refactor(kubeadm): including dns addon version to signature

[prometherion]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Although kubeadm has not been designed to run as a library, the code base can still be imported and used via public functions.

The kubeadm function GetAvailableUpgrades can be used to extract the available upgrades for the given Control Plane, and it allows some mocking using the VersionGetter interface, which allows extensibility (e.g.: in case of air-gapped environments, or the need to interact with specific network desigs).

The VersionGetter already reports the following signatures:

• ClusterVersion
• KubeadmVersion
• VersionFromCILabel
• KubeletVersions
• ComponentVersions

Some of the functions must interact with the API Server to extract information, and having them wrapped in an interface is vital; however, the GetAvailableUpgrades is not providing any interface to extract the CoreDNS version.

kubernetes/cmd/kubeadm/app/phases/upgrade/compute.go

Lines 148 to 151 in 8cd57a9

	dnsVersion, err := dns.DeployedDNSAddon(client)
	if err != nil {
	return nil, err
	}

The proposed changes extend the signature for the VersionGetter since it's responsible for retrieving component versions, and CoreDNS/DNS is part of those.

A positive outcome for implementing this would be able to reuse the function GetAvailableUpgrades by implementing a new VersionGetter when the state for the given cluster has been "frozen" (e.g.: Control Plane scaled to zero): otherwise, a fakeClient should be used since it's injected currently from the function signature, and it's just used for such a purpose. Thus, we could decrease the number of arguments and achieve more elegance and maintainability in the code.

Which issue(s) this PR is related to:

None

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

NONE

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: prometherion
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[neolit123]

i think this is generally fine @prometherion
added a few comments.

/ok-to-test

[neolit123]

is this needed?

[prometherion]

It was ready there:

kubernetes/cmd/kubeadm/app/phases/upgrade/compute.go

Line 23 in 8cd57a9

versionutil "k8s.io/apimachinery/pkg/util/version"

[neolit123]

should be at the bottom of the list of methods.

[neolit123]

instead of creating a client, this fake getter should just return a 'faked string' version for coredns like the rest of the methods here.

[prometherion]

We can't, since we have a test case which requires a failure (get stable version from CI label failed)

[neolit123]

the negative case can be simulated with a 'returnError bool` field from the feke version getter struct.
i think it's more consistent if you drop the fake client and return strings / fake errors.

[neolit123]

then here it could be just a string version instead of the client, IIUC.

[prometherion]

Given that we were using a client with the previous implementation, I kept the original test behaviour without changing its logic. We can refactor the entire suite if you prefer by returning the version and the error. However, this client simulates scenarios with multiple CoreDNS installations; in such cases, the upgrade computation should fail.

[neolit123]

any missing cases for the DNS getter should be added in TestDeployedDNSAddon, IMO