test(purchase): regression guard for parallel multi-account fan-out (#210)

[cristim]

Summary

Closes #210 — adds the parallelism regression guard for executeMultiAccount that the existing TestExecuteMultiAccount_PartialFailure_IsolatesAccounts doesn't catch.

The existing isolation test would still pass if the fan-out were refactored into a serial for loop. Spec acceptance criterion E-2 explicitly calls out parallelism as a separate promise — this test pins it.

Mechanics

• Two valid AWS accounts (A + B), both with valid mock credentials.
• Each PurchaseCommitment mock blocks for 300 ms.
• Wall-clock the whole executePurchase call.
  • Parallel fan-out: ~310 ms (one delay + scheduling overhead).
  • Serial loop: ~600 ms (sum of two delays).
  • Threshold: 500 ms — comfortably between the two.

Verified locally via mutation testing: rewrote executeMultiAccount to a serial for loop and ran the test:

"603.577875ms" is not less than "500ms"
--- FAIL: TestExecuteMultiAccount_RunsAccountsInParallel (0.60s)

After restoring the errgroup-style fan-out the test passes at ~310 ms.

What this does NOT cover (per #210 out-of-scope)

• Load testing of CUDLY_MAX_ACCOUNT_PARALLELISM (separate ticket if needed).
• Credential refresh failures mid-execution.
• Mocking the actual AWS purchase APIs end-to-end.

Test plan

• go test ./internal/purchase/ -run TestExecuteMultiAccount_RunsAccountsInParallel -v -count=1 — passes at ~310 ms.
• go test ./internal/purchase/ -count=1 — full package green.
• go build ./... clean.
• go vet ./... clean.
• Mutation test: serial-loop refactor causes the test to fail with the expected error message (verified locally; mutation reverted before commit).
• All pre-commit hooks pass (gofmt, govet, gocyclo, gosec, trivy, AWS secret scan, full Go tests).

Summary by CodeRabbit

• Tests
  • Added concurrency regression test for multi-account purchase execution to verify parallel processing and performance behavior.

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📥 Commits

Reviewing files that changed from the base of the PR and between c6588bb and 4833083.

📒 Files selected for processing (1)

• internal/purchase/execution_test.go

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📝 Walkthrough

Walkthrough

A concurrency regression test is added to verify that multi-account purchase execution runs in parallel across AWS accounts rather than serially. The test mocks two accounts with timed calls, asserts completion within a parallel upper bound, and verifies correct execution record counts and mock interactions.

Changes

Multi-Account Execution Parallelism Test

Layer / File(s)	Summary
Test Setup & Mock Configuration internal/purchase/execution_test.go	TestExecuteMultiAccount_RunsAccountsInParallel creates two mock AWS accounts and configures mocks for SavePurchaseExecution to record per-account executions, with provider/client mocks injecting delays via time.Sleep(perCallDelay) to simulate realistic call latencies.
Parallelism Assertion & Verification internal/purchase/execution_test.go	manager.executePurchase is invoked and completion time is measured against parallelMaxBound to catch serial fan-out regressions; test asserts no error, exactly two execution records saved, and all mock expectations met.

Sequence Diagram(s)

(Skipped: test-only change with no new feature or control flow modifications)

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• test(purchase): regression guard for parallel multi-account execution isolation (closes #210) #233: Both PRs add regression tests to internal/purchase/execution_test.go targeting manager.executePurchase's multi-account behavior; one guards parallel execution timing, the other enforces per-account failure isolation under mixed credential validity.

Poem

🐰 Hop, hop—two accounts now race!
No serial slowdown to embrace,
Our test keeps time with mighty care,
Ensures the parallelism's there! ✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Linked Issues check	⚠️ Warning	The PR adds a test with two accounts and parallel execution timing bounds, but does not fully implement the linked issue criteria: it lacks verification of independent result_data, proper error sanitization checks, and uses only valid accounts instead of one valid and one invalid.	Add assertions for independent result_data between accounts, verify error sanitization (no credential material), and include both valid and invalid mock credentials as specified in #210 acceptance criteria.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: adding a regression test for parallel multi-account fan-out execution, which directly matches the test addition in the changeset.
Out of Scope Changes check	✅ Passed	The PR correctly adds only a regression test for parallel multi-account execution without introducing load testing, credential refresh logic, or end-to-end AWS mocking, staying within defined scope.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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• Create PR with unit tests
• Commit unit tests in branch test/multi-account-isolation

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[cristim]

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