Add Convention Develocity Maven extension

[clayburn]

This PR adds a sample Maven extension that applies Develocity, Common Custom User Data Maven extension, and a reasonable set of defaults for configuring build scans and build caching. The goal is to provide a sample that makes it simple for organizations to create and host their own, customized version of this extension in order to more easily and consistently apply Develocity configurations across an organization.

[runningcode]

i agree with what Daz said about trying to dogfood this extension.

[jprinet]

Some minor comments

[runningcode]

Can maven extensions also be fetched from custom artifact repositories? Or do they only fetch from Maven central? I don't know the answer here but I am wondering since maven extensions are loaded possibly before artifact repositories can be set.

[clayburn]

I ran a test where I cleared maven local, deployed to a non-maven local file repo, and it was able to resolve from there when I declared that directory as a repository.

It seems to respect the repository definitions in the POM file and resolve extensions from those. It's hinted at in this documentation. I do think it would be good to be explicit about that in this README.

[erichaagdev]

We should do a full e2e test of this using a real repository. I wonder if we could publish to https://repo.grdev.net/ for a test. I have had one customer report problems with this. But I have another customer who hasn't.

[etiennestuder]

@clayburn I have not gotten to review this, yet, but please make sure all of this runs on Java 8, incl. the build itself. Similarly, the approach of this convention should work on Maven 3.6, 3.7, 3.8, and 3.9. thanks.

[jprinet]

Is the intention here to check that the convention extension has been applied to the build?

[clayburn]

It's a rudimentary check just to make sure that the build succeeds with the convention extension. It won't actually verify the extension has been applied I suppose.

[jprinet]

It could be a nice improvement.
I can think of a custom extension creating a dummy file for which presence would be checked to assess E2E validation.

[clayburn]

That is a good idea. I will look into adding something like that

[erichaagdev]

For Gradle I added a check that a build scan was published. The setup-gradle action makes this very easy.

develocity-build-config-samples/.github/workflows/convention-develocity-gradle-plugin-verification.yml

Lines 62 to 64 in 4e67808

	- name: Verify Build Scan published
	if: ${{ !steps.build.outputs.build-scan-url }}
	run: echo "::error ::No Build Scan published"; exit 1

It would be nice if maven-setup could do the same. https://github.com/gradle/develocity-actions/tree/main/maven-setup

[clayburn]

This needs some attention. I'm going to close it but leave the branch around for now until we want to prioritize it again.

[erichaagdev]

@clayburn do we need this, or can we align it with the common configuration sample? IntelliJ doesn't seem to like it and I'm not sure what the purpose is.

develocity-build-config-samples/common-develocity-maven-configuration/.mvn/extensions.xml

Line 1 in 7bba437

<?xml version="1.0" encoding="UTF-8"?>

Suggested change

	<?xml version="1.0" encoding="UTF-8"?>
	<extensions xmlns="http://maven.apache.org/EXTENSIONS/1.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/EXTENSIONS/1.0.0 http://maven.apache.org/xsd/core-extensions-1.0.0.xsd">
	<?xml version="1.0" encoding="UTF-8"?>