Ensure the order of undelegate entries

core/state_transition.go

@@ -394,7 +394,6 @@ func (st *StateTransition) applyDelegateTx(delegate *staking.Delegate) error {
 				// Firstly use the tokens in undelegation to delegate (redelegate)
 				undelegateAmount := big.NewInt(0).Set(delegate.Amount)
 				// Use the latest undelegated token first as it has the longest remaining locking time.
-				// TODO: always order the list by epoch.
 				i := len(delegation.Entries) - 1
 				for ; i >= 0; i-- {
 					if delegation.Entries[i].Amount.Cmp(undelegateAmount) <= 0 {

internal/chain/engine.go

@@ -187,7 +187,6 @@ func (e *engineImpl) Finalize(
 					delegation := wrapper.Delegations[i]
 					totalWithdraw := big.NewInt(0)
 					count := 0
-					// TODO: need ot make sure the entries are ordered by epoch
 					for j := range delegation.Entries {
 						if delegation.Entries[j].Epoch.Cmp(header.Epoch()) > 14 { // need to wait at least 14 epochs to withdraw;
 							totalWithdraw.Add(totalWithdraw, delegation.Entries[j].Amount)

staking/types/delegation.go

@@ -3,6 +3,7 @@ package types
 import (
 	"errors"
 	"math/big"
+	"sort"
 
 	"github.com/ethereum/go-ethereum/common"
 )
@@ -59,7 +60,14 @@ func (d *Delegation) Undelegate(epoch *big.Int, amt *big.Int) error {
 	if !exist {
 		item := UndelegationEntry{amt, epoch}
 		d.Entries = append(d.Entries, &item)
+
+		// Always sort the undelegate by epoch in increasing order
+		sort.SliceStable(
+			d.Entries,
+			func(i, j int) bool { return d.Entries[i].Epoch.Cmp(d.Entries[j].Epoch) < 0 },
+		)
 	}
+
 	return nil
 }