fix(nativehistograms): validation should fail on unsupported schemas

Histogram.Validate and FloatHistogram.Validate now return error on
unsupported schemas.

Scrape and remote-write handler reduces the schema to the maximum allowed
if it is above the maximum, but below theoretical maximum of 52.
For scrape the maximum is a configuration option, for remote-write it is 8.

Note: OTLP endpont already does the reduction, without checking that it is
below 52 as the spec does not specify a maximum.

Signed-off-by: György Krajcsovits <gyorgy.krajcsovits@grafana.com>

model/histogram/float_histogram.go

@@ -798,7 +798,8 @@ func (h *FloatHistogram) AllReverseBucketIterator() BucketIterator[float64] {
 // create false positives here.
 func (h *FloatHistogram) Validate() error {
 	var nCount, pCount float64
-	if h.UsesCustomBuckets() {
+	switch {
+	case IsCustomBucketsSchema(h.Schema):
 		if err := checkHistogramCustomBounds(h.CustomValues, h.PositiveSpans, len(h.PositiveBuckets)); err != nil {
 			return fmt.Errorf("custom buckets: %w", err)
 		}
@@ -814,7 +815,7 @@ func (h *FloatHistogram) Validate() error {
 		if len(h.NegativeBuckets) > 0 {
 			return errors.New("custom buckets: must not have negative buckets")
 		}
-	} else {
+	case IsExponentialSchema(h.Schema):
 		if err := checkHistogramSpans(h.PositiveSpans, len(h.PositiveBuckets)); err != nil {
 			return fmt.Errorf("positive side: %w", err)
 		}
@@ -828,6 +829,8 @@ func (h *FloatHistogram) Validate() error {
 		if h.CustomValues != nil {
 			return errors.New("histogram with exponential schema must not have custom bounds")
 		}
+	default:
+		return fmt.Errorf("schema %d: %w", h.Schema, ErrHistogramsInvalidSchema)
 	}
 	err := checkHistogramBuckets(h.PositiveBuckets, &pCount, false)
 	if err != nil {

model/histogram/generic.go

@@ -22,7 +22,9 @@ import (
 
 const (
 	ExponentialSchemaMax         int32 = 8
+	ExponentialSchemaMaxReserved int32 = 52
 	ExponentialSchemaMin         int32 = -4
+	ExponentialSchemaMinReserved int32 = -9
 	CustomBucketsSchema          int32 = -53
 )
 
@@ -37,6 +39,7 @@ var (
 	ErrHistogramCustomBucketsInfinite = errors.New("histogram custom bounds must be finite")
 	ErrHistogramsIncompatibleSchema   = errors.New("cannot apply this operation on histograms with a mix of exponential and custom bucket schemas")
 	ErrHistogramsIncompatibleBounds   = errors.New("cannot apply this operation on custom buckets histograms with different custom bounds")
+	ErrHistogramsInvalidSchema        = errors.New("histogram has an invalid schema, which must be between -4 and 8 for exponential buckets, or -53 for custom buckets")
 )
 
 func IsCustomBucketsSchema(s int32) bool {
@@ -47,6 +50,10 @@ func IsExponentialSchema(s int32) bool {
 	return s >= ExponentialSchemaMin && s <= ExponentialSchemaMax
 }
 
+func IsExponentialSchemaReserved(s int32) bool {
+	return s >= ExponentialSchemaMinReserved && s <= ExponentialSchemaMaxReserved
+}
+
 // BucketCount is a type constraint for the count in a bucket, which can be
 // float64 (for type FloatHistogram) or uint64 (for type Histogram).
 type BucketCount interface {

