Fix: IAP cannot be disabled

[yfuruyama]

Issue Overview

Currently when disabling IAP with the following BackendConfig,

apiVersion: cloud.google.com/v1
kind: BackendConfig
metadata:
  name: iap-backendconfig
spec:
  iap:
    enabled: false

it results with a following sync error:

Error syncing to GCP: error running backend syncing routine: Errored updating IAP Settings for service default/sample-svc: Cannot switch to default OAuth once IAP credentials have been set

Issue Details

This is because when spec.iap.enabled: false is applied, the validation is skipped and beConfig.Spec.Iap.OAuthClientCredentials won't be fulfilled properly (code), so switchingToDefault function always uses the blank OAuth client credentials and returns the error here.

This might be a regression that was introduced by this commit.

About this PR

This PR added the validation in switchingToDefault so that it can continue updating the IAP setting when it's disabled.

I tested this change with manual l7-lb-controller deployment and disabling IAP succeeded with the following logs.

[2024-07-15 21:10:39.058 JST] "Iap `enabled` setting changed: " logger="IngressController.SyncBackends.EnsureIAP" ingressKey="default/sample-ingress" syncId=750146048 backendServiceName="k8s1-f9080803-default-sample-svc-443-b069e533" backendVersion="ga" backendScope="global" port=0 service="default/sample-svc" from=true to=false 

[2024-07-15 21:10:39.058 JST] "Updated IAP settings" logger="IngressController.SyncBackends.EnsureIAP" ingressKey="default/sample-ingress" syncId=750146048 backendServiceName="k8s1-f9080803-default-sample-svc-443-b069e533" backendVersion="ga" backendScope="global" port=0 service="default/sample-svc" 

[2024-07-15 21:10:39.058 JST] "Updating ga BackendService" logger="IngressController.SyncBackends" ingressKey="default/sample-ingress" syncId=750146048 backendServiceName="k8s1-f9080803-default-sample-svc-443-b069e533" backendVersion="ga" backendScope="global" port=0 name="k8s1-f9080803-default-sample-svc-443-b069e533" 

[k8s-ci-robot]

Hi @yfuruyama. Thanks for your PR.

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: yfuruyama
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[swetharepakula]

/ok-to-test

[swetharepakula]

/hold

After some offline discussion, we are waiting on changes on the underlying Arcus API before proceeding with this PR. We may be able to remove the validation completely.

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