Close per-query querier in the remote read handler

CHANGELOG.md

@@ -88,6 +88,7 @@
 * [BUGFIX] Compactor: Fix spurious `bucket operation fail after retries` error logs emitted during partial block cleanup. #7749
 * [BUGFIX] Alertmanager: Fix panic in `validateAlertmanagerConfig` when receiver config traversal encounters nil interface values. #7751
 * [BUGFIX] Parquet Converter: Fix `auto_forget_delay` having no effect. The ring lifecycler was created without the auto-forget delegate, so unhealthy instances were never automatically removed from the ring. #7752
+* [BUGFIX] Querier: Close the per-query `storage.Querier` in the remote read handler once each sub-query completes. The handler was the only `storage.Querier` call site that did not call `Close()`, so it did not honor the interface contract and would leak any resources a querier releases in `Close()`. #7672
 
 ## 1.21.1 2026-06-04
 

pkg/querier/remote_read.go

@@ -46,6 +46,7 @@ func RemoteReadHandler(q storage.Queryable, logger log.Logger) http.Handler {
 					errors <- err
 					return
 				}
+				defer querier.Close()
 
 				params := &storage.SelectHints{
 					Start: int64(from),

pkg/querier/remote_read_test.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"testing"
+	"time"
 
 	"github.com/go-kit/log"
 	"github.com/gogo/protobuf/proto"
@@ -17,10 +18,12 @@ import (
 	"github.com/prometheus/prometheus/storage"
 	"github.com/prometheus/prometheus/util/annotations"
 	"github.com/stretchr/testify/require"
+	"go.uber.org/atomic"
 
 	"github.com/cortexproject/cortex/pkg/cortexpb"
 	"github.com/cortexproject/cortex/pkg/ingester/client"
 	"github.com/cortexproject/cortex/pkg/querier/series"
+	"github.com/cortexproject/cortex/pkg/util/test"
 )
 
 func TestRemoteReadHandler(t *testing.T) {
@@ -88,6 +91,37 @@ func TestRemoteReadHandler(t *testing.T) {
 	require.Equal(t, expected, response)
 }
 
+func TestRemoteReadHandler_Closes_Querier(t *testing.T) {
+	t.Parallel()
+	var closed atomic.Int64
+	q := storage.QueryableFunc(func(mint, maxt int64) (storage.Querier, error) {
+		return &closeCountingQuerier{closed: &closed}, nil
+	})
+	handler := RemoteReadHandler(q, log.NewNopLogger())
+
+	const numQueries = 3
+	queries := make([]*client.QueryRequest, numQueries)
+	for i := range queries {
+		queries[i] = &client.QueryRequest{StartTimestampMs: 0, EndTimestampMs: 10}
+	}
+	requestBody, err := proto.Marshal(&client.ReadRequest{Queries: queries})
+	require.NoError(t, err)
+	requestBody = snappy.Encode(nil, requestBody)
+	request, err := http.NewRequest("GET", "/query", bytes.NewReader(requestBody))
+	require.NoError(t, err)
+	request.Header.Set("X-Prometheus-Remote-Read-Version", "0.1.0")
+
+	recorder := httptest.NewRecorder()
+	handler.ServeHTTP(recorder, request)
+
+	require.Equal(t, 200, recorder.Result().StatusCode)
+	// Each per-query querier is closed by a deferred call in its own goroutine,
+	// which may run after ServeHTTP returns, so poll until every querier is closed.
+	test.Poll(t, time.Second, int64(numQueries), func() any {
+		return closed.Load()
+	})
+}
+
 type mockQuerier struct {
 	matrix model.Matrix
 }
@@ -110,3 +144,15 @@ func (m mockQuerier) LabelNames(ctx context.Context, _ *storage.LabelHints, matc
 func (mockQuerier) Close() error {
 	return nil
 }
+
+// closeCountingQuerier records how many times Close is called so a test can
+// assert the remote read handler releases every querier it creates.
+type closeCountingQuerier struct {
+	mockQuerier
+	closed *atomic.Int64
+}
+
+func (q *closeCountingQuerier) Close() error {
+	q.closed.Add(1)
+	return nil
+}