model/histogram/histogram.go

@@ -425,7 +425,8 @@ func resize[T any](items []T, n int) []T {
 // the total h.Count).
 func (h *Histogram) Validate() error {
 	var nCount, pCount uint64
-	if h.UsesCustomBuckets() {
+	switch {
+	case IsCustomBucketsSchema(h.Schema):
 		if err := checkHistogramCustomBounds(h.CustomValues, h.PositiveSpans, len(h.PositiveBuckets)); err != nil {
 			return fmt.Errorf("custom buckets: %w", err)
 		}
@@ -441,7 +442,7 @@ func (h *Histogram) Validate() error {
 		if len(h.NegativeBuckets) > 0 {
 			return errors.New("custom buckets: must not have negative buckets")
 		}
-	} else {
+	case IsExponentialSchema(h.Schema):
 		if err := checkHistogramSpans(h.PositiveSpans, len(h.PositiveBuckets)); err != nil {
 			return fmt.Errorf("positive side: %w", err)
 		}
@@ -455,6 +456,8 @@ func (h *Histogram) Validate() error {
 		if h.CustomValues != nil {
 			return errors.New("histogram with exponential schema must not have custom bounds")
 		}
+	default:
+		return fmt.Errorf("schema %d: %w", h.Schema, ErrHistogramsInvalidSchema)
 	}
 	err := checkHistogramBuckets(h.PositiveBuckets, &pCount, true)
 	if err != nil {

model/histogram/histogram_test.go

@@ -1565,6 +1565,18 @@ func TestHistogramValidation(t *testing.T) {
 				CustomValues:    []float64{1, 2, 3, 4, 5, 6, 7, 8},
 			},
 		},
+		"schema too high": {
+			h: &Histogram{
+				Schema: 10,
+			},
+			errMsg: `schema 10: histogram has an invalid schema, which must be between -4 and 8 for exponential buckets, or -53 for custom buckets`,
+		},
+		"schema too low": {
+			h: &Histogram{
+				Schema: -10,
+			},
+			errMsg: `schema -10: histogram has an invalid schema, which must be between -4 and 8 for exponential buckets, or -53 for custom buckets`,
+		},
 	}
 
 	for testName, tc := range tests {

scrape/target.go

@@ -414,12 +414,12 @@ type maxSchemaAppender struct {
 
 func (app *maxSchemaAppender) AppendHistogram(ref storage.SeriesRef, lset labels.Labels, t int64, h *histogram.Histogram, fh *histogram.FloatHistogram) (storage.SeriesRef, error) {
 	if h != nil {
-		if histogram.IsExponentialSchema(h.Schema) && h.Schema > app.maxSchema {
+		if histogram.IsExponentialSchemaReserved(h.Schema) && h.Schema > app.maxSchema {
 			h = h.ReduceResolution(app.maxSchema)
 		}
 	}
 	if fh != nil {
-		if histogram.IsExponentialSchema(fh.Schema) && fh.Schema > app.maxSchema {
+		if histogram.IsExponentialSchemaReserved(fh.Schema) && fh.Schema > app.maxSchema {
 			fh = fh.ReduceResolution(app.maxSchema)
 		}
 	}

storage/remote/otlptranslator/prometheusremotewrite/histograms.go

@@ -86,16 +86,16 @@ func (c *PrometheusConverter) addExponentialHistogramDataPoints(ctx context.Cont
 func exponentialToNativeHistogram(p pmetric.ExponentialHistogramDataPoint, temporality pmetric.AggregationTemporality) (*histogram.Histogram, annotations.Annotations, error) {
 	var annots annotations.Annotations
 	scale := p.Scale()
-	if scale < -4 {
+	if scale < histogram.ExponentialSchemaMin {
 		return nil, annots,
 			fmt.Errorf("cannot convert exponential to native histogram."+
-				" Scale must be >= -4, was %d", scale)
+				" Scale must be >= %d, was %d", histogram.ExponentialSchemaMin, scale)
 	}
 
 	var scaleDown int32
-	if scale > 8 {
+	if scale > histogram.ExponentialSchemaMax {
-		scaleDown = scale - 8
+		scaleDown = scale - histogram.ExponentialSchemaMax
-		scale = 8
+		scale = histogram.ExponentialSchemaMax
 	}
 
 	pSpans, pDeltas := convertBucketsLayout(p.Positive().BucketCounts().AsRaw(), p.Positive().Offset(), scaleDown, true)

storage/remote/write_handler.go

@@ -229,7 +229,7 @@ func (h *writeHandler) write(ctx context.Context, req *prompb.WriteRequest) (err
 	samplesWithInvalidLabels := 0
 	samplesAppended := 0
 
