Use older randomness for the PoSt commit on specs-actors version 2

After the upgrade, we're allowed to use randomness up to the challenge epoch.
This safe-guards us against very large reorgs where a miner computes a PoSt very
quickly.

storage/wdpost_run.go

@@ -12,6 +12,7 @@ import (
 	"github.com/filecoin-project/go-state-types/big"
 	"github.com/filecoin-project/go-state-types/crypto"
 	"github.com/filecoin-project/go-state-types/dline"
+	"github.com/filecoin-project/go-state-types/network"
 	"github.com/ipfs/go-cid"
 
 	"go.opencensus.io/trace"
@@ -151,7 +152,15 @@ func (s *WindowPoStScheduler) runSubmitPoST(
 	defer span.End()
 
 	// Get randomness from tickets
+	// use the challenge epoch if we've upgraded to network version 4
+	// (actors version 2). We want to go back as far as possible to be safe.
 	commEpoch := deadline.Open
+	if ver, err := s.api.StateNetworkVersion(ctx, types.EmptyTSK); err != nil {
+		log.Errorw("failed to get network version to determine PoSt epoch randomness lookback", "error", err)
+	} else if ver >= network.Version4 {
+		commEpoch = deadline.Challenge
+	}
+
 	commRand, err := s.api.ChainGetRandomnessFromTickets(ctx, ts.Key(), crypto.DomainSeparationTag_PoStChainCommit, commEpoch, nil)
 	if err != nil {
 		err = xerrors.Errorf("failed to get chain randomness from tickets for windowPost (ts=%d; deadline=%d): %w", ts.Height(), commEpoch, err)

storage/wdpost_run_test.go

@@ -23,6 +23,7 @@ import (
 	tutils "github.com/filecoin-project/specs-actors/v2/support/testing"
 
 	"github.com/filecoin-project/lotus/api"
+	"github.com/filecoin-project/lotus/build"
 	"github.com/filecoin-project/lotus/chain/actors/builtin/miner"
 	"github.com/filecoin-project/lotus/chain/types"
 	"github.com/filecoin-project/lotus/journal"
@@ -47,7 +48,7 @@ func (m *mockStorageMinerAPI) StateMinerInfo(ctx context.Context, a address.Addr
 }
 
 func (m *mockStorageMinerAPI) StateNetworkVersion(ctx context.Context, key types.TipSetKey) (network.Version, error) {
-	panic("implement me")
+	return build.NewestNetworkVersion, nil
 }
 
 func (m *mockStorageMinerAPI) ChainGetRandomnessFromTickets(ctx context.Context, tsk types.TipSetKey, personalization crypto.DomainSeparationTag, randEpoch abi.ChainEpoch, entropy []byte) (abi.Randomness, error) {