cleanup ingress class handling

[ElvinEfendi]

What this PR does / why we need it:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added tests to cover my changes.
• All new and existing tests passed.

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[ElvinEfendi]

This condition would never hit. IngressClass is set only when ingress-class is not empty. Which means the only time it is set, it is set to a non-empty value.

[ElvinEfendi]

So here we are saying if the class name is not set in the given ingress resource, and also ingress-class flag is not configured for the controller we then process the ingress. But this does not make sense. According to our docs if ingress-class is not set, then the controller should be handling all the ingresses in the given cluster regardless of their ingress class.

[k8s-ci-robot]

@ElvinEfendi: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-ingress-nginx-test	770673c	link	/test pull-ingress-nginx-test

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[cmluciano]

/cc @aledbf

[aledbf]

Closing. The interpretation I did of the new IngressClassName field is wrong.
https://github.com/kubernetes/kubernetes/blob/d911254debefb5b630498aa3552899ee2f99f22e/staging/src/k8s.io/api/networking/v1beta1/types.go#L292

We need:

• To keep using the annotation if it's defined in the ingress
• The new field requires k8s v1.18
• A new flag is required for the IngressClass (this replaces the ingress.class annotation)
• The controller needs to check the IngressClass exists on start

An example:

apiVersion: networking.k8s.io/v1beta1
kind: IngressClass
metadata:
  name: "internal"
  namespace: "ingress-nginx"
spec:
  controller: "k8s.io/ingress-nginx"