exp/orderbook: Reallocate offer slice after removals

The code in this PR reallocates offers slice after each offer removal.

This is done to prevent situation keeping a large chunks of allocated but unused
memory that can lead to OOM crash.

exp/orderbook/batch.go

@@ -87,15 +87,19 @@ func (tx *orderBookBatchedUpdates) apply(ledger uint32) error {
 		return errUnexpectedLedger
 	}
 
+	reallocatePairs := map[tradingPair]struct{}{}
+
 	for _, operation := range tx.operations {
 		switch operation.operationType {
 		case addOfferOperationType:
 			if err := tx.orderbook.addOffer(*operation.offer); err != nil {
 				panic(errors.Wrap(err, "could not apply update in batch"))
 			}
 		case removeOfferOperationType:
-			if _, ok := tx.orderbook.tradingPairForOffer[operation.offerID]; !ok {
+			if pair, ok := tx.orderbook.tradingPairForOffer[operation.offerID]; !ok {
 				continue
+			} else {
+				reallocatePairs[pair] = struct{}{}
 			}
 			if err := tx.orderbook.removeOffer(operation.offerID); err != nil {
 				panic(errors.Wrap(err, "could not apply update in batch"))
@@ -114,5 +118,9 @@ func (tx *orderBookBatchedUpdates) apply(ledger uint32) error {
 
 	tx.orderbook.lastLedger = ledger
 
+	for pair := range reallocatePairs {
+		tx.orderbook.venuesForSellingAsset[pair.sellingAsset].reallocate()
+		tx.orderbook.venuesForBuyingAsset[pair.buyingAsset].reallocate()
+	}
 	return nil
 }

exp/orderbook/edges.go

@@ -15,6 +15,10 @@ type edgeSet []edge
 type edge struct {
 	key   int32
 	value Venues
+
+	// reallocate is set to true when some offers were removed from the edge.
+	// See edgeSet.reallocate() godoc for more information.
+	reallocate bool
 }
 
 func (e edgeSet) find(key int32) int {
@@ -85,6 +89,7 @@ func (e edgeSet) removeOffer(key int32, offerID xdr.Int64) (edgeSet, bool) {
 		return slices.Delete(e, i, i+1), true
 	}
 	e[i].value.offers = updatedOffers
+	e[i].reallocate = true
 	return e, true
 }
 
@@ -101,3 +106,18 @@ func (e edgeSet) removePool(key int32) edgeSet {
 	e[i].value = Venues{offers: e[i].value.offers}
 	return e
 }
+
+// reallocate recreates offers slice when edge.reallocate is set to true and
+// this is true after an offer is removed.
+// Without periodic reallocations an arbitrary account could create 1000s of
+// offers in an orderbook, then remove them but the space occupied by these
+// offers would not be released by GC because an array used internally is
+// the same. This can lead to DoS attack by OOM.
+func (e edgeSet) reallocate() {
+	for i := 0; i < len(e); i++ {
+		if e[i].reallocate {
+			e[i].value.offers = append([]xdr.OfferEntry(nil), e[i].value.offers[:]...)
+			e[i].reallocate = false
+		}
+	}
+}

exp/orderbook/edges_test.go

@@ -0,0 +1,46 @@
+package orderbook
+
+import (
+	"runtime"
+	"testing"
+
+	"github.com/stellar/go/xdr"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestRemoveOffersMemoryUsage(t *testing.T) {
+	edges := edgeSet{}
+	for i := 0; i < 2000; i++ {
+		edges = edges.addOffer(1, xdr.OfferEntry{
+			SellerId: xdr.MustAddress("GCZFUQEPMLGUE2NB5RR7C3I2LTTLOEBM7GYD7PDKI4SU5HHWTDB553WD"),
+			OfferId:  xdr.Int64(i),
+		})
+	}
+
+	var afterAdded, afterRemoved, afterReallocate runtime.MemStats
+	runtime.ReadMemStats(&afterAdded)
+
+	t.Logf("after added: %d\n", afterAdded.HeapInuse)
+
+	// Remove all offers except one
+	for i := 0; i < 2000-1; i++ {
+		var removed bool
+		edges, removed = edges.removeOffer(1, xdr.Int64(i))
+		require.True(t, removed)
+
+	}
+
+	runtime.ReadMemStats(&afterRemoved)
+	t.Logf("after removed: %d\n", afterRemoved.HeapInuse)
+
+	require.True(t, edges[0].reallocate)
+	edges.reallocate()
+	runtime.GC()
+
+	runtime.ReadMemStats(&afterReallocate)
+	t.Logf("after reallocate: %d\n", afterReallocate.HeapInuse)
+
+	assert.Less(t, afterReallocate.HeapInuse, afterAdded.HeapInuse)
+	assert.Less(t, afterReallocate.HeapInuse, afterRemoved.HeapInuse)
+}

