Rejects empty transactions in the example kvstore (#9823) (#9841)

* Rejects empty transactions in the example kvstore

* Add code for rejected transaction; Add test for txn rejection;

* Apply suggestions from code review

Co-authored-by: Sergio Mena <sergio@informal.systems>
(cherry picked from commit 49502dae92)

Co-authored-by: Lasaro Camargos <lasaro@informal.systems>

abci/example/code/code.go

@@ -8,4 +8,5 @@ const (
 	CodeTypeUnauthorized  uint32 = 3
 	CodeTypeUnknownError  uint32 = 4
 	CodeTypeExecuted      uint32 = 5
+	CodeTypeRejected      uint32 = 6
 )

abci/example/kvstore/kvstore.go

@@ -122,6 +122,10 @@ func (app *Application) DeliverTx(req types.RequestDeliverTx) types.ResponseDeli
 }
 
 func (app *Application) CheckTx(req types.RequestCheckTx) types.ResponseCheckTx {
+	if len(req.Tx) == 0 {
+		return types.ResponseCheckTx{Code: code.CodeTypeRejected}
+	}
+
 	if req.Type == types.CheckTxType_Recheck {
 		if _, ok := app.txToRemove[string(req.Tx)]; ok {
 			return types.ResponseCheckTx{Code: code.CodeTypeExecuted, GasWanted: 1}

abci/example/kvstore/kvstore_test.go

@@ -70,6 +70,24 @@ func TestKVStoreKV(t *testing.T) {
 	testKVStore(t, kvstore, tx, key, value)
 }
 
+func TestPersistentKVStoreEmptyTX(t *testing.T) {
+	dir, err := os.MkdirTemp("/tmp", "abci-kvstore-test") // TODO
+	if err != nil {
+		t.Fatal(err)
+	}
+	kvstore := NewPersistentKVStoreApplication(dir)
+	tx := []byte("")
+	reqCheck := types.RequestCheckTx{Tx: tx}
+	resCheck := kvstore.CheckTx(reqCheck)
+	require.Equal(t, resCheck.Code, code.CodeTypeRejected)
+
+	txs := make([][]byte, 0, 4)
+	txs = append(txs, []byte("key=value"), []byte("key"), []byte(""), []byte("kee=value"))
+	reqPrepare := types.RequestPrepareProposal{Txs: txs, MaxTxBytes: 10 * 1024}
+	resPrepare := kvstore.PrepareProposal(reqPrepare)
+	require.Equal(t, len(reqPrepare.Txs), len(resPrepare.Txs)+1, "Empty transaction not properly removed")
+}
+
 func TestPersistentKVStoreKV(t *testing.T) {
 	dir, err := os.MkdirTemp("/tmp", "abci-kvstore-test") // TODO
 	if err != nil {

abci/example/kvstore/persistent_kvstore.go

@@ -324,11 +324,15 @@ func (app *PersistentKVStoreApplication) execPrepareTx(tx []byte) types.Response
 }
 
 // substPrepareTx substitutes all the transactions prefixed with 'prepare' in the
-// proposal for transactions with the prefix stripped.
+// proposal for transactions with the prefix stripped, while discarding invalid empty transactions.
 func (app *PersistentKVStoreApplication) substPrepareTx(blockData [][]byte, maxTxBytes int64) [][]byte {
 	txs := make([][]byte, 0, len(blockData))
 	var totalBytes int64
 	for _, tx := range blockData {
+		if len(tx) == 0 {
+			continue
+		}
+
 		txMod := tx
 		if isPrepareTx(tx) {
 			txMod = bytes.Replace(tx, []byte(PreparePrefix), []byte(ReplacePrefix), 1)