Enable consumer POM flattening for Maven 4 RC-6 readiness

[ascheman]

Heads-up: PLC4X will stop building with Maven 4.0.0-RC-6

Since apache/maven#11780 (backported to 4.0.x via #12236, so first shipping with RC-6), install/deploy of a POM-packaged project fails fast when its consumer POM cannot be downgraded to model 4.0.0. The PLC4X parent uses 4.1.0-only features (<subprojects>, build <sources>), so with a current 4.0.0-SNAPSHOT (pre-RC-6) every build dies in the very first reactor module:

maven-install-plugin:3.1.4:install on project plc4x-parent: The consumer POM …
cannot be downgraded to model version 4.0.0 … Since consumer POM flattening
is disabled, the parent reference is preserved …

Fix

One line in .mvn/maven.config: -Dmaven.consumer.pom.flatten=true.

Side benefit beyond staying buildable: with flattening enabled, the jar module consumer POMs become self-contained model-4.0.0 (no <parent> reference), which fixes Maven 3/Gradle consumption of published PLC4X artifacts — the very issue behind apache/maven#11772 (rc-5 silently publishes model-4.1.0 parent POMs that Maven 3/Gradle cannot parse).

Verified locally

• 4.0.0-rc-5 (current wrapper pin): full reactor BUILD SUCCESS — property exists and is safe on rc-5
• current 4.0.0-SNAPSHOT (pre-RC-6): full reactor (113 modules) BUILD SUCCESS; without this change: fail-fast as above
• installed jar consumer POMs are model 4.0.0 without parent reference; note the POM-packaged parents themselves currently remain model 4.1.0 even with flattening — raised upstream with the Maven folks

Found while testing PLC4X against pre-RC-6 Maven for the Maven 4 adoption effort — happy to keep sending such findings your way if useful. 🙏

🤖 Generated with Claude Code