Fix nil when ingesterQueryMaxAttempts > 1 (#7369)

* Trigger nil with test

Signed-off-by: Friedrich Gonzalez <1517449+friedrichg@users.noreply.github.com>

* Fix nil results

Signed-off-by: Friedrich Gonzalez <1517449+friedrichg@users.noreply.github.com>

* fix changelog

Signed-off-by: Friedrich Gonzalez <1517449+friedrichg@users.noreply.github.com>

---------

Signed-off-by: Friedrich Gonzalez <1517449+friedrichg@users.noreply.github.com>

Author: friedrichg

CHANGELOG.md

@@ -5,6 +5,7 @@
 ## 1.21.0 in progress
 
 * [BUGFIX] KV store: Fix false-positive `status_code="500"` metrics for HA tracker CAS operations when using memberlist. #7408
+* [BUGFIX] Fix nil when ingester_query_max_attempts > 1. #7369
 
 * [CHANGE] Ruler: Graduate Ruler API from experimental. #7312
   * Flag: Renamed `-experimental.ruler.enable-api` to `-ruler.enable-api`. The old flag is kept as deprecated.

pkg/querier/distributor_queryable.go

@@ -166,6 +166,16 @@ func (q *distributorQuerier) streamingSelect(ctx context.Context, sortSeries, pa
 		return storage.ErrSeriesSet(err)
 	}
 
+	// Guard against nil results. This can happen when queryWithRetry's
+	// backoff loop exits without executing (e.g., context cancelled before
+	// the first attempt), returning (nil, nil). See #7364.
+	if results == nil {
+		if err != nil {
+			return storage.ErrSeriesSet(err)
+		}
+		return storage.EmptySeriesSet()
+	}
+
 	serieses := make([]storage.Series, 0, len(results.Chunkseries))
 	for _, result := range results.Chunkseries {
 		// Sometimes the ingester can send series that have no data.

pkg/querier/distributor_queryable_test.go

@@ -387,6 +387,70 @@ func TestDistributorQuerier_Retry(t *testing.T) {
 	}
 }
 
+// TestDistributorQuerier_Select_CancelledContext_NoRetry verifies that with
+// ingesterQueryMaxAttempts=1, a cancelled context does not panic because the
+// direct code path (no retry loop) is used.
+func TestDistributorQuerier_Select_CancelledContext_NoRetry(t *testing.T) {
+	t.Parallel()
+
+	ctx := user.InjectOrgID(context.Background(), "0")
+	ctx, cancel := context.WithCancel(ctx)
+	cancel()
+
+	d := &MockDistributor{}
+	d.On("QueryStream", mock.Anything, mock.Anything, mock.Anything, mock.Anything).Return(&client.QueryStreamResponse{}, context.Canceled)
+
+	ingesterQueryMaxAttempts := 1
+	queryable := newDistributorQueryable(d, true, true, batch.NewChunkMergeIterator, 0, func(string) bool {
+		return true
+	}, ingesterQueryMaxAttempts)
+	querier, err := queryable.Querier(mint, maxt)
+	require.NoError(t, err)
+
+	require.NotPanics(t, func() {
+		seriesSet := querier.Select(ctx, true, &storage.SelectHints{Start: mint, End: maxt})
+		_ = seriesSet.Err()
+	})
+}
+
+// TestDistributorQuerier_Select_CancelledContext reproduces the panic described
+// in https://github.com/cortexproject/cortex/issues/7364.
+//
+// When ingesterQueryMaxAttempts > 1 and the context is cancelled before the
+// retry loop starts (e.g. query timeout or another querier goroutine failing),
+// backoff.Ongoing() returns false immediately. The result variable stays nil,
+// queryWithRetry returns (nil, nil), and streamingSelect dereferences the nil
+// result at line 169 → panic.
+func TestDistributorQuerier_Select_CancelledContext(t *testing.T) {
+	t.Parallel()
+
+	// Create a context that is already cancelled.
+	ctx := user.InjectOrgID(context.Background(), "0")
+	ctx, cancel := context.WithCancel(ctx)
+	cancel()
+
+	d := &MockDistributor{}
+	// No mock expectations needed — QueryStream should never be called
+	// because the context is already cancelled.
+
+	ingesterQueryMaxAttempts := 2
+	queryable := newDistributorQueryable(d, true, true, batch.NewChunkMergeIterator, 0, func(string) bool {
+		return true
+	}, ingesterQueryMaxAttempts)
+	querier, err := queryable.Querier(mint, maxt)
+	require.NoError(t, err)
+
+	// This should NOT panic. Before the fix, the cancelled context causes
+	// queryWithRetry to return (nil, nil), and streamingSelect dereferences
+	// the nil result: panic: runtime error: invalid memory address or nil
+	// pointer dereference [signal SIGSEGV ... addr=0x8]
+	require.NotPanics(t, func() {
+		seriesSet := querier.Select(ctx, true, &storage.SelectHints{Start: mint, End: maxt})
+		// With a cancelled context, we expect either an error or an empty result.
+		_ = seriesSet.Err()
+	})
+}
+
 func TestDistributorQuerier_LabelNames(t *testing.T) {
 	t.Parallel()