Pass limit param as hint to storage.Querier

Signed-off-by: 🌲 Harry 🌊 John 🏔 <johrry@amazon.com>

docs/querying/api.md

@@ -256,7 +256,7 @@ URL query parameters:
   series to return. At least one `match[]` argument must be provided.
 - `start=<rfc3339 | unix_timestamp>`: Start timestamp.
 - `end=<rfc3339 | unix_timestamp>`: End timestamp.
-- `limit=<number>`: Maximum number of returned series. Optional.
+- `limit=<number>`: Maximum number of returned series. Optional. 0 means disabled.
 
 You can URL-encode these parameters directly in the request body by using the `POST` method and
 `Content-Type: application/x-www-form-urlencoded` header. This is useful when specifying a large
@@ -307,7 +307,7 @@ URL query parameters:
 - `end=<rfc3339 | unix_timestamp>`: End timestamp. Optional.
 - `match[]=<series_selector>`: Repeated series selector argument that selects the
   series from which to read the label names. Optional.
-- `limit=<number>`: Maximum number of returned series. Optional.
+- `limit=<number>`: Maximum number of returned series. Optional. 0 means disabled.
 
 
 The `data` section of the JSON response is a list of string label names.
@@ -358,7 +358,7 @@ URL query parameters:
 - `end=<rfc3339 | unix_timestamp>`: End timestamp. Optional.
 - `match[]=<series_selector>`: Repeated series selector argument that selects the
   series from which to read the label values. Optional.
-- `limit=<number>`: Maximum number of returned series. Optional.
+- `limit=<number>`: Maximum number of returned series. Optional. 0 means disabled.
 
 
 The `data` section of the JSON response is a list of string label values.

promql/engine_test.go

@@ -236,11 +236,11 @@ func (q *errQuerier) Select(context.Context, bool, *storage.SelectHints, ...*lab
 	return errSeriesSet{err: q.err}
 }
 
-func (*errQuerier) LabelValues(context.Context, string, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (*errQuerier) LabelValues(context.Context, string, *storage.LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	return nil, nil, nil
 }
 
-func (*errQuerier) LabelNames(context.Context, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (*errQuerier) LabelNames(context.Context, *storage.LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	return nil, nil, nil
 }
 func (*errQuerier) Close() error { return nil }

storage/fanout_test.go

@@ -236,11 +236,11 @@ func (errQuerier) Select(context.Context, bool, *storage.SelectHints, ...*labels
 	return storage.ErrSeriesSet(errSelect)
 }
 
-func (errQuerier) LabelValues(context.Context, string, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (errQuerier) LabelValues(context.Context, string, *storage.LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	return nil, nil, errors.New("label values error")
 }
 
-func (errQuerier) LabelNames(context.Context, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (errQuerier) LabelNames(context.Context, *storage.LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	return nil, nil, errors.New("label names error")
 }
 

storage/interface.go

@@ -122,11 +122,11 @@ type MockQuerier struct {
 	SelectMockFunction func(sortSeries bool, hints *SelectHints, matchers ...*labels.Matcher) SeriesSet
 }
 
-func (q *MockQuerier) LabelValues(context.Context, string, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (q *MockQuerier) LabelValues(context.Context, string, *LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	return nil, nil, nil
 }
 
-func (q *MockQuerier) LabelNames(context.Context, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (q *MockQuerier) LabelNames(context.Context, *LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	return nil, nil, nil
 }
 
@@ -161,12 +161,12 @@ type LabelQuerier interface {
 	// It is not safe to use the strings beyond the lifetime of the querier.
 	// If matchers are specified the returned result set is reduced
 	// to label values of metrics matching the matchers.
-	LabelValues(ctx context.Context, name string, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error)
+	LabelValues(ctx context.Context, name string, hints *LabelHints, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error)
 
 	// LabelNames returns all the unique label names present in the block in sorted order.
 	// If matchers are specified the returned result set is reduced
 	// to label names of metrics matching the matchers.
-	LabelNames(ctx context.Context, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error)
+	LabelNames(ctx context.Context, hints *LabelHints, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error)
 
 	// Close releases the resources of the Querier.
 	Close() error
@@ -190,6 +190,9 @@ type SelectHints struct {
 	Start int64 // Start time in milliseconds for this select.
 	End   int64 // End time in milliseconds for this select.
 
