Validate empty labels in push

CHANGELOG.md

@@ -24,6 +24,7 @@
 * [FEATURE] Querier: Support for configuring query optimizers and enabling XFunctions in the Thanos engine. #6873
 * [FEATURE] Query Frontend: Add support /api/v1/format_query API for formatting queries. #6893
 * [FEATURE] Query Frontend: Add support for /api/v1/parse_query API (experimental) to parse a PromQL expression and return it as a JSON-formatted AST (abstract syntax tree). #6978
+* [ENHANCEMENT] Distributor: Emit an error with a 400 status code when empty labels are found before the relabelling or label dropping process. #7052
 * [ENHANCEMENT] Parquet Storage: Add support for additional sort columns during Parquet file generation #7003
 * [ENHANCEMENT] Modernizes the entire codebase by using go modernize tool. #7005
 * [ENHANCEMENT] Overrides Exporter: Expose all fields that can be converted to float64. Also, the label value `max_local_series_per_metric` got renamed to `max_series_per_metric`, and `max_local_series_per_user` got renamed to `max_series_per_user`. #6979

pkg/distributor/distributor.go

@@ -1012,6 +1012,10 @@ func (d *Distributor) prepareSeriesKeys(ctx context.Context, req *cortexpb.Write
 	// check each sample and discard if outside limits.
 	skipLabelNameValidation := d.cfg.SkipLabelNameValidation || req.GetSkipLabelNameValidation()
 	for _, ts := range req.Timeseries {
+		if len(ts.Labels) == 0 {
+			return nil, nil, nil, nil, 0, 0, 0, nil, httpgrpc.Errorf(http.StatusBadRequest, "%s", "empty labels found")
+		}
+
 		if limits.AcceptHASamples && limits.AcceptMixedHASamples {
 			cluster, replica := findHALabels(limits.HAReplicaLabel, limits.HAClusterLabel, ts.Labels)
 			if cluster != "" && replica != "" {

pkg/distributor/distributor_test.go

@@ -849,6 +849,69 @@ func TestDistributor_PushIngestionRateLimiter_Histograms(t *testing.T) {
 
 }
 
+func TestPush_EmptyLabels(t *testing.T) {
+	t.Parallel()
+
+	var limits validation.Limits
+	flagext.DefaultValues(&limits)
+
+	limits.IngestionRate = math.MaxFloat64
+
+	dists, _, _, _ := prepare(t, prepConfig{
+		numDistributors: 1,
+		numIngesters:    3,
+		happyIngesters:  3,
+	})
+
+	ctx := user.InjectOrgID(context.Background(), "user")
+
+	d := dists[0]
+	ts := time.Now().UnixMilli()
+
+	tests := []struct {
+		desc    string
+		request *cortexpb.WriteRequest
+		isErr   bool
+	}{
+		{
+			desc: "1 series, a series has empty labels",
+			request: &cortexpb.WriteRequest{
+				Timeseries: []cortexpb.PreallocTimeseries{
+					makeWriteRequestTimeseries(
+						[]cortexpb.LabelAdapter{}, ts, 3, false),
+				},
+			},
+			isErr: true,
+		},
+		{
+			desc: "2 series, one series has empty labels",
+			request: &cortexpb.WriteRequest{
+				Timeseries: []cortexpb.PreallocTimeseries{
+					makeWriteRequestTimeseries(
+						[]cortexpb.LabelAdapter{}, ts, 3, false),
+					makeWriteRequestTimeseries(
+						[]cortexpb.LabelAdapter{
+							{Name: model.MetricNameLabel, Value: "foo"},
+							{Name: "bar", Value: "baz"},
+						}, ts, 3, false),
+				},
+			},
+			isErr: true,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.desc, func(t *testing.T) {
+			_, err := d.Push(ctx, test.request)
+			require.Error(t, err)
+			s, ok := status.FromError(err)
+			require.True(t, ok)
+			require.Equal(t, codes.Code(400), s.Code())
+			require.Equal(t, "empty labels found", s.Message())
+		})
+	}
+}
+
 func TestPush_QuorumError(t *testing.T) {
 	t.Parallel()