTheConfiguredRelativePath(siblingPom, null));
+    }
+}

src/test/java/org/apache/maven/plugins/shade/pom/PomWriterRelativePathTest.java

@@ -0,0 +1,72 @@
+/*
+ * 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.shade.pom;
+
+import java.io.StringWriter;
+
+import org.apache.maven.model.Model;
+import org.apache.maven.model.Parent;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+public class PomWriterRelativePathTest {
+    @Test
+    public void emptyRelativePathIsWritten() throws Exception {
+        Model model = new Model();
+        model.setModelVersion("4.0.0");
+        model.setGroupId("g");
+        model.setArtifactId("a");
+        model.setVersion("1.0");
+        Parent parent = new Parent();
+        parent.setGroupId("pg");
+        parent.setArtifactId("pa");
+        parent.setVersion("1.0");
+        parent.setRelativePath("");
+        model.setParent(parent);
+
+        StringWriter writer = new StringWriter();
+        PomWriter.write(writer, model, true);
+        String xml = writer.toString();
+
+        assertTrue(xml.contains("<relativePath"), xml);
+        assertFalse(xml.contains("<relativePath>../pom.xml</relativePath>"), xml);
+    }
+
+    @Test
+    public void defaultRelativePathIsOmitted() throws Exception {
+        Model model = new Model();
+        model.setModelVersion("4.0.0");
+        model.setGroupId("g");
+        model.setArtifactId("a");
+        model.setVersion("1.0");
+        Parent parent = new Parent();
+        parent.setGroupId("pg");
+        parent.setArtifactId("pa");
+        parent.setVersion("1.0");
+        model.setParent(parent);
+
+        StringWriter writer = new StringWriter();
+        PomWriter.write(writer, model, true);
+        String xml = writer.toString();
+
+        assertFalse(xml.contains("relativePath"), xml);
+    }
+}