clusterDNS cannot change when DNS is enabled

[neoaggelos]

Summary

Found this while working on #375

k8s set dns.service-ip cannot be allowed to change the clusterDNS address when DNS is enabled, as this produces an error. Solves the following error:

// Apr 25 23:50:27 u1 k8s.k8sd[6839]: 2024/04/25 23:50:27 error updating the resource "coredns":
// Apr 25 23:50:27 u1 k8s.k8sd[6839]:          cannot patch "coredns" with kind Service: Service "coredns" is invalid: 
spec.clusterIPs[0]: Invalid value: []string{"10.152.183.168"}: may not change once set
// Apr 25 23:50:27 u1 k8s.k8sd[6839]: 2024/04/25 23:50:27 Looks like there are no changes for Deployment "coredns"
// Apr 25 23:50:27 u1 k8s.k8sd[6839]: 2024/04/25 23:50:27 warning: Upgrade "ck-dns" failed: cannot patch "coredns" with 
kind Service: Service "coredns" is invalid: spec.clusterIPs[0]: Invalid value: []string{"10.152.183.168"}: may not 
change once set
// Apr 25 23:50:27 u1 k8s.k8sd[6839]: 2024/04/25 23:50:27 failed to reconcile DNS configuration, will retry in 5 
seconds: failed to apply configuration: failed to apply DNS configuration: failed to apply coredns: failed to upgrade 
ck-dns: cannot patch "coredns" with kind Service: Service "coredns" is invalid: spec.clusterIPs[0]: Invalid value: []
string{"10.152.183.168"}: may not change once set

We still want to allow changing the clusterDNS service if DNS Is disabled (e.g. an external DNS is used)

Changes

• Allow ClusterDNS updates only if DNS is disabled, similar to the handling of LocalStorage.LocalPath
• Add extensive unit tests to verify this behaviour

[github-actions]

Package	Line Rate
github.com/canonical/k8s/api/v1	35%
github.com/canonical/k8s/cmd/k8s-apiserver-proxy	0%
github.com/canonical/k8s/cmd/k8s	33%
github.com/canonical/k8s/cmd/k8sd	0%
github.com/canonical/k8s/cmd	0%
github.com/canonical/k8s/cmd/util	15%
github.com/canonical/k8s/pkg/client/dqlite	48%
github.com/canonical/k8s/pkg/k8s/client	0%
github.com/canonical/k8s/pkg/k8s/client/mock	8%
github.com/canonical/k8s/pkg/k8sd/api	13%
github.com/canonical/k8s/pkg/k8sd/api/impl	0%
github.com/canonical/k8s/pkg/k8sd/app	7%
github.com/canonical/k8s/pkg/k8sd/controllers	62%
github.com/canonical/k8s/pkg/k8sd/database	48%
github.com/canonical/k8s/pkg/k8sd/database/util	0%
github.com/canonical/k8s/pkg/k8sd/features	0%
github.com/canonical/k8s/pkg/k8sd/pki	52%
github.com/canonical/k8s/pkg/k8sd/setup	77%
github.com/canonical/k8s/pkg/k8sd/types	78%
github.com/canonical/k8s/pkg/proxy	6%
github.com/canonical/k8s/pkg/snap/mock	80%
github.com/canonical/k8s/pkg/snap	18%
github.com/canonical/k8s/pkg/snap/util	90%
github.com/canonical/k8s/pkg/utils	68%
github.com/canonical/k8s/pkg/utils/control	100%
github.com/canonical/k8s/pkg/utils/errors	100%
github.com/canonical/k8s/pkg/utils/k8s	69%
github.com/canonical/k8s/pkg/utils/node	0%
github.com/canonical/k8s/pkg/utils/shims	0%
Summary	37% (2429 / 6645)

[bschimke95]

LGTM

[addyess]

@neoaggelos can we disable and change cluster dns at the same time?

canonical/k8s-operator#35

[neoaggelos]

@neoaggelos can we disable and change cluster dns at the same time?

canonical/k8s-operator#35

Yes, that works just fine