refactor how no backends are modeled in NLB / ALB Gateways

[zac-nixon]

Description

follow up from: #4267

Key changes:
1/ No longer filter out weight = 0 backends in the route builder. Instead, we let the model builder handle the backend depending on which LB type we are working with.

NLB -> Only 1 TG allowed in a Listener, don't provision listener when the TG has weight = 0
ALB -> Propagate all weight = 0 target groups. We special case all target groups having weight = 0, we will instead insert a 503 fixed response rule. HOWEVER, the target groups and targets are still provisioned meaning that customers don't need to re-provision these resources later.

2/ When an NLB / ALB Listener has no backends, we follow the same rules as outlined above.

I was able to manually test NLB and ALB. Further, I've updated the E2E tests to also test that the behavior I have described above is actually implemented.

I noticed that the ListenerRules builder had no UT, so I've gone ahead and created two basic ones for my own scenarios I have added.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[shuqz]

do we wanna check backend.Weight != nil here?

[zac-nixon]

This can't be nil. The route loader forces all nil values to be 1 per Gateway API spec:
https://github.com/kubernetes-sigs/aws-load-balancer-controller/blob/main/pkg/gateway/routeutils/backend.go#L205-L219

[shuqz]

i might missed it, but where is this part being implemented?
"ALB - We special case all target groups having weight = 0, we will instead insert a 503 fixed response rule."

[zac-nixon]

actions are built here (IIF there are going to be target groups to send to)

aws-load-balancer-controller/pkg/gateway/model/model_build_listener.go

Lines 224 to 226 in ebefb4f

	if shouldProvisionActions(targetGroupTuples) {
	actions = buildL7ListenerForwardActions(targetGroupTuples, nil)
	}

when we generate no forward actions:

aws-load-balancer-controller/pkg/gateway/model/model_build_listener.go

Lines 242 to 245 in ebefb4f

	if len(actions) == 0 {
	l.logger.Info("Filling in no backend actions with fixed 503")
	actions = buildL7ListenerNoBackendActions()
	}

[shraddhabang]

/lgtm

[shuqz]

/lgtm /approved

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: shuqz, zac-nixon

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [zac-nixon]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment