Flaky test: TestCompactor_DeleteLocalSyncFiles (arm64) recurs after #7567 — 30s CompactionRunsCompleted>=2 poll times out

[sandy2008]

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Describe the bug

TestCompactor_DeleteLocalSyncFiles (pkg/compactor) is flaky again on arm64, despite the fix in #7567 (which closed #7565). It now times out on the post-fix poll that #7567 added:

--- FAIL: TestCompactor_DeleteLocalSyncFiles (32.80s)
    compactor_test.go:1855: expected true, got false
FAIL	github.com/cortexproject/cortex/pkg/compactor

The poll added by #7567:

// pkg/compactor/compactor_test.go:1855
// Wait for at least two completed cycles so we sample after a steady-state
// ownership cycle ...
cortex_testutil.Poll(t, 30*time.Second, true, func() any {
	return prom_testutil.ToFloat64(c2.CompactionRunsCompleted) >= 2 &&
		len(c2.listTenantsWithMetaSyncDirectories()) > 0
})

On arm64 the second compactor does not reliably reach 2 completed cycles with at least one owned tenant within 30s, so the poll added to fix the original flake itself times out. The original symptom (#7565: "Should not be zero, but was 0") is gone, but the test is still non-deterministic on slow/loaded arm64 runners.

To Reproduce

Steps to reproduce the behavior:

1. Start Cortex (master @ 40a27ad or later — i.e. with fix(compactor): fix flaky TestCompactor_DeleteLocalSyncFiles on arm64 #7567 merged)
2. Run repeatedly on arm64 (flaky):

   go test -count=20 -run TestCompactor_DeleteLocalSyncFiles ./pkg/compactor/
   

Expected behavior

The test passes deterministically on arm64; the second compactor reaches the required steady-state ownership cycles within the poll window (or the wait condition is made robust to arm64 CI timing).

Environment:

• Infrastructure: GitHub Actions CI, ubuntu-24.04-arm (arm64), test-no-race job
• Deployment tool: N/A (Go unit test)

Additional Context

Observed on master CI run (2026-06-08), i.e. after #7567 merged: https://github.com/cortexproject/cortex/actions/runs/27123579544 (job test-no-race (arm64)).

Related: #7565 (original report, closed), #7567 (fix that proved insufficient).

Filed from CI failure-log analysis with AI assistance; the run link and compactor_test.go:1855 were reviewed and verified against master before submitting.