core/txpool: disallow future churn by remote txs (#26907)

Prior to this change, it was possible that transactions are erroneously deemed as 'future' although they are in fact 'pending', causing them to be dropped due to 'future' not being allowed to replace 'pending'. 

This change fixes that, by doing a more in-depth inspection of the queue.

core/txpool/list.go

@@ -270,10 +270,10 @@ func newList(strict bool) *list {
 	}
 }
 
-// Overlaps returns whether the transaction specified has the same nonce as one
-// already contained within the list.
-func (l *list) Overlaps(tx *types.Transaction) bool {
-	return l.txs.Get(tx.Nonce()) != nil
+// Contains returns whether the  list contains a transaction
+// with the provided nonce.
+func (l *list) Contains(nonce uint64) bool {
+	return l.txs.Get(nonce) != nil
 }
 
 // Add tries to insert a new transaction into the list, returning whether the

core/txpool/txpool.go

@@ -745,11 +745,11 @@ func (pool *TxPool) add(tx *types.Transaction, local bool) (replaced bool, err e
 		}
 
 		// If the new transaction is a future transaction it should never churn pending transactions
-		if !isLocal && pool.isFuture(from, tx) {
+		if !isLocal && pool.isGapped(from, tx) {
 			var replacesPending bool
 			for _, dropTx := range drop {
 				dropSender, _ := types.Sender(pool.signer, dropTx)
-				if list := pool.pending[dropSender]; list != nil && list.Overlaps(dropTx) {
+				if list := pool.pending[dropSender]; list != nil && list.Contains(dropTx.Nonce()) {
 					replacesPending = true
 					break
 				}
@@ -774,7 +774,7 @@ func (pool *TxPool) add(tx *types.Transaction, local bool) (replaced bool, err e
 	}
 
 	// Try to replace an existing transaction in the pending pool
-	if list := pool.pending[from]; list != nil && list.Overlaps(tx) {
+	if list := pool.pending[from]; list != nil && list.Contains(tx.Nonce()) {
 		// Nonce already pending, check if required price bump is met
 		inserted, old := list.Add(tx, pool.config.PriceBump)
 		if !inserted {
@@ -817,18 +817,26 @@ func (pool *TxPool) add(tx *types.Transaction, local bool) (replaced bool, err e
 	return replaced, nil
 }
 
-// isFuture reports whether the given transaction is immediately executable.
-func (pool *TxPool) isFuture(from common.Address, tx *types.Transaction) bool {
-	list := pool.pending[from]
-	if list == nil {
-		return pool.pendingNonces.get(from) != tx.Nonce()
+// isGapped reports whether the given transaction is immediately executable.
+func (pool *TxPool) isGapped(from common.Address, tx *types.Transaction) bool {
+	// Short circuit if transaction matches pending nonce and can be promoted
+	// to pending list as an executable transaction.
+	next := pool.pendingNonces.get(from)
+	if tx.Nonce() == next {
+		return false
 	}
-	// Sender has pending transactions.
-	if old := list.txs.Get(tx.Nonce()); old != nil {
-		return false // It replaces a pending transaction.
+	// The transaction has a nonce gap with pending list, it's only considered
+	// as executable if transactions in queue can fill up the nonce gap.
+	queue, ok := pool.queue[from]
+	if !ok {
+		return true
 	}
-	// Not replacing, check if parent nonce exists in pending.
-	return list.txs.Get(tx.Nonce()-1) == nil
+	for nonce := next; nonce < tx.Nonce(); nonce++ {
+		if !queue.Contains(nonce) {
+			return true // txs in queue can't fill up the nonce gap
+		}
+	}
+	return false
 }
 
 // enqueueTx inserts a new transaction into the non-executable transaction queue.

core/txpool/txpool2_test.go

@@ -42,7 +42,7 @@ func count(t *testing.T, pool *TxPool) (pending int, queued int) {
 	return pending, queued
 }
 
-func fillPool(t *testing.T, pool *TxPool) {
+func fillPool(t testing.TB, pool *TxPool) {
 	t.Helper()
 	// Create a number of test accounts, fund them and make transactions
 	executableTxs := types.Transactions{}
@@ -189,7 +189,7 @@ func TestTransactionZAttack(t *testing.T) {
 		key, _ := crypto.GenerateKey()
 		pool.currentState.AddBalance(crypto.PubkeyToAddress(key.PublicKey), big.NewInt(100000000000))
 		for j := 0; j < int(pool.config.GlobalSlots); j++ {
-			overDraftTxs = append(overDraftTxs, pricedValuedTransaction(uint64(j), 60000000000, 21000, big.NewInt(500), key))
+			overDraftTxs = append(overDraftTxs, pricedValuedTransaction(uint64(j), 600000000000, 21000, big.NewInt(500), key))
 		}
 	}
 	pool.AddRemotesSync(overDraftTxs)
@@ -210,3 +210,27 @@ func TestTransactionZAttack(t *testing.T) {
 			newIvPending, ivPending, pool.config.GlobalSlots, newQueued)
 	}
 }
+
+func BenchmarkFutureAttack(b *testing.B) {
+	// Create the pool to test the limit enforcement with
+	statedb, _ := state.New(common.Hash{}, state.NewDatabase(rawdb.NewMemoryDatabase()), nil)
+	blockchain := newTestBlockChain(1000000, statedb, new(event.Feed))
+	config := testTxPoolConfig
+	config.GlobalQueue = 100
+	config.GlobalSlots = 100
+	pool := NewTxPool(config, eip1559Config, blockchain)
+	defer pool.Stop()
+	fillPool(b, pool)
+
+	key, _ := crypto.GenerateKey()
+	pool.currentState.AddBalance(crypto.PubkeyToAddress(key.PublicKey), big.NewInt(100000000000))
+	futureTxs := types.Transactions{}
+
+	for n := 0; n < b.N; n++ {
+		futureTxs = append(futureTxs, pricedTransaction(1000+uint64(n), 100000, big.NewInt(500), key))
+	}
+	b.ResetTimer()
+	for i := 0; i < 5; i++ {
+		pool.AddRemotesSync(futureTxs)
+	}
+}