feat(l4): Add support for enabling L4 NetLB Regional Backend Services by default

[08volt]

feat(l4): Enable L4 NetLB Regional Backend Services by default

This change introduces a feature flag --enable-l4-netlb-rbs-by-default to enable the use of L4 NetLB Regional Backend Services (RBS) by default for newly created L4 NetLB services, instead of relying on CCM as default implementation.

Changes:

• Added EnableL4NetLBRBSByDefault field to ControllerContextConfig in pkg/context/context.go.
• Added the --enable-l4-netlb-rbs-by-default boolean flag in pkg/flags/flags.go, defaulting to false.
• Updated L4NetLBController in pkg/l4lb/l4netlbcontroller.go to use the EnableL4NetLBRBSByDefault setting from the context.
• Modified isRBSBasedService logic to return true if the enableRBSDefault flag is set, in addition to existing checks (annotations, finalizers, existing FR).
• Updated unit tests in pkg/l4lb/l4netlbcontroller_test.go to test the new default behavior.

This change allows clusters to default to using the more modern RBS for L4 NetLBs, while still allowing individual services to use the legacy instance-based backend services if explicitly setting the LoadBalancerClass spec in the Service.

#L4NetLB #RBS

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[k8s-ci-robot]

Hi @08volt. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[TortillaZHawaii]

/ok-to-test

[08volt]

/retest

[TortillaZHawaii]

What about other testcases? We need to check if this doesn't modify existing Target pool LBs. I think it might be easier to add expectRBSServiceWithDefault to existing testcases and run with flag "true" and "false"

[08volt]

I added those cases and added the check for ForwardingRule legacy in the controller code

[08volt]

@TortillaZHawaii

[k8s-ci-robot]

PR needs rebase.

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