fix(drand): add null HistoricalBeaconClient for old beacons

Fixes: #11802
Ref: #11802 (comment)

chain/beacon/drand/drand.go

@@ -3,6 +3,7 @@ package drand
 import (
 	"bytes"
 	"context"
+	"fmt"
 	"time"
 
 	dchain "github.com/drand/drand/chain"
@@ -100,7 +101,9 @@ func NewDrandBeacon(genesisTs, interval uint64, ps *pubsub.PubSub, config dtypes
 		}
 		hc.(DrandHTTPClient).SetUserAgent("drand-client-lotus/" + build.NodeBuildVersion)
 		clients = append(clients, hc)
-
+	}
+	if len(clients) == 0 {
+		clients = append(clients, HistoricalBeaconClient{})
 	}
 
 	opts := []dclient.Option{
@@ -239,13 +242,42 @@ var _ beacon.RandomBeacon = (*DrandBeacon)(nil)
 
 func BeaconScheduleFromDrandSchedule(dcs dtypes.DrandSchedule, genesisTime uint64, ps *pubsub.PubSub) (beacon.Schedule, error) {
 	shd := beacon.Schedule{}
-	for _, dc := range dcs {
+	for i, dc := range dcs {
 		bc, err := NewDrandBeacon(genesisTime, buildconstants.BlockDelaySecs, ps, dc.Config)
 		if err != nil {
-			return nil, xerrors.Errorf("creating drand beacon: %w", err)
+			fmt.Printf("failed to create drand beacon: %+v\n", dc)
+			return nil, xerrors.Errorf("%d creating drand beacon: %w", i, err)
 		}
 		shd = append(shd, beacon.BeaconPoint{Start: dc.Start, Beacon: bc})
 	}
 
 	return shd, nil
 }
+
+var _ dclient.Client = HistoricalBeaconClient{}
+
+// HistoricalBeaconClient is a drand client that doesn't actually do anything. It's used when
+// we don't have a drand network to connect to but still need to provide a beacon client.
+// We don't expect calls through to the client to be made since we should only be verifying old
+// randomness, not fetching it.
+type HistoricalBeaconClient struct{}
+
+func (h HistoricalBeaconClient) Get(ctx context.Context, round uint64) (dclient.Result, error) {
+	return nil, xerrors.Errorf("no historical randomness available")
+}
+
+func (h HistoricalBeaconClient) Watch(ctx context.Context) <-chan dclient.Result {
+	return nil
+}
+
+func (h HistoricalBeaconClient) Info(ctx context.Context) (*dchain.Info, error) {
+	return nil, xerrors.Errorf("no historical randomness available")
+}
+
+func (h HistoricalBeaconClient) RoundAt(time.Time) uint64 {
+	return 0
+}
+
+func (h HistoricalBeaconClient) Close() error {
+	return nil
+}