Azure cluster defaulting webhook should add default subnet CIDRs when subnet CIDRs already exist

[nawazkh]

/kind bug

[Before submitting an issue, have you checked the Troubleshooting Guide self-managed & managed?]

What steps did you take and what happened:
[A clear and concise description of what the bug is.]

• Upon deploying a self managed cluster, the azure cluster validation webhook complains about subnet CIDR being inadmissible.
• Error:

  E0214 20:07:00.141000       8 controller.go:316] "Reconciler error" err="failed to patch AzureCluster default/default-32152: admission webhook \"validation.azurecluster.infrastructure.cluster.x-k8s.io\" denied the request: AzureCluster.infrastructure.cluster.x-k8s.io \"default-32152\" is invalid: [spec.networkSpec.subnets[0].CIDRBlocks: Invalid value: []string{\"10.0.0.0/16\"}: field is immutable, spec.networkSpec.subnets[1].CIDRBlocks: Invalid value: []string{\"10.1.0.0/16\"}: field is immutable]" controller="azurecluster" controllerGroup="infrastructure.cluster.x-k8s.io" controllerKind="AzureCluster" AzureCluster="default/default-32152" namespace="default" name="default-32152" reconcileID="23226a8b-c26d-43ea-8eee-29e3ae9a1a69"
  E0214 20:07:00.652249       8 controller.go:316] "Reconciler error" err="failed to patch AzureCluster default/default-32152: admission webhook \"validation.azurecluster.infrastructure.cluster.x-k8s.io\" denied the request: AzureCluster.infrastructure.cluster.x-k8s.io \"default-32152\" is invalid: [spec.networkSpec.subnets[0].CIDRBlocks: Invalid value: []string{\"10.0.0.0/16\"}: field is immutable, spec.networkSpec.subnets[1].CIDRBlocks: Invalid value: []string{\"10.1.0.0/16\"}: field is immutable]" controller="azurecluster" controllerGroup="infrastructure.cluster.x-k8s.io" controllerKind="AzureCluster" AzureCluster="default/default-32152" namespace="default" name="default-32152" reconcileID="d01128b1-b335-433a-8ee7-4ec5a950d30f"
  

• This seems to happen because of the defaults being set at

  cluster-api-provider-azure/api/v1beta1/azurecluster_default.go

  Line 97 in 21ff484

  func (c *AzureCluster) setSubnetDefaults() {

  • This commit can be used to see the source of this error : 36078c1

What did you expect to happen:

• Azure cluster validation webhook to not complain about the CIDRs.
• Azure cluster defaulting webhook should not add extra CIDRs.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• cluster-api-provider-azure version: v1.18
• Kubernetes version: (use kubectl version): v1.30.2
• OS (e.g. from /etc/os-release):