do snapshot for validators at last block of epoch (harmony-one#2736)

core/blockchain.go

@@ -2544,7 +2544,7 @@ func (bc *BlockChain) writeDelegationsByDelegator(
 // Note: this should only be called within the blockchain insert process.
 func (bc *BlockChain) UpdateStakingMetaData(
 	batch rawdb.DatabaseWriter, txns staking.StakingTransactions,
-	state *state.DB, epoch *big.Int,
+	state *state.DB, epoch *big.Int, isNewEpoch bool,
 ) (newValidators []common.Address, err error) {
 	newValidators, newDelegations, err := bc.prepareStakingMetaData(txns, state)
 	if err != nil {
@@ -2574,6 +2574,14 @@ func (bc *BlockChain) UpdateStakingMetaData(
 			if err := rawdb.WriteValidatorSnapshot(batch, validator, epoch); err != nil {
 				return newValidators, err
 			}
+			// For validator created at exactly the last block of an epoch, we should create the snapshot
+			// for next epoch too.
+			if isNewEpoch {
+				newEpoch := new(big.Int).Add(epoch, common.Big1)
+				if err := rawdb.WriteValidatorSnapshot(batch, validator, newEpoch); err != nil {
+					return newValidators, err
+				}
+			}
 		}
 
 		// Update validator list

core/offchain.go

@@ -94,7 +94,7 @@ func (bc *BlockChain) CommitOffChainData(
 
 	// Do bookkeeping for new staking txns
 	newVals, err := bc.UpdateStakingMetaData(
-		batch, block.StakingTransactions(), state, epoch,
+		batch, block.StakingTransactions(), state, epoch, isNewEpoch,
 	)
 	if err != nil {
 		utils.Logger().Err(err).Msg("UpdateStakingMetaData failed")