Problem: mempool iteration is not thread safe

CHANGELOG.md

@@ -66,6 +66,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 ### Bug Fixes
 
 * (x/bank) [#20028](https://github.com/cosmos/cosmos-sdk/pull/20028) Align query with multi denoms for send-enabled.
+* (baseapp) [#699](https://github.com/crypto-org-chain/cosmos-sdk/pull/699) Fix data race in mempool iteration.
 
 ## [Unreleased-Upstream]
 

baseapp/abci_utils.go

@@ -284,14 +284,18 @@ func (h *DefaultProposalHandler) PrepareProposalHandler() sdk.PrepareProposalHan
 			return &abci.ResponsePrepareProposal{Txs: h.txSelector.SelectedTxs(ctx)}, nil
 		}
 
-		iterator := h.mempool.Select(ctx, req.Txs)
 		selectedTxsSignersSeqs := make(map[string]uint64)
-		var selectedTxsNums int
-		for iterator != nil {
-			memTx := iterator.Tx()
+		var (
+			resError        error
+			selectedTxsNums int
+			invalidTxs      []sdk.Tx // invalid txs to be removed after the iteration
+		)
+		h.mempool.SelectBy(ctx, req.Txs, func(memTx mempool.Tx) bool {
 			signerData, err := h.signerExtAdapter.GetSigners(memTx.Tx)
 			if err != nil {
-				return nil, err
+				// propagate the error to the caller
+				resError = err
+				return false
 			}
 
 			// If the signers aren't in selectedTxsSignersSeqs then we haven't seen them before
@@ -315,8 +319,7 @@ func (h *DefaultProposalHandler) PrepareProposalHandler() sdk.PrepareProposalHan
 				txSignersSeqs[signer.Signer.String()] = signer.Sequence
 			}
 			if !shouldAdd {
-				iterator = iterator.Next()
-				continue
+				return true
 			}
 
 			// NOTE: Since transaction verification was already executed in CheckTx,
@@ -325,14 +328,11 @@ func (h *DefaultProposalHandler) PrepareProposalHandler() sdk.PrepareProposalHan
 			// check again.
 			txBz, err := h.txVerifier.PrepareProposalVerifyTx(memTx.Tx)
 			if err != nil {
-				err := h.mempool.Remove(memTx.Tx)
-				if err != nil && !errors.Is(err, mempool.ErrTxNotFound) {
-					return nil, err
-				}
+				invalidTxs = append(invalidTxs, memTx.Tx)
 			} else {
 				stop := h.txSelector.SelectTxForProposal(ctx, uint64(req.MaxTxBytes), maxBlockGas, memTx.Tx, txBz, memTx.GasWanted)
 				if stop {
-					break
+					return false
 				}
 
 				txsLen := len(h.txSelector.SelectedTxs(ctx))
@@ -353,7 +353,18 @@ func (h *DefaultProposalHandler) PrepareProposalHandler() sdk.PrepareProposalHan
 				selectedTxsNums = txsLen
 			}
 
-			iterator = iterator.Next()
+			return true
+		})
+
+		if resError != nil {
+			return nil, resError
+		}
+
+		for _, tx := range invalidTxs {
+			err := h.mempool.Remove(tx)
+			if err != nil && !errors.Is(err, mempool.ErrTxNotFound) {
+				return nil, err
+			}
 		}
 
 		return &abci.ResponsePrepareProposal{Txs: h.txSelector.SelectedTxs(ctx)}, nil

types/mempool/mempool.go

@@ -32,10 +32,12 @@ type Mempool interface {
 	InsertWithGasWanted(context.Context, sdk.Tx, uint64) error
 
 	// Select returns an Iterator over the app-side mempool. If txs are specified,
-	// then they shall be incorporated into the Iterator. The Iterator must
-	// closed by the caller.
+	// then they shall be incorporated into the Iterator. The Iterator is not thread-safe to use.
 	Select(context.Context, [][]byte) Iterator
 
+	// SelectBy use callback to iterate over the mempool.
+	SelectBy(context.Context, [][]byte, func(Tx) bool)
+
 	// CountTx returns the number of transactions currently in the mempool.
 	CountTx() int
 

types/mempool/noop.go

@@ -19,5 +19,6 @@ type NoOpMempool struct{}
 func (NoOpMempool) Insert(context.Context, sdk.Tx) error                      { return nil }
 func (NoOpMempool) InsertWithGasWanted(context.Context, sdk.Tx, uint64) error { return nil }
 func (NoOpMempool) Select(context.Context, [][]byte) Iterator                 { return nil }
+func (NoOpMempool) SelectBy(context.Context, [][]byte, func(Tx) bool)         {}
 func (NoOpMempool) CountTx() int                                              { return 0 }
 func (NoOpMempool) Remove(sdk.Tx) error                                       { return nil }

types/mempool/priority_nonce.go

@@ -361,9 +361,13 @@ func (i *PriorityNonceIterator[C]) Tx() Tx {
 //
 // NOTE: It is not safe to use this iterator while removing transactions from
 // the underlying mempool.
-func (mp *PriorityNonceMempool[C]) Select(_ context.Context, _ [][]byte) Iterator {
+func (mp *PriorityNonceMempool[C]) Select(ctx context.Context, txs [][]byte) Iterator {
 	mp.mtx.Lock()
 	defer mp.mtx.Unlock()
+	return mp.doSelect(ctx, txs)
+}
+
+func (mp *PriorityNonceMempool[C]) doSelect(_ context.Context, _ [][]byte) Iterator {
 	if mp.priorityIndex.Len() == 0 {
 		return nil
 	}
@@ -378,6 +382,16 @@ func (mp *PriorityNonceMempool[C]) Select(_ context.Context, _ [][]byte) Iterato
 	return iterator.iteratePriority()
 }
 
