Skip stale postings in /active_series (#9580)

* Skip stale postings in `/active_series`

* changelog

# Conflicts:
#	CHANGELOG.md

CHANGELOG.md

@@ -50,6 +50,7 @@
 * [BUGFIX] Fix issue where `metric might not be a counter, name does not end in _total/_sum/_count/_bucket` annotation would be emitted even if `rate` or `increase` did not have enough samples to compute a result. #9508
 * [BUGFIX] Fix issue where sharded queries could return annotations with incorrect or confusing position information. #9536
 * [BUGFIX] Fix issue where downstream consumers may not generate correct cache keys for experimental error caching. #9644
+* [BUGFIX] Fix issue where active series requests error when encountering a stale posting. #9580
 
 ### Mixin
 

pkg/ingester/active_series.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/go-kit/log/level"
 	"github.com/grafana/dskit/tenant"
+	"github.com/pkg/errors"
 	"github.com/prometheus/prometheus/model/labels"
 	"github.com/prometheus/prometheus/storage"
 	"github.com/prometheus/prometheus/tsdb"
@@ -77,6 +78,10 @@ func (i *Ingester) ActiveSeries(request *client.ActiveSeriesRequest, stream clie
 		seriesRef, count := postings.AtBucketCount()
 		err = idx.Series(seriesRef, &buf, nil)
 		if err != nil {
+			// Postings may be stale. Skip if no underlying series exists.
+			if errors.Is(err, storage.ErrNotFound) {
+				continue
+			}
 			return fmt.Errorf("error getting series: %w", err)
 		}
 		m := &mimirpb.Metric{Labels: mimirpb.FromLabelsToLabelAdapters(buf.Labels())}