exp/orderbook/graph_test.go

@@ -463,22 +463,26 @@ func TestAddOffersOrderBook(t *testing.T) {
 				{
 					assetStringToID[usdAsset.String()],
 					makeVenues(quarterOffer, fiftyCentsOffer, dollarOffer),
+					false,
 				},
 				{
 					assetStringToID[eurAsset.String()],
 					makeVenues(eurOffer, twoEurOffer, threeEurOffer),
+					false,
 				},
 			},
 			{
 				{
 					assetStringToID[eurAsset.String()],
 					makeVenues(eurUsdOffer, otherEurUsdOffer),
+					false,
 				},
 			},
 			{
 				{
 					assetStringToID[usdAsset.String()],
 					makeVenues(usdEurOffer),
+					false,
 				},
 			},
 		},
@@ -488,20 +492,24 @@ func TestAddOffersOrderBook(t *testing.T) {
 				{
 					assetStringToID[eurAsset.String()],
 					makeVenues(usdEurOffer),
+					false,
 				},
 				{
 					assetStringToID[nativeAsset.String()],
 					makeVenues(quarterOffer, fiftyCentsOffer, dollarOffer),
+					false,
 				},
 			},
 			{
 				{
 					assetStringToID[usdAsset.String()],
 					makeVenues(eurUsdOffer, otherEurUsdOffer),
+					false,
 				},
 				{
 					assetStringToID[nativeAsset.String()],
 					makeVenues(eurOffer, twoEurOffer, threeEurOffer),
+					false,
 				},
 			},
 		},
@@ -705,22 +713,26 @@ func TestUpdateOfferOrderBook(t *testing.T) {
 				{
 					assetStringToID[usdAsset.String()],
 					makeVenues(quarterOffer, fiftyCentsOffer, dollarOffer),
+					false,
 				},
 				{
 					assetStringToID[eurAsset.String()],
 					makeVenues(eurOffer, twoEurOffer, threeEurOffer),
+					false,
 				},
 			},
 			{
 				{
 					assetStringToID[eurAsset.String()],
 					makeVenues(otherEurUsdOffer, eurUsdOffer),
+					false,
 				},
 			},
 			{
 				{
 					assetStringToID[usdAsset.String()],
 					makeVenues(usdEurOffer),
+					false,
 				},
 			},
 		},
@@ -730,20 +742,24 @@ func TestUpdateOfferOrderBook(t *testing.T) {
 				{
 					assetStringToID[nativeAsset.String()],
 					makeVenues(quarterOffer, fiftyCentsOffer, dollarOffer),
+					false,
 				},
 				{
 					assetStringToID[eurAsset.String()],
 					makeVenues(usdEurOffer),
+					false,
 				},
 			},
 			{
 				{
 					assetStringToID[nativeAsset.String()],
 					makeVenues(eurOffer, twoEurOffer, threeEurOffer),
+					false,
 				},
 				{
 					assetStringToID[usdAsset.String()],
 					makeVenues(otherEurUsdOffer, eurUsdOffer),
+					false,
 				},
 			},
 		},
@@ -876,16 +892,19 @@ func TestRemoveOfferOrderBook(t *testing.T) {
 				{
 					assetStringToID[usdAsset.String()],
 					makeVenues(quarterOffer, fiftyCentsOffer),
+					false,
 				},
 				{
 					assetStringToID[eurAsset.String()],
 					makeVenues(eurOffer, twoEurOffer, threeEurOffer),
+					false,
 				},
 			},
 			{
 				{
 					assetStringToID[eurAsset.String()],
 					makeVenues(eurUsdOffer),
+					false,
 				},
 			},
 			{},
@@ -896,16 +915,19 @@ func TestRemoveOfferOrderBook(t *testing.T) {
 				{
 					assetStringToID[nativeAsset.String()],
 					makeVenues(quarterOffer, fiftyCentsOffer),
+					false,
 				},
 			},
 			{
 				{
 					assetStringToID[nativeAsset.String()],
 					makeVenues(eurOffer, twoEurOffer, threeEurOffer),
+					false,
 				},
 				{
 					assetStringToID[usdAsset.String()],
 					makeVenues(eurUsdOffer),
+					false,
 				},
 			},
 		},