-	app := &timeLimitAppender{
+	app := &remoteWriteAppender{
 		Appender: h.appendable.Appender(ctx),
 		maxTime:  timestamp.FromTime(time.Now().Add(maxAheadTime)),
 	}
@@ -344,7 +344,7 @@ func (h *writeHandler) appendV1Histograms(app storage.Appender, hh []prompb.Hist
 // NOTE(bwplotka): TSDB storage is NOT idempotent, so we don't allow "partial retry-able" errors.
 // Once we have 5xx type of error, we immediately stop and rollback all appends.
 func (h *writeHandler) writeV2(ctx context.Context, req *writev2.Request) (_ WriteResponseStats, errHTTPCode int, _ error) {
-	app := &timeLimitAppender{
+	app := &remoteWriteAppender{
 		Appender: h.appendable.Appender(ctx),
 		maxTime:  timestamp.FromTime(time.Now().Add(maxAheadTime)),
 	}
@@ -616,7 +616,7 @@ type rwExporter struct {
 
 func (rw *rwExporter) ConsumeMetrics(ctx context.Context, md pmetric.Metrics) error {
 	otlpCfg := rw.config().OTLPConfig
-	app := &timeLimitAppender{
+	app := &remoteWriteAppender{
 		Appender: rw.appendable.Appender(ctx),
 		maxTime:  timestamp.FromTime(time.Now().Add(maxAheadTime)),
 	}
@@ -719,13 +719,13 @@ func hasDelta(md pmetric.Metrics) bool {
 	return false
 }
 
-type timeLimitAppender struct {
+type remoteWriteAppender struct {
 	storage.Appender
 
 	maxTime int64
 }
 
-func (app *timeLimitAppender) Append(ref storage.SeriesRef, lset labels.Labels, t int64, v float64) (storage.SeriesRef, error) {
+func (app *remoteWriteAppender) Append(ref storage.SeriesRef, lset labels.Labels, t int64, v float64) (storage.SeriesRef, error) {
 	if t > app.maxTime {
 		return 0, fmt.Errorf("%w: timestamp is too far in the future", storage.ErrOutOfBounds)
 	}
@@ -737,11 +737,18 @@ func (app *timeLimitAppender) Append(ref storage.SeriesRef, lset labels.Labels,
 	return ref, nil
 }
 
-func (app *timeLimitAppender) AppendHistogram(ref storage.SeriesRef, l labels.Labels, t int64, h *histogram.Histogram, fh *histogram.FloatHistogram) (storage.SeriesRef, error) {
+func (app *remoteWriteAppender) AppendHistogram(ref storage.SeriesRef, l labels.Labels, t int64, h *histogram.Histogram, fh *histogram.FloatHistogram) (storage.SeriesRef, error) {
 	if t > app.maxTime {
 		return 0, fmt.Errorf("%w: timestamp is too far in the future", storage.ErrOutOfBounds)
 	}
 
+	if h != nil && histogram.IsExponentialSchemaReserved(h.Schema) && h.Schema > histogram.ExponentialSchemaMax {
+		h = h.ReduceResolution(histogram.ExponentialSchemaMax)
+	}
+	if fh != nil && histogram.IsExponentialSchemaReserved(fh.Schema) && fh.Schema > histogram.ExponentialSchemaMax {
+		fh = fh.ReduceResolution(histogram.ExponentialSchemaMax)
+	}
+
 	ref, err := app.Appender.AppendHistogram(ref, l, t, h, fh)
 	if err != nil {
 		return 0, err
@@ -749,7 +756,7 @@ func (app *timeLimitAppender) AppendHistogram(ref storage.SeriesRef, l labels.La
 	return ref, nil
 }
 
