Disable consumer POM flattening by default and add an opt-in feature

[gnodet]

Summary

This PR introduces a new feature flag maven.consumer.pom.flatten that allows users to control whether consumer POMs are flattened by removing dependency management sections. This addresses dependency management inheritance scenarios and provides better control over consumer POM generation.

The consumer POM are NOT flattened anymore by default.

Changes

Core Implementation

• Features.java: Added consumerPomFlatten() method to check the feature flag (defaults to false)
• DefaultConsumerPomBuilder: Enhanced to conditionally apply flattening based on the flag
• Constants.java: Added comprehensive documentation for the new feature flag and improved existing documentation
• Dependency management control: When disabled, consumer POMs preserve dependency management like parent POMs

Integration Tests

• Add ability to disable consumer POM flattening to preserve dependency management inheritance #11346-dependency-management-override: New comprehensive test demonstrating dependency management inheritance scenarios
• Updated existing tests: Modified several integration tests to explicitly enable flattening where needed for backward compatibility

Feature Behavior

When maven.consumer.pom.flatten=false (Default)

• Dependency management preserved: Consumer POMs keep dependency management sections like parent POMs
• Inheritance enabled: Allows dependency management to be inherited by consumers
• Non-POM projects: Processed without flattening dependency management
• Backward compatible: Maintains existing dependency resolution behavior

When maven.consumer.pom.flatten=true (Opt-in)

• Dependency management removed: Consumer POMs are flattened by removing dependency management
• Transitive dependencies only: Only direct dependencies with transitive scopes are kept
• Traditional behavior: Matches the previous Maven consumer POM behavior

Use Case Example

Consider this dependency scenario:

• A 1.0 depends on B 1.0 and manages C to 1.2
• B 2.0 depends on C 1.1
• D depends on A 1.0 and manages B to 2.0

With maven.consumer.pom.flatten=false (default):

• D gets C 1.2 (A's dependency management applies to D's transitive dependencies)
• Consumer POMs preserve dependency management for inheritance

With maven.consumer.pom.flatten=true:

• Traditional flattened consumer POM behavior
• Dependency management sections are removed

Benefits

• Better dependency management: Allows dependency management inheritance from consumer POMs
• Flexible control: Users can choose between flattened and non-flattened consumer POMs
• Improved transitive dependency control: Dependency management can properly control transitive dependency versions
• Opt-in safety: Users can choose when to enable the previous flattening behavior

Fixes #11346 and addresses dependency management inheritance scenarios

[gnodet]

@Bukama fwiw, if we go this way, this will require an update to https://maven.apache.org/whatsnewinmaven4.html#Consumer_POM and will require a 4.0.0-rc-5 release imho.

[gnodet]

💚 All backports created successfully

Status	Branch	Result
✅	maven-4.0.x

Questions ?

Please refer to the Backport tool documentation

[jglick]

This says true is the default, but the code and the PR description says false is the default…?

[jglick]

Do I understand correctly that this can only be controlled by a -D flag? So you cannot easily enable flattening on specific modules. I guess you could add -D to .mvn/maven.config if you wished to enable flattening on all modules in a given reactor (such as one Git repository). Has any thought been given to controlling this mode based on a flag set in the POM itself (if that is even possible)? Or as a configuration option to the install mojo, etc.?