Convert isValidClientBodyBufferSize to something more generic

[wayt]

What this PR does / why we need it:

This re-use the isValidClientBodyBufferSize for client_max_body_size configuration.

I'm make isValidClientBodyBufferSize more generic for that.

Recently, we've had nginx configuration issue because of client_max_body_size being set to an empty value.

As mentioned in #2494 this is caused by a race condition somewhere, making https://github.com/kubernetes/ingress-nginx/blob/master/internal/ingress/controller/store/store.go#L675 return an empty annotations.Ingress.

Which issue this PR fixes

Special notes for your reviewer:

This does not solve the underlying race condition but fixes the reload issue it causes.

[wayt]

Cc @ElvinEfendi

[ElvinEfendi]

I'm OK with this change but I think the more important issue is how we handle when getting annotations for given ingress fails:

ingress-nginx/internal/ingress/controller/controller.go

Line 262 in 0232611

if err != nil {

Currently we still continue generating configuration for that ingress that we can not get annotations of, maybe we should skip that ingress?

There's always going to be race since listing ingresses and getting their annotations is not an atomic operation. That being said we can make the situation better by introducing another function that returns both the list of ingresses as well as their annotations in a map:

func (s k8sStore) ListIngressesAndAnnotations() ([]*extensions.Ingress, map[string]*annotations.Ingress)

[ElvinEfendi]

/lgtm
/approve

[aledbf]

@ElvinEfendi reading the travis-ci e2e log I think the reason for the invalid configuration happens when there is no endpoints available or an error getting information about the service.

[aledbf]

/lgtm

[k8s-ci-robot]

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This pull-request has been approved by: aledbf, ElvinEfendi, wayt

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