From 4bfe2dcb4f15afc62be7106dbb9cf259c489f4a6 Mon Sep 17 00:00:00 2001
From: "Piotr P. Karwasz" <pkarwasz-github@apache.org>
Date: Thu, 4 Sep 2025 08:03:52 +0200
Subject: [PATCH] feat: flatten non-BOM artifacts

This change introduces a new execution of the Flatten Maven Plugin for non-BOM modules, aimed at simplifying their published POMs while preserving important metadata and build integrity.

### Key changes

* Applies `clean` mode, removing most optional POM elements.
* Retains key elements:

  * `<name>`, `<description>`, and `<url>` are expanded to preserve meaningful metadata and prevent undesirable URL inheritance (i.e., appending `artifactId` to the parent URL).
  * `<license>` is preserved in each module for clarity and utility.
* Keeps the `<parent>` element so modules continue to inherit metadata and `dependencyManagement` from `log4j-bom` and their direct ancestors.
* `<properties>` are removed, but `<dependencies>` and `<dependencyManagement>` are retained with interpolated values to avoid unresolved variables in the flattened POMs.

### Differences from the `clean` mode

* Keeps `name` and `description` per module to aid discoverability.
* Delegates all contact and project metadata (organization, developers, mailing lists, SCM, issue tracker, CI config, etc.) to the centralized `log4j-bom` POM.
* Preserves `dependencyManagement` sections by interpolating versions, rather than removing them.
* Retains non-transitive scopes (`provided`, `test`) to expose compile/test-time dependencies, even if consumers don't require them.

This flattening strategy reduces noise in published POMs while maintaining enough structure for clarity and downstream tool compatibility.
---
 log4j-mongodb/pom.xml |  7 -------
 log4j-parent/pom.xml  | 46 +++++++++++++++++++++++++++++++++++++++++++
 pom.xml               | 46 +++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 92 insertions(+), 7 deletions(-)

diff --git a/log4j-mongodb/pom.xml b/log4j-mongodb/pom.xml
index 452ac0a5829..7ccc14fc779 100644
--- a/log4j-mongodb/pom.xml
+++ b/log4j-mongodb/pom.xml
@@ -128,13 +128,6 @@
         <configuration>
           <skip>true</skip>
         </configuration>
-        <dependencies>
-          <dependency>
-            <groupId>org.junit.jupiter</groupId>
-            <artifactId>junit-jupiter-engine</artifactId>
-            <version>${junit-jupiter.version}</version>
-          </dependency>
-        </dependencies>
       </plugin>
 
     </plugins>
diff --git a/log4j-parent/pom.xml b/log4j-parent/pom.xml
index 22802a52068..48027a3cdc4 100644
--- a/log4j-parent/pom.xml
+++ b/log4j-parent/pom.xml
@@ -1140,6 +1140,52 @@
         </executions>
       </plugin>
 
+      <!--
+       ~ Drop the <build> section from flattened POMs.
+       ~ Keeps the consumer POM minimal and prevents flattened build-only properties
+       ~ (e.g., `error-prone.version`) from causing resolution problems to users.
+       ~
+       ~ Ref: https://github.com/apache/logging-log4j2/issues/3779
+      -->
+      <plugin>
+        <groupId>org.codehaus.mojo</groupId>
+        <artifactId>flatten-maven-plugin</artifactId>
+        <executions>
+          <!-- First use the special `resolveCiFriendliesOnly` mode to replace the special `${revision}` property with the actual version.
+               This is necessary because `flatten` mode does not support `ci-friendly` versions. -->
+          <execution>
+            <id>flatten-revision</id>
+          </execution>
+          <execution>
+            <id>flatten</id>
+            <goals>
+              <goal>flatten</goal>
+            </goals>
+            <phase>process-resources</phase>
+            <configuration>
+              <!-- Use 'clean' as a base, then selectively retain key elements -->
+              <flattenMode>clean</flattenMode>
+              <pomElements>
+                <!-- Keep parent to maintain inheritance from log4j-bom and ancestors -->
+                <parent>keep</parent>
+
+                <!-- Retain meaningful module metadata -->
+                <name>expand</name>
+                <description>expand</description>
+
+                <!-- Expand URL to prevent default behavior of appending artifactId -->
+                <url>expand</url>
+
+                <!-- Keep dependencyManagement and dependencies, interpolating property-based versions -->
+                <dependencyManagement>interpolate</dependencyManagement>
+                <dependencies>interpolate</dependencies>
+              </pomElements>
+            </configuration>
+          </execution>
+
+        </executions>
+      </plugin>
+
     </plugins>
   </build>
 
diff --git a/pom.xml b/pom.xml
index df51f7d4a0d..80ab7100ad8 100644
--- a/pom.xml
+++ b/pom.xml
@@ -630,6 +630,37 @@
         </configuration>
       </plugin>
 
+      <!--
+        ~ Memento rules to remove overrides from child modules, whenever the changes are moved to `logging-parent`.
+        ~
+        ~ When empty, please comment this execution out, but do not delete it!
+        -->
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-enforcer-plugin</artifactId>
+        <executions>
+          <execution>
+            <id>remove-overrides</id>
+            <goals>
+              <goal>enforce</goal>
+            </goals>
+            <!-- Wait until we retrieve `parent.version` property in `build-helper-maven-plugin` below -->
+            <phase>initialize</phase>
+            <inherited>false</inherited>
+            <configuration>
+              <rules>
+                <requireProperty>
+                  <property>parent.version</property>
+                  <regex>12\.1\.1</regex>
+                  <regexMessage>Remember to remove overrides from child modules, whenever the changes are moved to `logging-parent`:
+                    * Remove the override `flatten-maven-plugin` configuration from `log4j-parent/pom.xml`.</regexMessage>
+                </requireProperty>
+              </rules>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+
       <!-- ███████ ████████  █████  ██████  ████████        ███████ ██ ████████ ███████
            ██         ██    ██   ██ ██   ██    ██    ██     ██      ██    ██    ██
            ███████    ██    ███████ ██████     ██           ███████ ██    ██    █████
@@ -657,6 +688,21 @@
               <locale>en_US</locale>
             </configuration>
           </execution>
+          <!-- Used by `maven-enforcer-plugin` above -->
+          <execution>
+            <id>capture-parent-version</id>
+            <goals>
+              <goal>regex-property</goal>
+            </goals>
+            <phase>validate</phase>
+            <inherited>false</inherited>
+            <configuration>
+              <name>parent.version</name>
+              <value>${project.parent.version}</value>
+              <regex>^(.+)$</regex>
+              <replacement>$1</replacement>
+            </configuration>
+          </execution>
         </executions>
       </plugin>