comment on return value

pkg/distributor/distributor.go

@@ -1832,13 +1832,17 @@ func (d *Distributor) ActiveSeries(ctx context.Context, matchers []*labels.Match
 	res := newActiveSeriesResponse(d.hashCollisionCount, maxResponseSize)
 
 	ingesterQuery := func(ctx context.Context, client ingester_client.IngesterClient) (any, error) {
+		// This function is invoked purely for its side effects on the captured
+		// activeSeriesResponse, its return value is never used.
+		type ignored struct{}
+
 		log, ctx := spanlogger.NewWithLogger(ctx, d.log, "Distributor.ActiveSeries.queryIngester")
 		defer log.Finish()
 
 		stream, err := client.ActiveSeries(ctx, req)
 		if err != nil {
 			if errors.Is(err, context.Canceled) {
-				return nil, nil
+				return ignored{}, nil
 			}
 			level.Error(log).Log("msg", "error creating active series response stream", "err", err)
 			ext.Error.Set(log.Span, true)
@@ -1859,7 +1863,7 @@ func (d *Distributor) ActiveSeries(ctx context.Context, matchers []*labels.Match
 					break
 				}
 				if errors.Is(err, context.Canceled) {
-					return nil, nil
+					return ignored{}, nil
 				}
 				level.Error(log).Log("msg", "error receiving active series response", "err", err)
 				ext.Error.Set(log.Span, true)
@@ -1872,7 +1876,7 @@ func (d *Distributor) ActiveSeries(ctx context.Context, matchers []*labels.Match
 			}
 		}
 
-		return nil, nil
+		return ignored{}, nil
 	}
 
 	_, err = forReplicationSet(ctx, d, replicationSet, ingesterQuery)