feat(source/service): watch headless endpoints

[tufitko]

Description

Call event handler for service source when headless service's endpoints change.

Fixes #4113

Checklist

• Unit tests updated
• End user documentation updated

[k8s-ci-robot]

Welcome @tufitko!

It looks like this is your first PR to kubernetes-sigs/external-dns 🎉. Please refer to our pull request process documentation to help your PR have a smooth ride to approval.

You will be prompted by a bot to use commands during the review process. Do not be afraid to follow the prompts! It is okay to experiment. Here is the bot commands documentation.

You can also check if kubernetes-sigs/external-dns has its own contribution guidelines.

You may want to refer to our testing guide if you run into trouble with your tests not passing.

If you are having difficulty getting your pull request seen, please follow the recommended escalation practices. Also, for tips and tricks in the contribution process you may want to read the Kubernetes contributor cheat sheet. We want to make sure your contribution gets all the attention it needs!

Thank you, and welcome to Kubernetes. 😃

[k8s-ci-robot]

Hi @tufitko. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please assign mloiseleur for approval. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[mloiseleur]

Thanks. This PR looks good.
/ok-to-test

[mloiseleur]

/lgtm

[szuecs]

I would rather like to see an opt-in approach rather than having this added.

[tufitko]

hmm, I can add label selector to a shared informer factory, but I need to create another one, something like this:

	headlessLabelSelector, err := labels.NewRequirement(v1.IsHeadlessService, selection.Exists, nil)
	if err != nil {
		return nil, err
	}

	headlessEndpointInformerFactory := kubeinformers.NewSharedInformerFactoryWithOptions(kubeClient, 0,
		kubeinformers.WithNamespace(namespace),
		kubeinformers.WithTweakListOptions(func(options *metav1.ListOptions) {
			options.LabelSelector = headlessLabelSelector.String()
		}),
	)
	
	endpointsInformer := headlessEndpointInformerFactory.Core().V1().Endpoints()
	
	// duplicate Start and waitForCacheSync funcs for this factory

Is that OK?