fix: implement LeaderElectionRunnable for CostAccrual

[Sarthak-Shreshtha01]

What this PR does / why we need it

CostAccrual ran only on the leader because that is the manager's default for a plain Runnable. Nothing in the code said so, so a change to that default could let every replica add up the same costs.

This PR adds NeedLeaderElection(), which returns true, and a compile-time check that CostAccrual implements manager.LeaderElectionRunnable. I also updated the comment in cmd/main.go, which said the loop relied on not having this method.

Which issue(s) this PR fixes

Fixes #95

Special notes for your reviewer

I added TestCostAccrualNeedsLeaderElection. The related tests pass.

Does this PR introduce a user-facing change?

NONE

Summary by CodeRabbit

• Bug Fixes
  • Cost accrual now runs on the elected leader, preventing multiple replicas from accruing costs at the same time. This helps keep cost totals consistent across deployments.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7bf4809 and e51a242.

📒 Files selected for processing (3)

• cmd/main.go
• internal/controller/cost_accrual.go
• internal/controller/cost_accrual_test.go

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Advanced

Run ID: 3c5f03e2-01ea-4f67-b52a-2b73759c57e9

📥 Commits

Reviewing files that changed from the base of the PR and between 7bf4809 and e51a242.

📒 Files selected for processing (3)

• cmd/main.go
• internal/controller/cost_accrual.go
• internal/controller/cost_accrual_test.go

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📝 Walkthrough

Walkthrough

CostAccrual now implements manager.LeaderElectionRunnable and returns true from NeedLeaderElection. The change also updates comments about leader election and adds a test for the method.

Changes

Cost Accrual Leader Election

Layer / File(s)	Summary
Leader-election contract and validation internal/controller/cost_accrual.go, cmd/main.go, internal/controller/cost_accrual_test.go	CostAccrual now implements the leader-election runnable interface and requires leader election. The comments describe this requirement, and a test asserts that NeedLeaderElection() returns true.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: kerthcet

Merge Risk: ⚪ Minimal · up to e51a2

In the supported deployment, cost accrual runs only on the elected manager, preventing replicas from accruing concurrently. No actionable merge risk remains.

Security Architecture Review

Security architecture risk: 🔵 Low · up to e51a2

The change reinforces the intended single-leader behavior rather than expanding access. Its protection still depends on leader election being enabled wherever multiple manager replicas run.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — Leader-only execution is relevant to the fleet-wide cost ledger and per-replica metrics, but the reviewed change pins the existing execution intent rather than increasing the loop's authority or reachable assets.

Trust Boundaries and Controls

• inferred — The method opts into the manager's leader-election control, conditional on that control being enabled. A false leader-election setting predates this PR and is not newly caused by the method.

Resilience and Maintainability Implications

• observed — The unchanged write ordering limits duplicate durable charging after a conflict; context cancellation stops the ticker loop. An in-flight write during leader handoff is not covered by the reviewed test.

Hardening Proposals

• proposed — Verify leader election is enabled in any multi-replica deployment and exercise leadership loss during an in-flight accrual write.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Issue #95 requires CostAccrual to implement NeedLeaderElection() bool and return true. The PR adds this method, adds the manager.LeaderElectionRunnable compile-time assertion, and adds `TestCo…
Out of Scope Changes check	✅ Passed	The changes stay within issue #95. The implementation, compile-time assertion, test, and comment update all support leader election for CostAccrual. No unrelated change is shown.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: implementing LeaderElectionRunnable for CostAccrual.

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create a new PR

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[Sarthak-Shreshtha01]

/kind bug

[kerthcet]

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[kerthcet]

@coderabbitai full review

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Pull request base or head changed.

[kerthcet]

/lgtm
/approve

Thanks @Sarthak-Shreshtha01

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Approved: PR has both lgtm and approved labels