+	// Maximum number of results returned. Use a value of 0 to disable.
+	Limit int
+
 	Step int64  // Query step size in milliseconds.
 	Func string // String representation of surrounding function or aggregation.
 
@@ -217,6 +220,13 @@ type SelectHints struct {
 	DisableTrimming bool
 }
 
+// LabelHints specifies hints passed for label reads.
+// This is used only as an option for implementation to use.
+type LabelHints struct {
+	// Maximum number of results returned. Use a value of 0 to disable.
+	Limit int
+}
+
 // TODO(bwplotka): Move to promql/engine_test.go?
 // QueryableFunc is an adapter to allow the use of ordinary functions as
 // Queryables. It follows the idea of http.HandlerFunc.

storage/merge.go

@@ -161,8 +161,8 @@ func (l labelGenericQueriers) SplitByHalf() (labelGenericQueriers, labelGenericQ
 // LabelValues returns all potential values for a label name.
 // If matchers are specified the returned result set is reduced
 // to label values of metrics matching the matchers.
-func (q *mergeGenericQuerier) LabelValues(ctx context.Context, name string, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (q *mergeGenericQuerier) LabelValues(ctx context.Context, name string, hints *LabelHints, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
-	res, ws, err := q.lvals(ctx, q.queriers, name, matchers...)
+	res, ws, err := q.lvals(ctx, q.queriers, name, hints, matchers...)
 	if err != nil {
 		return nil, nil, fmt.Errorf("LabelValues() from merge generic querier for label %s: %w", name, err)
 	}
@@ -170,22 +170,22 @@ func (q *mergeGenericQuerier) LabelValues(ctx context.Context, name string, matc
 }
 
 // lvals performs merge sort for LabelValues from multiple queriers.
-func (q *mergeGenericQuerier) lvals(ctx context.Context, lq labelGenericQueriers, n string, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (q *mergeGenericQuerier) lvals(ctx context.Context, lq labelGenericQueriers, n string, hints *LabelHints, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	if lq.Len() == 0 {
 		return nil, nil, nil
 	}
 	if lq.Len() == 1 {
-		return lq.Get(0).LabelValues(ctx, n, matchers...)
+		return lq.Get(0).LabelValues(ctx, n, hints, matchers...)
 	}
 	a, b := lq.SplitByHalf()
 
 	var ws annotations.Annotations
-	s1, w, err := q.lvals(ctx, a, n, matchers...)
+	s1, w, err := q.lvals(ctx, a, n, hints, matchers...)
 	ws.Merge(w)
 	if err != nil {
 		return nil, ws, err
 	}
-	s2, ws, err := q.lvals(ctx, b, n, matchers...)
+	s2, ws, err := q.lvals(ctx, b, n, hints, matchers...)
 	ws.Merge(w)
 	if err != nil {
 		return nil, ws, err
@@ -221,13 +221,13 @@ func mergeStrings(a, b []string) []string {
 }
 
 // LabelNames returns all the unique label names present in all queriers in sorted order.
-func (q *mergeGenericQuerier) LabelNames(ctx context.Context, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (q *mergeGenericQuerier) LabelNames(ctx context.Context, hints *LabelHints, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	var (
 		labelNamesMap = make(map[string]struct{})
 		warnings      annotations.Annotations
 	)
 	for _, querier := range q.queriers {
-		names, wrn, err := querier.LabelNames(ctx, matchers...)
+		names, wrn, err := querier.LabelNames(ctx, hints, matchers...)
 		if wrn != nil {
 			// TODO(bwplotka): We could potentially wrap warnings.
 			warnings.Merge(wrn)

storage/merge_test.go

@@ -1361,7 +1361,7 @@ func (m *mockGenericQuerier) Select(_ context.Context, b bool, _ *SelectHints, _
 	return &mockGenericSeriesSet{resp: m.resp, warnings: m.warnings, err: m.err}
 }
 