+func (mp *PriorityNonceMempool[C]) SelectBy(ctx context.Context, txs [][]byte, callback func(Tx) bool) {
+	mp.mtx.Lock()
+	defer mp.mtx.Unlock()
+
+	iter := mp.doSelect(ctx, txs)
+	for iter != nil && callback(iter.Tx()) {
+		iter = iter.Next()
+	}
+}
+
 type reorderKey[C comparable] struct {
 	deleteKey txMeta[C]
 	insertKey txMeta[C]

types/mempool/priority_nonce_test.go

@@ -1,9 +1,11 @@
 package mempool_test
 
 import (
+	"context"
 	"fmt"
 	"math"
 	"math/rand"
+	"sync"
 	"testing"
 	"time"
 
@@ -396,6 +398,89 @@ func (s *MempoolTestSuite) TestIterator() {
 	}
 }
 
+func (s *MempoolTestSuite) TestIteratorConcurrency() {
+	t := s.T()
+	ctx := sdk.NewContext(nil, cmtproto.Header{}, false, log.NewNopLogger())
+	accounts := simtypes.RandomAccounts(rand.New(rand.NewSource(0)), 2)
+	sa := accounts[0].Address
+	sb := accounts[1].Address
+
+	tests := []struct {
+		txs  []txSpec
+		fail bool
+	}{
+		{
+			txs: []txSpec{
+				{p: 20, n: 1, a: sa},
+				{p: 15, n: 1, a: sb},
+				{p: 6, n: 2, a: sa},
+				{p: 21, n: 4, a: sa},
+				{p: 8, n: 2, a: sb},
+			},
+		},
+		{
+			txs: []txSpec{
+				{p: 20, n: 1, a: sa},
+				{p: 15, n: 1, a: sb},
+				{p: 6, n: 2, a: sa},
+				{p: 21, n: 4, a: sa},
+				{p: math.MinInt64, n: 2, a: sb},
+			},
+		},
+	}
+
+	for i, tt := range tests {
+		t.Run(fmt.Sprintf("case %d", i), func(t *testing.T) {
+			pool := mempool.DefaultPriorityMempool()
+
+			// create test txs and insert into mempool
+			for i, ts := range tt.txs {
+				tx := testTx{id: i, priority: int64(ts.p), nonce: uint64(ts.n), address: ts.a}
+				c := ctx.WithPriority(tx.priority)
+				err := pool.Insert(c, tx)
+				require.NoError(t, err)
+			}
+
+			// iterate through txs
+			stdCtx, cancel := context.WithCancel(context.Background())
+			var wg sync.WaitGroup
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+
+				id := len(tt.txs)
+				for {
+					select {
+					case <-stdCtx.Done():
+						return
+					default:
+						id++
+						tx := testTx{id: id, priority: int64(rand.Intn(100)), nonce: uint64(id), address: sa}
+						c := ctx.WithPriority(tx.priority)
+						err := pool.Insert(c, tx)
+						require.NoError(t, err)
+					}
+				}
+			}()
+
+			var i int
+			pool.SelectBy(ctx, nil, func(memTx mempool.Tx) bool {
+				tx := memTx.Tx.(testTx)
+				if tx.id < len(tt.txs) {
+					require.Equal(t, tt.txs[tx.id].p, int(tx.priority))
+					require.Equal(t, tt.txs[tx.id].n, int(tx.nonce))
+					require.Equal(t, tt.txs[tx.id].a, tx.address)
+					i++
+				}
+				return i < len(tt.txs)
+			})
+			require.Equal(t, i, len(tt.txs))
+			cancel()
+			wg.Wait()
+		})
+	}
+}
+
 func (s *MempoolTestSuite) TestPriorityTies() {
 	ctx := sdk.NewContext(nil, cmtproto.Header{}, false, log.NewNopLogger())
 	accounts := simtypes.RandomAccounts(rand.New(rand.NewSource(0)), 3)

types/mempool/sender_nonce.go

@@ -169,9 +169,13 @@ func (snm *SenderNonceMempool) Insert(ctx context.Context, tx sdk.Tx) error {
 //
 // NOTE: It is not safe to use this iterator while removing transactions from
 // the underlying mempool.
-func (snm *SenderNonceMempool) Select(_ context.Context, _ [][]byte) Iterator {
+func (snm *SenderNonceMempool) Select(ctx context.Context, txs [][]byte) Iterator {
 	snm.mtx.Lock()
 	defer snm.mtx.Unlock()
+	return snm.doSelect(ctx, txs)
+}
+
+func (snm *SenderNonceMempool) doSelect(_ context.Context, _ [][]byte) Iterator {
 	var senders []string
 
 	senderCursors := make(map[string]*skiplist.Element)
@@ -199,6 +203,16 @@ func (snm *SenderNonceMempool) Select(_ context.Context, _ [][]byte) Iterator {
 	return iter.Next()
 }
 
+func (snm *SenderNonceMempool) SelectBy(ctx context.Context, txs [][]byte, callback func(Tx) bool) {
+	snm.mtx.Lock()
+	defer snm.mtx.Unlock()
+
+	iter := snm.doSelect(ctx, txs)
+	for iter != nil && callback(iter.Tx()) {
+		iter = iter.Next()
+	}
+}
+
 // CountTx returns the total count of txs in the mempool.
 func (snm *SenderNonceMempool) CountTx() int {
 	snm.mtx.Lock()