Thanks for the PR.

Could you please add an integration test (see https://github.com/pmd/pmd/tree/master/pmd-compat6/src/it ), so that we have a reproducer and a verifier, that this fixes the problem and so that it is not reintroduced in the future.

I assume, this is about printing processing errors (here https://github.com/apache/maven-pmd-plugin/blob/9ce35a43fef8f50d84782eb8a960ceb832d386ec/src/main/java/org/apache/maven/plugins/pmd/exec/PmdExecutor.java#L265 ).
One simple way to provoke a processing error (and therefore to reproduce the problem) is by creating a custom ruleset with one custom XPath-based rule, which uses an invalid xpath expression.

Thanks for the update!

A rule with an invalid XPath expression will only generate an
error row in the log during initialization of the rule:

"Exception while initializing rule , the rule will not be run"

Oh, yes, my bad.

To produce a processing error, the rule needs to be initialized
successfully and then throw an exception during execution. So
instead I resorted to writing a rule in Java which just throws an
exception. And to use it I had to create a test-jar and include it
as a dependency to the pmd maven plugin.

Instead of creating a java based rule, we can still write a XPath rule using the error() function. This ends up as a processing error.
See my comment on the ruleset file.

So we don't need ExceptionThrowingRule and don't need to create a test-jar and this additional dependency.

Also, ProcessingError.getFile() is only called when using the
pmd:pmd goal and not when using pmd:check, so that had to be added
as well.

That's due to MPMD-236 - pmd:check is executing pmd:pmd, but doesn't apply the configuration from the specific execution. See my comment for the pom.xml.

Since we now have a processing error, we need to make sure, that we don't stop maven-pmd-plugin - therefore we need to "skipPmdError":

<skipPmdError>true</skipPmdError> <!-- we want to capture processing errors -->

Otherwise no report we be created and the verification will fail.

Also, we should verify, that the processing error is indeed ending up in the report (in target/pmd.xml). Please add the following lines in verify.bsh (at line 36):

if (!pmdXml.contains("<error filename=\"" + mainFile.getAbsolutePath()) || !pmdXml.contains(mainFile + "\" msg=\"PmdXPathException")) {
    throw new RuntimeException("Processing error has not been reported");
}

I'm not sure this is the best solution, so feel free to suggest a better approach!

I'd prefer to use the XPath-based approach, so that we don't need to create this extra test rule class.

Note: the failing CI build is unrelated to your change, it looks like intermittent NPE (#4757)

Thanks!