-func (m *mockGenericQuerier) LabelValues(_ context.Context, name string, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (m *mockGenericQuerier) LabelValues(_ context.Context, name string, hints *LabelHints, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	m.mtx.Lock()
 	m.labelNamesRequested = append(m.labelNamesRequested, labelNameRequest{
 		name:     name,
@@ -1371,7 +1371,7 @@ func (m *mockGenericQuerier) LabelValues(_ context.Context, name string, matcher
 	return m.resp, m.warnings, m.err
 }
 
-func (m *mockGenericQuerier) LabelNames(context.Context, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (m *mockGenericQuerier) LabelNames(context.Context, *LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	m.mtx.Lock()
 	m.labelNamesCalls++
 	m.mtx.Unlock()
@@ -1560,7 +1560,7 @@ func TestMergeGenericQuerierWithSecondaries_ErrorHandling(t *testing.T) {
 				}
 			})
 			t.Run("LabelNames", func(t *testing.T) {
-				res, w, err := q.LabelNames(ctx)
+				res, w, err := q.LabelNames(ctx, nil)
 				require.Subset(t, tcase.expectedWarnings, w)
 				require.ErrorIs(t, err, tcase.expectedErrs[1], "expected error doesn't match")
 				require.Equal(t, tcase.expectedLabels, res)
@@ -1575,7 +1575,7 @@ func TestMergeGenericQuerierWithSecondaries_ErrorHandling(t *testing.T) {
 				}
 			})
 			t.Run("LabelValues", func(t *testing.T) {
-				res, w, err := q.LabelValues(ctx, "test")
+				res, w, err := q.LabelValues(ctx, "test", nil)
 				require.Subset(t, tcase.expectedWarnings, w)
 				require.ErrorIs(t, err, tcase.expectedErrs[2], "expected error doesn't match")
 				require.Equal(t, tcase.expectedLabels, res)
@@ -1591,7 +1591,7 @@ func TestMergeGenericQuerierWithSecondaries_ErrorHandling(t *testing.T) {
 			})
 			t.Run("LabelValuesWithMatchers", func(t *testing.T) {
 				matcher := labels.MustNewMatcher(labels.MatchEqual, "otherLabel", "someValue")
-				res, w, err := q.LabelValues(ctx, "test2", matcher)
+				res, w, err := q.LabelValues(ctx, "test2", nil, matcher)
 				require.Subset(t, tcase.expectedWarnings, w)
 				require.ErrorIs(t, err, tcase.expectedErrs[3], "expected error doesn't match")
 				require.Equal(t, tcase.expectedLabels, res)

storage/noop.go

@@ -31,11 +31,11 @@ func (noopQuerier) Select(context.Context, bool, *SelectHints, ...*labels.Matche
 	return NoopSeriesSet()
 }
 
-func (noopQuerier) LabelValues(context.Context, string, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (noopQuerier) LabelValues(context.Context, string, *LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	return nil, nil, nil
 }
 
-func (noopQuerier) LabelNames(context.Context, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (noopQuerier) LabelNames(context.Context, *LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	return nil, nil, nil
 }
 
@@ -54,11 +54,11 @@ func (noopChunkQuerier) Select(context.Context, bool, *SelectHints, ...*labels.M
 	return NoopChunkedSeriesSet()
 }
 
-func (noopChunkQuerier) LabelValues(context.Context, string, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (noopChunkQuerier) LabelValues(context.Context, string, *LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	return nil, nil, nil
 }
 
-func (noopChunkQuerier) LabelNames(context.Context, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (noopChunkQuerier) LabelNames(context.Context, *LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	return nil, nil, nil
 }
 

storage/remote/read.go

@@ -210,13 +210,13 @@ func (q querier) addExternalLabels(ms []*labels.Matcher) ([]*labels.Matcher, []s
 }
 
 // LabelValues implements storage.Querier and is a noop.
-func (q *querier) LabelValues(context.Context, string, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (q *querier) LabelValues(context.Context, string, *storage.LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	// TODO: Implement: https://github.com/prometheus/prometheus/issues/3351
 	return nil, nil, errors.New("not implemented")
 }
 
 // LabelNames implements storage.Querier and is a noop.
-func (q *querier) LabelNames(context.Context, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (q *querier) LabelNames(context.Context, *storage.LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	// TODO: Implement: https://github.com/prometheus/prometheus/issues/3351
 	return nil, nil, errors.New("not implemented")
 }

storage/secondary.go

@@ -49,16 +49,16 @@ func newSecondaryQuerierFromChunk(cq ChunkQuerier) genericQuerier {
 	return &secondaryQuerier{genericQuerier: newGenericQuerierFromChunk(cq)}
 }
 
