Add best-effort validation for prometheus scrape interval

viz/metrics-api/grpc_server.go

@@ -15,6 +15,7 @@ import (
 	promv1 "github.com/prometheus/client_golang/api/prometheus/v1"
 	log "github.com/sirupsen/logrus"
 	"google.golang.org/grpc"
+	"gopkg.in/yaml.v2"
 	corev1 "k8s.io/api/core/v1"
 	"k8s.io/apimachinery/pkg/labels"
 )
@@ -237,3 +238,44 @@ func (s *grpcServer) ListServices(ctx context.Context, req *pb.ListServicesReque
 
 	return &pb.ListServicesResponse{Services: svcs}, nil
 }
+
+// validateTimeWindow returns an error if the Prometheus scrape interval
+// is longer than the query time window. This is an opportunistic, best-effort
+// validation: if we cannot determine the Prometheus scrape interval for any
+// reason, we do not return an error.
+func (s *grpcServer) validateTimeWindow(ctx context.Context, window string) error {
+	config, err := s.prometheusAPI.Config(ctx)
+	if err != nil {
+		return nil
+	}
+
+	type PrometheusConfig struct {
+		Global map[string]string
+	}
+
+	var prom PrometheusConfig
+	err = yaml.Unmarshal([]byte(config.YAML), &prom)
+	if err != nil {
+		return nil
+	}
+
+	scrape_interval_str, found := prom.Global["scrape_interval"]
+	if !found {
+		return nil
+	}
+
+	scrape_interval, err := time.ParseDuration(scrape_interval_str)
+	if err != nil {
+		return nil
+	}
+
+	t, err := time.ParseDuration(window)
+	if err != nil {
+		return err
+	}
+
+	if t < scrape_interval {
+		return fmt.Errorf("Time window (%s) must be at least as long as the Prometheus scrape interval (%s)", window, scrape_interval)
+	}
+	return nil
+}

viz/metrics-api/stat_summary.go

@@ -102,6 +102,11 @@ func (s *grpcServer) StatSummary(ctx context.Context, req *pb.StatSummaryRequest
 		}
 	}
 
+	err := s.validateTimeWindow(ctx, req.TimeWindow)
+	if err != nil {
+		return statSummaryError(req, fmt.Sprintf("invalid time window: %s", err)), nil
+	}
+
 	statTables := make([]*pb.StatTable, 0)
 
 	var resourcesToQuery []string

viz/metrics-api/top_routes.go

@@ -63,6 +63,11 @@ func (s *grpcServer) TopRoutes(ctx context.Context, req *pb.TopRoutesRequest) (*
 		return topRoutesError(req, "Authority cannot be the target of a routes query; try using an authority in the --to flag instead"), nil
 	}
 
+	err = s.validateTimeWindow(ctx, req.TimeWindow)
+	if err != nil {
+		return topRoutesError(req, fmt.Sprintf("invalid time window: %s", err)), nil
+	}
+
 	// Non-authority resource
 	objects, err := s.k8sAPI.GetObjects(targetResource.Namespace, targetResource.Type, targetResource.Name, labelSelector)
 	if err != nil {