Every model problem is reported twice since 4.0.0-rc-6

[slachiewicz]

Since 4.0.0-rc-6 every model problem is printed twice: once in a new grouped
per-file block, and again in the ProjectBuildingException summary that Maven
has always emitted.

Reproduction

A single pom, no other files:

<project xmlns="http://maven.apache.org/POM/4.0.0">
  <modelVersion>99.0.0</modelVersion>
  <groupId>test</groupId>
  <artifactId>fail-build</artifactId>
  <version>0.1-SNAPSHOT</version>
  <packaging>jar</packaging>
  <invalidElementShouldFailBuild/>
</project>

mvn -B validate

4.0.0-rc-5 reports each problem once:

[ERROR] Some problems were encountered while processing the POMs
[ERROR] The build could not read 1 project -> [Help 1]
[ERROR]
[ERROR]   The project (…/pom.xml) has 3 errors
[ERROR]     'modelVersion' of '99.0.0' is newer than the versions supported by this version of Maven: [4.0.0, 4.1.0]. …
[ERROR]     Malformed POM …: Unrecognised tag: '…invalidElementShouldFailBuild': ParseError at [row,col]:[8,3]
[ERROR]     Message: Unrecognised tag: '…invalidElementShouldFailBuild'
[ERROR]     Failed to load project …

4.0.0-rc-6 reports each problem twice — the new block first, then the same
three problems again in the old one:

[ERROR] 3 problems were encountered while processing the POMs (3 errors):

[pom.xml]
  [FATAL] 'modelVersion' of '99.0.0' is newer than the versions supported by this Maven version (4.0.0-rc-6). …
  [ERROR] Malformed POM …: Unrecognised tag: '…invalidElementShouldFailBuild' @ …/pom.xml
  [ERROR] Failed to load project …/pom.xml

[ERROR] The build could not read 1 project -> [Help 1]
[ERROR]
[ERROR]   The project (…/pom.xml) has 3 errors
[ERROR]     'modelVersion' of '99.0.0' is newer than the versions supported by this Maven version (4.0.0-rc-6). …
[ERROR]     Malformed POM …: Unrecognised tag: '…invalidElementShouldFailBuild': ParseError at [row,col]:[8,3]
[ERROR]     Message: Unrecognised tag: '…invalidElementShouldFailBuild'
[ERROR]     Failed to load project …: 2 problems were encountered while building the effective model
[ERROR]         - [FATAL] 'modelVersion' of '99.0.0' …
[ERROR]         - [ERROR] Malformed POM …

Cause

cb9c4f1 ("Improve ProjectBuildingException error messages with detailed
problem reporting", #10975) changed
ProjectBuildingException.createMessage(List<ProjectBuildingResult>) from
returning the one-line "Some problems were encountered while processing the POMs" to rendering the full per-project detail into the exception message.

That message reaches the log because DefaultGraphBuilder wraps the exception
in a DefaultModelProblem:

} catch (final ProjectBuildingException | DuplicateProjectException | MavenExecutionException e) {
    return Result.error(Collections.singletonList(new DefaultModelProblem(null, null, null, null, 0, 0, e)));
}

Meanwhile DefaultExceptionHandler.handle() never uses that message — for a
ProjectBuildingException it discards it and rebuilds the same information from
getResults():

if (exception instanceof ProjectBuildingException projectBuildingException) {
    …
    message = "The build could not read " + children.size() + " project" + (children.size() == 1 ? "" : "s");
}

So the detail added to the message is not a replacement for the existing
rendering, it is a second copy of it.

Still present on master.

Impact

Beyond the duplicated output, this breaks anything that counts occurrences of a
diagnostic. maven-invoker-plugin's fail-build-streamLogsOnFailures IT asserts
that a [FATAL] line appears twice across two forked runs and now sees four
(apache/maven-invoker-plugin#744 makes that assertion version-independent, but
the duplicated output stands on its own.)

Suggested fix

Pick one renderer. Either revert createMessage to the short form and keep the
ExceptionSummary tree, or have DefaultExceptionHandler use the exception's
message and stop rebuilding the children.

[Mazen050]

Hello I'd like to work on this if no one else is already.

After looking into it, I think reverting the ProjectBuildingException message change is the best fix. The detailed message is already rendered by DefaultExceptionHandler, while DefaultMaven.buildGraph() logs exception.getMessage() earlier, which is what causes the duplicated output after the recent change. Reverting the exception message should restore the previous behavior without affecting the existing CLI rendering.

I'll open a PR shortly if that approach sounds good.