-func (s *secondaryQuerier) LabelValues(ctx context.Context, name string, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (s *secondaryQuerier) LabelValues(ctx context.Context, name string, hints *LabelHints, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
-	vals, w, err := s.genericQuerier.LabelValues(ctx, name, matchers...)
+	vals, w, err := s.genericQuerier.LabelValues(ctx, name, hints, matchers...)
 	if err != nil {
 		return nil, w.Add(err), nil
 	}
 	return vals, w, nil
 }
 
-func (s *secondaryQuerier) LabelNames(ctx context.Context, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (s *secondaryQuerier) LabelNames(ctx context.Context, hints *LabelHints, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
-	names, w, err := s.genericQuerier.LabelNames(ctx, matchers...)
+	names, w, err := s.genericQuerier.LabelNames(ctx, hints, matchers...)
 	if err != nil {
 		return nil, w.Add(err), nil
 	}

tsdb/db_test.go

@@ -1001,7 +1001,7 @@ func TestWALFlushedOnDBClose(t *testing.T) {
 	q, err := db.Querier(0, 1)
 	require.NoError(t, err)
 
-	values, ws, err := q.LabelValues(ctx, "labelname")
+	values, ws, err := q.LabelValues(ctx, "labelname", nil)
 	require.NoError(t, err)
 	require.Empty(t, ws)
 	require.Equal(t, []string{"labelvalue"}, values)
@@ -1976,7 +1976,7 @@ func TestQuerierWithBoundaryChunks(t *testing.T) {
 	defer q.Close()
 
 	// The requested interval covers 2 blocks, so the querier's label values for blockID should give us 2 values, one from each block.
-	b, ws, err := q.LabelValues(ctx, "blockID")
+	b, ws, err := q.LabelValues(ctx, "blockID", nil)
 	require.NoError(t, err)
 	var nilAnnotations annotations.Annotations
 	require.Equal(t, nilAnnotations, ws)
@@ -2288,7 +2288,7 @@ func TestDB_LabelNames(t *testing.T) {
 		q, err := db.Querier(math.MinInt64, math.MaxInt64)
 		require.NoError(t, err)
 		var ws annotations.Annotations
-		labelNames, ws, err = q.LabelNames(ctx)
+		labelNames, ws, err = q.LabelNames(ctx, nil)
 		require.NoError(t, err)
 		require.Empty(t, ws)
 		require.NoError(t, q.Close())

tsdb/querier.go

@@ -77,12 +77,12 @@ func newBlockBaseQuerier(b BlockReader, mint, maxt int64) (*blockBaseQuerier, er
 	}, nil
 }
 
-func (q *blockBaseQuerier) LabelValues(ctx context.Context, name string, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (q *blockBaseQuerier) LabelValues(ctx context.Context, name string, hints *storage.LabelHints, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	res, err := q.index.SortedLabelValues(ctx, name, matchers...)
 	return res, nil, err
 }
 
-func (q *blockBaseQuerier) LabelNames(ctx context.Context, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (q *blockBaseQuerier) LabelNames(ctx context.Context, hints *storage.LabelHints, matchers ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	res, err := q.index.LabelNames(ctx, matchers...)
 	return res, nil, err
 }

tsdb/querier_test.go

@@ -3022,7 +3022,7 @@ func TestQuerierIndexQueriesRace(t *testing.T) {
 				q, err := db.Querier(math.MinInt64, math.MaxInt64)
 				require.NoError(t, err)
 
-				values, _, err := q.LabelValues(ctx, "seq", c.matchers...)
+				values, _, err := q.LabelValues(ctx, "seq", nil, c.matchers...)
 				require.NoError(t, err)
 				require.Emptyf(t, values, `label values for label "seq" should be empty`)
 

web/api/v1/api.go

@@ -658,6 +658,10 @@ func (api *API) labelNames(r *http.Request) apiFuncResult {
 		return apiFuncResult{nil, &apiError{errorBadData, err}, nil, nil}
 	}
 
