Fix ReplicaLabelRemover panic when replicaLabels are not specified

CHANGELOG.md

@@ -19,6 +19,7 @@ We use *breaking* word for marking changes that are not backward compatible (rel
 - [#2866](https://github.com/thanos-io/thanos/pull/2866) Receive, Querier: Fixed leaks on receive and querier Store API Series, which were leaking on errors.
 - [#2895](https://github.com/thanos-io/thanos/pull/2895) Compact: Fix increment of `thanos_compact_downsample_total` metric for downsample of 5m resolution blocks.
 - [#2858](https://github.com/thanos-io/thanos/pull/2858) Store: Fix `--store.grpc.series-sample-limit` implementation. The limit is now applied to the sum of all samples fetched across all queried blocks via a single Series call, instead of applying it individually to each block.
+- [#2936](https://github.com/thanos-io/thanos/pull/2936) Compact: Fix ReplicaLabelRemover panic when replicaLabels are not specified.
 
 ### Added
 

pkg/block/fetcher.go

@@ -683,6 +683,10 @@ func NewReplicaLabelRemover(logger log.Logger, replicaLabels []string) *ReplicaL
 
 // Modify modifies external labels of existing blocks, it removes given replica labels from the metadata of blocks that have it.
 func (r *ReplicaLabelRemover) Modify(_ context.Context, metas map[ulid.ULID]*metadata.Meta, modified *extprom.TxGaugeVec) error {
+	if len(r.replicaLabels) == 0 {
+		return nil
+	}
+
 	for u, meta := range metas {
 		l := meta.Thanos.Labels
 		for _, replicaLabel := range r.replicaLabels {

pkg/block/fetcher_test.go

@@ -878,13 +878,13 @@ func TestDeduplicateFilter_Filter(t *testing.T) {
 func TestReplicaLabelRemover_Modify(t *testing.T) {
 	ctx, cancel := context.WithTimeout(context.Background(), 120*time.Second)
 	defer cancel()
-	rm := NewReplicaLabelRemover(log.NewNopLogger(), []string{"replica", "rule_replica"})
 
 	for _, tcase := range []struct {
-		name     string
-		input    map[ulid.ULID]*metadata.Meta
-		expected map[ulid.ULID]*metadata.Meta
-		modified float64
+		name                string
+		input               map[ulid.ULID]*metadata.Meta
+		expected            map[ulid.ULID]*metadata.Meta
+		modified            float64
+		replicaLabelRemover *ReplicaLabelRemover
 	}{
 		{
 			name: "without replica labels",
@@ -898,7 +898,8 @@ func TestReplicaLabelRemover_Modify(t *testing.T) {
 				ULID(2): {Thanos: metadata.Thanos{Labels: map[string]string{"message": "something"}}},
 				ULID(3): {Thanos: metadata.Thanos{Labels: map[string]string{"message": "something1"}}},
 			},
-			modified: 0,
+			modified:            0,
+			replicaLabelRemover: NewReplicaLabelRemover(log.NewNopLogger(), []string{"replica", "rule_replica"}),
 		},
 		{
 			name: "with replica labels",
@@ -914,11 +915,27 @@ func TestReplicaLabelRemover_Modify(t *testing.T) {
 				ULID(3): {Thanos: metadata.Thanos{Labels: map[string]string{"message": "something"}}},
 				ULID(4): {Thanos: metadata.Thanos{Labels: map[string]string{"replica": "deduped"}}},
 			},
-			modified: 5.0,
+			modified:            5.0,
+			replicaLabelRemover: NewReplicaLabelRemover(log.NewNopLogger(), []string{"replica", "rule_replica"}),
+		},
+		{
+			name: "no replica label specified in the ReplicaLabelRemover",
+			input: map[ulid.ULID]*metadata.Meta{
+				ULID(1): {Thanos: metadata.Thanos{Labels: map[string]string{"message": "something"}}},
+				ULID(2): {Thanos: metadata.Thanos{Labels: map[string]string{"message": "something"}}},
+				ULID(3): {Thanos: metadata.Thanos{Labels: map[string]string{"message": "something1"}}},
+			},
+			expected: map[ulid.ULID]*metadata.Meta{
+				ULID(1): {Thanos: metadata.Thanos{Labels: map[string]string{"message": "something"}}},
+				ULID(2): {Thanos: metadata.Thanos{Labels: map[string]string{"message": "something"}}},
+				ULID(3): {Thanos: metadata.Thanos{Labels: map[string]string{"message": "something1"}}},
+			},
+			modified:            0,
+			replicaLabelRemover: NewReplicaLabelRemover(log.NewNopLogger(), []string{}),
 		},
 	} {
 		m := newTestFetcherMetrics()
-		testutil.Ok(t, rm.Modify(ctx, tcase.input, m.modified))
+		testutil.Ok(t, tcase.replicaLabelRemover.Modify(ctx, tcase.input, m.modified))
 
 		testutil.Equals(t, tcase.modified, promtest.ToFloat64(m.modified.WithLabelValues(replicaRemovedMeta)))
 		testutil.Equals(t, tcase.expected, tcase.input)