fix(compactor): de-flake sharded-ownership compactor tests

TestCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEnabledAndMultipleInstancesRunning
and its partition twin seed one shared bucket.ClientMock with 100
tenants (~3,208/~3,707 registered testify expectations). Every bucket
operation from both compactors and both blocks cleaners holds the
mock's single mutex through an O(expectations) reflective scan (~38%
of CPU in two independent profiles), so the first compaction cycle
alone takes ~20s under -race on an idle machine and 6-7x longer on
starved CI runners. Each per-tenant meta sync additionally runs under
a deadline equal to -compactor.compaction-interval (5s in these tests,
pkg/compactor/compactor.go:1085), so starved syncs turn whole runs
into failures and CompactionRunsCompleted can stay pinned at 0 longer
than ANY poll budget: in CI run 26632776611 the partition test failed
its 60s poll AND the non-partition sibling burned its full 120s poll,
in both attempts. The ring itself was ACTIVE and stable before the
poll started (starting() waits for both), correcting the issue's
original ring-convergence framing.

Fix (test-only):
- Cut numUsers from 100 to 20 in both tests (~25x less quadratic
  mock-matching work; the per-user ownership assertions are
  tenant-count independent).
- Align the partition test's run-completion poll budget with its
  non-partition sibling (60s -> 120s) as a seatbelt.
- Set WaitActiveInstanceTimeout=30s in both tests (same hardening as
  #7503): prepareConfig's 5s ACTIVE ceiling is within reach of the
  observed CI starvation envelope.

The shuffle-sharding twins are deliberately untouched: their live
arm64 failure in the same run is an ownership-exclusivity violation
(compactor_test.go:1318), a different failure class that needs its own
root-causing.

Before/after on Apple Silicon, -race: 21.00s/24.01s -> 3.57s/3.55s.

Fixes #7607

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Sandy Chen <Yuxuan.Chen@morganstanley.com>

Author: sandy2008

CHANGELOG.md

@@ -57,6 +57,7 @@
 * [BUGFIX] Query Frontend: Fix native histogram responses not being handled correctly in `minTime()` sort ordering for split_by_interval merge. #7555
 * [BUGFIX] Distributor: Release the push worker pool goroutines on shutdown by stopping the async executor during the stopping phase when `-distributor.num-push-workers` is set. #7602
 * [BUGFIX] Querier: Fix unbounded resource leak in the bucket-scan blocks finder (used when the bucket index is disabled). Per-tenant metadata fetchers, their Prometheus registries, and on-disk meta caches are now evicted once a tenant is no longer active, instead of being retained for the lifetime of the process. #7573
+* [BUGFIX] Compactor: Fix flake in `TestCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEnabledAndMultipleInstancesRunning` and its partition twin: shrink the 100-tenant mock-bucket fixture, whose quadratic testify-mock matching could starve the first compaction run past the poll budget on loaded CI runners, and align the partition test's run-completion poll timeout with its non-partition sibling. #7607
 
 ## 1.21.0 2026-04-24
 

pkg/compactor/compactor_paritioning_test.go

@@ -1142,7 +1142,7 @@ func TestPartitionCompactor_ShouldCompactAllUsersOnShardingEnabledButOnlyOneInst
 
 func TestPartitionCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEnabledAndMultipleInstancesRunning(t *testing.T) {
 
-	numUsers := 100
+	numUsers := 20
 
 	// Setup user IDs
 	userIDs := make([]string, 0, numUsers)
@@ -1192,6 +1192,7 @@ func TestPartitionCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEn
 		cfg.ShardingRing.InstanceAddr = fmt.Sprintf("127.0.0.%d", i)
 		cfg.ShardingRing.WaitStabilityMinDuration = time.Second
 		cfg.ShardingRing.WaitStabilityMaxDuration = 5 * time.Second
+		cfg.ShardingRing.WaitActiveInstanceTimeout = 30 * time.Second // Give the ring ACTIVE wait headroom on slow CI (#7503).
 		cfg.ShardingRing.KVStore.Mock = kvstore
 
 		c, _, tsdbPlanner, l, _ := prepareForPartitioning(t, cfg, bucketClient, nil, nil)
@@ -1214,7 +1215,7 @@ func TestPartitionCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEn
 
 	// Wait until a run has been completed on each compactor
 	for _, c := range compactors {
-		cortex_testutil.Poll(t, 60*time.Second, true, func() any {
+		cortex_testutil.Poll(t, 120*time.Second, true, func() any {
 			return prom_testutil.ToFloat64(c.CompactionRunsCompleted) >= 1
 		})
 	}

pkg/compactor/compactor_test.go

@@ -1091,7 +1091,7 @@ func TestCompactor_ShouldCompactAllUsersOnShardingEnabledButOnlyOneInstanceRunni
 func TestCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEnabledAndMultipleInstancesRunning(t *testing.T) {
 	t.Parallel()
 
-	numUsers := 100
+	numUsers := 20
 
 	// Setup user IDs
 	userIDs := make([]string, 0, numUsers)
@@ -1139,6 +1139,7 @@ func TestCompactor_ShouldCompactOnlyUsersOwnedByTheInstanceOnShardingEnabledAndM
 		cfg.ShardingRing.InstanceAddr = fmt.Sprintf("127.0.0.%d", i)
 		cfg.ShardingRing.WaitStabilityMinDuration = time.Second
 		cfg.ShardingRing.WaitStabilityMaxDuration = 5 * time.Second
+		cfg.ShardingRing.WaitActiveInstanceTimeout = 30 * time.Second // Give the ring ACTIVE wait headroom on slow CI (#7503).
 		cfg.ShardingRing.KVStore.Mock = kvstore
 
 		c, _, tsdbPlanner, l, _ := prepare(t, cfg, bucketClient, nil)