+	hints := &storage.LabelHints{
+		Limit: toHintLimit(limit),
+	}
+
 	q, err := api.Queryable.Querier(timestamp.FromTime(start), timestamp.FromTime(end))
 	if err != nil {
 		return apiFuncResult{nil, returnAPIError(err), nil, nil}
@@ -672,7 +676,7 @@ func (api *API) labelNames(r *http.Request) apiFuncResult {
 		labelNamesSet := make(map[string]struct{})
 
 		for _, matchers := range matcherSets {
-			vals, callWarnings, err := q.LabelNames(r.Context(), matchers...)
+			vals, callWarnings, err := q.LabelNames(r.Context(), hints, matchers...)
 			if err != nil {
 				return apiFuncResult{nil, returnAPIError(err), warnings, nil}
 			}
@@ -694,7 +698,7 @@ func (api *API) labelNames(r *http.Request) apiFuncResult {
 		if len(matcherSets) == 1 {
 			matchers = matcherSets[0]
 		}
-		names, warnings, err = q.LabelNames(r.Context(), matchers...)
+		names, warnings, err = q.LabelNames(r.Context(), hints, matchers...)
 		if err != nil {
 			return apiFuncResult{nil, &apiError{errorExec, err}, warnings, nil}
 		}
@@ -704,7 +708,7 @@ func (api *API) labelNames(r *http.Request) apiFuncResult {
 		names = []string{}
 	}
 
-	if len(names) > limit {
+	if limit > 0 && len(names) > limit {
 		names = names[:limit]
 		warnings = warnings.Add(errors.New("results truncated due to limit"))
 	}
@@ -738,6 +742,10 @@ func (api *API) labelValues(r *http.Request) (result apiFuncResult) {
 		return apiFuncResult{nil, &apiError{errorBadData, err}, nil, nil}
 	}
 
+	hints := &storage.LabelHints{
+		Limit: toHintLimit(limit),
+	}
+
 	q, err := api.Queryable.Querier(timestamp.FromTime(start), timestamp.FromTime(end))
 	if err != nil {
 		return apiFuncResult{nil, &apiError{errorExec, err}, nil, nil}
@@ -762,7 +770,7 @@ func (api *API) labelValues(r *http.Request) (result apiFuncResult) {
 		var callWarnings annotations.Annotations
 		labelValuesSet := make(map[string]struct{})
 		for _, matchers := range matcherSets {
-			vals, callWarnings, err = q.LabelValues(ctx, name, matchers...)
+			vals, callWarnings, err = q.LabelValues(ctx, name, hints, matchers...)
 			if err != nil {
 				return apiFuncResult{nil, &apiError{errorExec, err}, warnings, closer}
 			}
@@ -781,7 +789,7 @@ func (api *API) labelValues(r *http.Request) (result apiFuncResult) {
 		if len(matcherSets) == 1 {
 			matchers = matcherSets[0]
 		}
-		vals, warnings, err = q.LabelValues(ctx, name, matchers...)
+		vals, warnings, err = q.LabelValues(ctx, name, hints, matchers...)
 		if err != nil {
 			return apiFuncResult{nil, &apiError{errorExec, err}, warnings, closer}
 		}
@@ -793,7 +801,7 @@ func (api *API) labelValues(r *http.Request) (result apiFuncResult) {
 
 	slices.Sort(vals)
 
-	if len(vals) > limit {
+	if limit > 0 && len(vals) > limit {
 		vals = vals[:limit]
 		warnings = warnings.Add(errors.New("results truncated due to limit"))
 	}
@@ -863,6 +871,7 @@ func (api *API) series(r *http.Request) (result apiFuncResult) {
 		Start: timestamp.FromTime(start),
 		End:   timestamp.FromTime(end),
 		Func:  "series", // There is no series function, this token is used for lookups that don't need samples.
+		Limit: toHintLimit(limit),
 	}
 	var set storage.SeriesSet
 
@@ -889,7 +898,7 @@ func (api *API) series(r *http.Request) (result apiFuncResult) {
 		}
 		metrics = append(metrics, set.At().Labels())
 
-		if len(metrics) > limit {
+		if limit > 0 && len(metrics) > limit {
 			metrics = metrics[:limit]
 			warnings.Add(errors.New("results truncated due to limit"))
 			return apiFuncResult{metrics, nil, warnings, closer}
@@ -1898,8 +1907,8 @@ OUTER:
 	return matcherSets, nil
 }
 
