using block after nullrounds to find randomness

chain/beacon/beacon.go

@@ -26,6 +26,7 @@ type RandomBeacon interface {
 	Entry(context.Context, uint64) <-chan Response
 	VerifyEntry(types.BeaconEntry, types.BeaconEntry) error
 	MaxBeaconRoundForEpoch(abi.ChainEpoch, types.BeaconEntry) uint64
+	MinDrandEntryEpoch(filEpoch abi.ChainEpoch) abi.ChainEpoch
 }
 
 func ValidateBlockValues(b RandomBeacon, h *types.BlockHeader, prevEntry types.BeaconEntry) error {

chain/beacon/drand/drand.go

@@ -3,6 +3,7 @@ package drand
 import (
 	"bytes"
 	"context"
+	"math"
 	"sync"
 	"time"
 
@@ -190,8 +191,29 @@ func (db *DrandBeacon) VerifyEntry(curr types.BeaconEntry, prev types.BeaconEntr
 func (db *DrandBeacon) MaxBeaconRoundForEpoch(filEpoch abi.ChainEpoch, prevEntry types.BeaconEntry) uint64 {
 	// TODO: sometimes the genesis time for filecoin is zero and this goes negative
 	latestTs := ((uint64(filEpoch) * db.filRoundTime) + db.filGenTime) - db.filRoundTime
-	dround := (latestTs - db.drandGenTime) / uint64(db.interval.Seconds())
+	dround := (latestTs-db.drandGenTime)/uint64(db.interval.Seconds()) + 1
 	return dround
 }
 
+// MinDrandEntryEpoch returns the minimum filecoin epoch where the drand entry
+// that is associated with `filEpoch` could have been inserted in the chain.
+// Given filecoin and drand can have different frequency, a drand entry can be
+// associated to one or multiple subsequent filecoin epochs. When searching the
+// chain for the drand entry corresponding to filEpoch, we want to start
+// searching from the minimum filecoin epoch where this drand entry should /
+// could have been inserted and move onwards (for example if it falls on a null
+// blok). This function returns the first such filecoin epoch.
+// Note that if filecoin and drand frequency are the same, filEpoch and the
+// returned epoch are the same.
+func (db *DrandBeacon) MinDrandEntryEpoch(filEpoch abi.ChainEpoch) abi.ChainEpoch {
+	drandRound := db.MaxBeaconRoundForEpoch(filEpoch, types.BeaconEntry{})
+	drandTs := db.drandGenTime + (drandRound-1)*uint64(db.interval.Seconds())
+	// compute the minimum filecoin epoch associated with that timestamp
+	// We do the ceiling such that if we find on a decimal epoch number, we take
+	// the epoch number after this one - we don't include a drand entry at round
+	// 9.4, but at round 10 because we can't know it at the time of round 9
+	minEpoch := math.Ceil(float64((drandTs - db.filGenTime)) / float64(db.filRoundTime))
+	return abi.ChainEpoch(minEpoch)
+}
+
 var _ beacon.RandomBeacon = (*DrandBeacon)(nil)

chain/beacon/drand/drand_test.go

@@ -3,12 +3,15 @@ package drand
 import (
 	"os"
 	"testing"
+	"time"
 
 	dchain "github.com/drand/drand/chain"
 	hclient "github.com/drand/drand/client/http"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 
 	"github.com/filecoin-project/lotus/build"
+	"github.com/filecoin-project/specs-actors/actors/abi"
 )
 
 func TestPrintGroupInfo(t *testing.T) {
@@ -23,3 +26,26 @@ func TestPrintGroupInfo(t *testing.T) {
 	err = chain.ToJSON(os.Stdout)
 	assert.NoError(t, err)
 }
+
+func TestDrandBeaconMinDrandEntry(t *testing.T) {
+	db := DrandBeacon{
+		interval:     30 * time.Second,
+		drandGenTime: 666,
+		filGenTime:   1337,
+		filRoundTime: 30,
+	}
+
+	// test for equal frequency
+	in := abi.ChainEpoch(10)
+	res := db.MinDrandEntryEpoch(in)
+	require.Equal(t, in-1, res)
+
+	// test for different frequency
+	// since we give an odd epoch number, the drand entry should have been
+	// inserted in the even epoch number given filecoin is twice faster than
+	// drand
+	db.interval = 60 * time.Second
+	in = abi.ChainEpoch(11)
+	res = db.MinDrandEntryEpoch(in)
+	require.Equal(t, in-1, res)
+}

chain/beacon/mock.go

@@ -61,4 +61,8 @@ func (mb *mockBeacon) MaxBeaconRoundForEpoch(epoch abi.ChainEpoch, prevEntry typ
 	return uint64(epoch)
 }
 
+func (mb *mockBeacon) MinDrandEntryEpoch(filEpoch abi.ChainEpoch) abi.ChainEpoch {
+	return filEpoch
+}
+
 var _ RandomBeacon = (*mockBeacon)(nil)

chain/store/store.go

@@ -1040,7 +1040,7 @@ func (cs *ChainStore) GetBeaconRandomness(ctx context.Context, blks []cid.Cid, p
 		searchHeight = 0
 	}
 
-	randTs, err := cs.GetTipsetByHeight(ctx, searchHeight, ts, true)
+	randTs, err := cs.GetTipsetByHeight(ctx, searchHeight, ts, false)
 	if err != nil {
 		return nil, err
 	}