cleanup(flowcontrol): Refactor registry with generic leasing and atomic GC

[LukeAVanDrie]

What type of PR is this?

/kind cleanup
/kind test

What this PR does / why we need it:

This PR refactors the FlowRegistry to extract the optimistic resource leasing and garbage collection logic into a dedicated, generic leasing.go file. Previously, the "Check-Pin-Retry" loops and "Mark-and-Sweep" GC logic were duplicated for both Flows and Priority Bands. This duplication obscured the critical concurrency safety invariants.

Key Changes:

• Introduced leasedState and pinLeasedResource. This encapsulates the complex lock-free retry loops, sync.Once handling, etc. into a single, reusable primitive.
• Leveraged generics to deduplicate the GC logic (collectLeasedResources). This ensures that Flows and Priority Bands adhere to identical lifecycle safety contracts.
• Added leasing_test.go with unit tests for the leasedState state machine and a high-contention chaos test verifying that "Pin vs GC" races result in deterministic behavior without data corruption.

Which issue(s) this PR fixes:

Cleanup following #2127 and #2097. Pure refactoring for maintainability, not tracked in any issue.

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

@LukeAVanDrie: The label(s) kind/test cannot be applied, because the repository doesn't have them.

Details

In response to this:

What type of PR is this?

/kind cleanup
/kind test

What this PR does / why we need it:

This PR refactors the FlowRegistry to extract the optimistic resource leasing and garbage collection logic into a dedicated, generic leasing.go file. Previously, the "Check-Pin-Retry" loops and "Mark-and-Sweep" GC logic were duplicated for both Flows and Priority Bands. This duplication obscured the critical concurrency safety invariants.

Key Changes:

• Introduced leasedState and pinLeasedResource. This encapsulates the complex lock-free retry loops, sync.Once handling, etc. into a single, reusable primitive.
• Leveraged generics to deduplicate the GC logic (collectLeasedResources). This ensures that Flows and Priority Bands adhere to identical lifecycle safety contracts.
• Added leasing_test.go with unit tests for the leasedState state machine and a high-contention chaos test verifying that "Pin vs GC" races result in deterministic behavior without data corruption.

Which issue(s) this PR fixes:

Cleanup following #2127 and #2097. Pure refactoring for maintainability, not tracked in any issue.

Does this PR introduce a user-facing change?:

NONE

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[LukeAVanDrie]

/assign @evacchi
/assign @aishukamal

This is purely a cleanup effort based on some of the discussion on the flow and band leasing PRs we deferred. It should be functionally equivalent but nearly fully deduped between these resource types now. Trying to make this pattern a bit more maintainable and reduce cognitive load. leasing.go is still quite complex, but it is at least well-encapsulated now.

[k8s-ci-robot]

@LukeAVanDrie: GitHub didn't allow me to assign the following users: evacchi, aishukamal.

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Details

In response to this:

/assign @evacchi
/assign @aishukamal

This is purely a cleanup effort based on some of the discussion on the flow and band leasing PRs we deferred. It should be functionally equivalent but nearly fully deduped between these resource types now. Trying to make this pattern a bit more maintainable and reduce cognitive load. leasing.go is still quite complex, but it is at least well-encapsulated now.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[ahg-g]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ahg-g, LukeAVanDrie

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