+// parseLimitParam returning 0 means no limit is to be applied.
 func parseLimitParam(limitStr string) (limit int, err error) {
-	limit = math.MaxInt
 	if limitStr == "" {
 		return limit, nil
 	}
@@ -1908,9 +1917,19 @@ func parseLimitParam(limitStr string) (limit int, err error) {
 	if err != nil {
 		return limit, err
 	}
-	if limit <= 0 {
+	if limit < 0 {
-		return limit, errors.New("limit must be positive")
+		return limit, errors.New("limit must be non-negative")
 	}
 
 	return limit, nil
 }
+
+// toHintLimit increases the API limit, as returned by parseLimitParam, by 1.
+// This allows for emitting warnings when the results are truncated.
+func toHintLimit(limit int) int {
+	// 0 means no limit and avoid int overflow
+	if limit > 0 && limit < math.MaxInt {
+		return limit + 1
+	}
+	return limit
+}

web/api/v1/api_test.go

@@ -739,13 +739,16 @@ func TestLabelNames(t *testing.T) {
 	api := &API{
 		Queryable: storage,
 	}
-	request := func(method string, matchers ...string) (*http.Request, error) {
+	request := func(method, limit string, matchers ...string) (*http.Request, error) {
 		u, err := url.Parse("http://example.com")
 		require.NoError(t, err)
 		q := u.Query()
 		for _, matcher := range matchers {
 			q.Add("match[]", matcher)
 		}
+		if limit != "" {
+			q.Add("limit", limit)
+		}
 		u.RawQuery = q.Encode()
 
 		r, err := http.NewRequest(method, u.String(), nil)
@@ -759,6 +762,7 @@ func TestLabelNames(t *testing.T) {
 		name              string
 		api               *API
 		matchers          []string
+		limit             string
 		expected          []string
 		expectedErrorType errorType
 	}{
@@ -773,6 +777,13 @@ func TestLabelNames(t *testing.T) {
 			expected: []string{"__name__", "abc", "foo", "xyz"},
 			api:      api,
 		},
+		{
+			name:     "non empty label matcher with limit",
+			matchers: []string{`{foo=~".+"}`},
+			expected: []string{"__name__", "abc"},
+			limit:    "2",
+			api:      api,
+		},
 		{
 			name:     "exact label matcher",
 			matchers: []string{`{foo="boo"}`},
@@ -805,7 +816,7 @@ func TestLabelNames(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) {
 			for _, method := range []string{http.MethodGet, http.MethodPost} {
 				ctx := context.Background()
-				req, err := request(method, tc.matchers...)
+				req, err := request(method, tc.limit, tc.matchers...)
 				require.NoError(t, err)
 				res := tc.api.labelNames(req.WithContext(ctx))
 				assertAPIError(t, res.err, tc.expectedErrorType)
@@ -1430,6 +1441,15 @@ func testEndpoints(t *testing.T, api *API, tr *testTargetRetriever, es storage.E
 			responseLen:   2, // API does not specify which particular value will come back.
 			warningsCount: 0, // No warnings if limit isn't exceeded.
 		},
+		{
+			endpoint: api.series,
+			query: url.Values{
+				"match[]": []string{"test_metric1"},
+				"limit":   []string{"0"},
+			},
+			responseLen:   2, // API does not specify which particular value will come back.
+			warningsCount: 0, // No warnings if limit isn't exceeded.
+		},
 		// Missing match[] query params in series requests.
 		{
 			endpoint: api.series,

web/api/v1/errors_test.go

@@ -170,11 +170,11 @@ type errorTestQuerier struct {
 	err error
 }
 
-func (t errorTestQuerier) LabelValues(context.Context, string, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (t errorTestQuerier) LabelValues(context.Context, string, *storage.LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	return nil, nil, t.err
 }
 
-func (t errorTestQuerier) LabelNames(context.Context, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
+func (t errorTestQuerier) LabelNames(context.Context, *storage.LabelHints, ...*labels.Matcher) ([]string, annotations.Annotations, error) {
 	return nil, nil, t.err
 }