Maven 4 consumer pom does not carry over mvn xml namespace

[stevenschlansker]

Affected version

4.0.0-rc-5

Bug description

I am attempting to upgrade a project to Maven 4. The project includes a build parent pom, which is a sibling module to others that use the parent.

jdbi/
  pom.xml
  jdbi-build-parent/
    pom.xml
  jdbi-core/
    pom.xml (inherit build-parent)
    src/...
  jdbi-other-integration/...

The build parent pom itself inherits from a generic template pom ( basepom )
In order to combine e.g. compiler arguments in our build parent and basepom, in our build parent we declare:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:mvn="http://maven.apache.org/POM/4.0.0" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
...
<plugins>
    <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <executions>
            <execution>
                <id>default-compile</id>
                <configuration>
                    <compilerArgs mvn:combine.children="append">
                        <arg>-Xlint:deprecation</arg>
                        <arg>-Xlint:unchecked</arg>
                    </compilerArgs>
                </configuration>
            </execution>
    </plugin>
...

We added the mvn namespace since it seemed to be the upgrade path from the old combine.children directive.

When building the multi-module project, all is well.

% mvn clean install
...
[INFO] BUILD SUCCESS

However, if you then try to use an individual project alone, without the full reactor build, you get a surprising error:

% mvn test -pl :jdbi-other-integration
...

[WARNING] The POM for org.jdbi:jdbi-core:jar:tests:4.0.0-SNAPSHOT is invalid, transitive dependencies (if any) will not be available: 1 problem was 
    - [FATAL] Non-parseable POM org.jdbi:jdbi-build-parent:4.0.0-SNAPSHOT: Unable to read model: Undeclared namespace prefix "mvn" (for attribute "combine.children")
 at [row,col {unknown-source}]: [786,64]

...

[ERROR] org.jdbi.guava.codec.TypeResolvingCodecFactoryTest.testCollectionCodec -- Time elapsed: 0.101 s <<< ERROR!
java.lang.NoClassDefFoundError: io/leangen/geantyref/GenericTypeReflector

Sure enough, the installed consumer pom lost the xmlns:mvn on the opening <project> tag:

% head ~/.m2/repository/org/jdbi/jdbi-build-parent/4.0.0-SNAPSHOT/jdbi-build-parent-4.0.0-SNAPSHOT.pom 
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">

So it looks like Maven, while transforming our build parent pom into its consumer version for installation to the local repository, drops the <project xmlns:mvn declaration and then writes an invalid POM file to the local repo.

The source is available, although it's not a minimal example: jdbi/jdbi#2935

[achyuth8055]

Hai @stevenschlansker, I would like to work on this issue and investigate why the mvn namespace
is not preserved when generating the consumer POM I will try to reproduce the issue and propose a fix.

Thanks!

[stevenschlansker]

Thank you @achyuth8055 for your excitement to look at this issue! Please let me know if there's any additional information or testing I can provide to help us move toward a solution.

Another thought I had while musing about this problem in the shower - given that the intermediate build-parent POM is actually intended to be, like its name suggests, a build-parent pom, does this mean that we should somehow treat it differently in Maven and skip writing the consumer-pom entirely? The only point of this module is to publish a build-pom internally to the project itself, and nobody should ever be using this module as a published consumer-pom.

I guess I am not sure in Maven 4 how this is supposed to work, when you want to install a build-pom to be reused for future internal-only builds, which is different than publishing a consumer-pom for external users to consume.