-func (app *timeLimitAppender) AppendExemplar(ref storage.SeriesRef, l labels.Labels, e exemplar.Exemplar) (storage.SeriesRef, error) {
+func (app *remoteWriteAppender) AppendExemplar(ref storage.SeriesRef, l labels.Labels, e exemplar.Exemplar) (storage.SeriesRef, error) {
 	if e.Ts > app.maxTime {
 		return 0, fmt.Errorf("%w: timestamp is too far in the future", storage.ErrOutOfBounds)
 	}

storage/remote/write_handler_test.go

@@ -1134,3 +1134,100 @@ func (m *mockAppendable) AppendCTZeroSample(_ storage.SeriesRef, l labels.Labels
 	m.samples = append(m.samples, mockSample{l, ct, 0})
 	return storage.SeriesRef(hash), nil
 }
+
+var (
+	highSchemaHistogram = &histogram.Histogram{
+		Schema: 10,
+		PositiveSpans: []histogram.Span{
+			{
+				Offset: -1,
+				Length: 2,
+			},
+		},
+		PositiveBuckets: []int64{1, 2},
+		NegativeSpans: []histogram.Span{
+			{
+				Offset: 0,
+				Length: 1,
+			},
+		},
+		NegativeBuckets: []int64{1},
+	}
+	reducedSchemaHistogram = &histogram.Histogram{
+		Schema: 8,
+		PositiveSpans: []histogram.Span{
+			{
+				Offset: 0,
+				Length: 1,
+			},
+		},
+		PositiveBuckets: []int64{4},
+		NegativeSpans: []histogram.Span{
+			{
+				Offset: 0,
+				Length: 1,
+			},
+		},
+		NegativeBuckets: []int64{1},
+	}
+)
+
+func TestHistogramsReduction(t *testing.T) {
+	for _, protoMsg := range []config.RemoteWriteProtoMsg{config.RemoteWriteProtoMsgV1, config.RemoteWriteProtoMsgV2} {
+		t.Run(string(protoMsg), func(t *testing.T) {
+			appendable := &mockAppendable{}
+			handler := NewWriteHandler(promslog.NewNopLogger(), nil, appendable, []config.RemoteWriteProtoMsg{protoMsg}, false)
+
+			var (
+				err     error
+				payload []byte
+			)
+
+			if protoMsg == config.RemoteWriteProtoMsgV1 {
+				payload, _, _, err = buildWriteRequest(nil, []prompb.TimeSeries{
+					{
+						Labels:     []prompb.Label{{Name: "__name__", Value: "test_metric1"}},
+						Histograms: []prompb.Histogram{prompb.FromIntHistogram(1, highSchemaHistogram)},
+					},
+					{
+						Labels:     []prompb.Label{{Name: "__name__", Value: "test_metric2"}},
+						Histograms: []prompb.Histogram{prompb.FromFloatHistogram(2, highSchemaHistogram.ToFloat(nil))},
+					},
+				}, nil, nil, nil, nil, "snappy")
+			} else {
+				payload, _, _, err = buildV2WriteRequest(promslog.NewNopLogger(), []writev2.TimeSeries{
+					{
+						LabelsRefs: []uint32{0, 1},
+						Histograms: []writev2.Histogram{writev2.FromIntHistogram(1, highSchemaHistogram)},
+					},
+					{
+						LabelsRefs: []uint32{0, 2},
+						Histograms: []writev2.Histogram{writev2.FromFloatHistogram(2, highSchemaHistogram.ToFloat(nil))},
+					},
+				}, []string{"__name__", "test_metric1", "test_metric2"},
+					nil, nil, nil, "snappy")
+			}
+			require.NoError(t, err)
+
+			req, err := http.NewRequest("", "", bytes.NewReader(payload))
+			require.NoError(t, err)
+
+			req.Header.Set("Content-Type", remoteWriteContentTypeHeaders[protoMsg])
+
+			recorder := httptest.NewRecorder()
+			handler.ServeHTTP(recorder, req)
+
+			resp := recorder.Result()
+			body, err := io.ReadAll(resp.Body)
+			require.NoError(t, err)
+			require.Equal(t, http.StatusNoContent, resp.StatusCode)
+			require.Empty(t, body)
+
+			require.Len(t, appendable.histograms, 2)
+			require.Equal(t, int64(1), appendable.histograms[0].t)
+			require.Equal(t, reducedSchemaHistogram, appendable.histograms[0].h)
+			require.Equal(t, int64(2), appendable.histograms[1].t)
+			require.Equal(t, reducedSchemaHistogram.ToFloat(nil), appendable.histograms[1].fh)
+		})
+	}
+}