Flaky test: TestPartitionCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEnabledAndMultipleInstancesRunning — 60s CompactionRunsCompleted poll times out

[sandy2008]

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Describe the bug

TestPartitionCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEnabledAndMultipleInstancesRunning (pkg/compactor) intermittently fails when a compactor does not complete a compaction run within the 60s poll window:

--- FAIL: TestPartitionCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEnabledAndMultipleInstancesRunning (77.67s)
    compactor_paritioning_test.go:1217: expected true, got false

The assertion:

// pkg/compactor/compactor_paritioning_test.go:1217
cortex_testutil.Poll(t, 60*time.Second, true, func() any {
	return prom_testutil.ToFloat64(c.CompactionRunsCompleted) >= 1
})

With multiple compactors under shuffle-sharding, ring convergence at startup is sometimes slow enough on loaded CI runners that a compactor does not reach one completed run within 60s. (Note the non-partition sibling test, TestCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEnabledAndMultipleInstancesRunning, uses a 120s poll for the same check.) This is the same class of sharding/ring-convergence startup-timing flake as the repeatedly-adjusted compactor ownership tests (e.g. #7486, #7503), but the partition variant has no dedicated issue.

To Reproduce

Steps to reproduce the behavior:

1. Start Cortex (recent master)
2. Run repeatedly (flaky):

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

Expected behavior

Each compactor reliably completes at least one compaction run within the poll window and the test passes deterministically (or the poll timeout accounts for realistic ring-convergence time, matching the non-partition variant's 120s).

Environment:

• Infrastructure: GitHub Actions CI, ubuntu-24.04 (amd64), test job
• Deployment tool: N/A (Go unit test)

Additional Context

Observed on CI (2026-05-30): https://github.com/cortexproject/cortex/actions/runs/26632776611 (job test (amd64)). The failure is intermittent — the test passes on the large majority of runs.

Filed from CI failure-log analysis with AI assistance; the run link and compactor_paritioning_test.go:1217 were reviewed and verified against master before submitting.