dao: fix transaction application log decoding

Conflict record stub has value of 5 bytes length: 1 byte for
storage.ExecTransaction prefix and 4 bytes for the block index LE. This
scheme was implemented in #3138, and this commit should be a part of
this PR.

Also, transaction.DummyVersion is removed since it's unused anymore.

Close #3426. The reason of `failed to locate application log: EOF` error
during genesis AER request is in the following: genesis executable was
overwritten by conflict record stub produced by transaction
0x289c235dcdab8be7426d05f0fbb5e86c619f81481ea136493fa95deee5dbb7cc (ref.
 #3427). As a consequence, an attempt to decode transaction AER was
initited, but conflict record scheme was changed in #3138.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

pkg/core/dao/dao.go

@@ -323,7 +323,7 @@ func (dao *Simple) GetTxExecResult(hash util.Uint256) (uint32, *transaction.Tran
 // decodeTxAndExecResult decodes transaction, its height and execution result from
 // the given executable bytes. It performs no executable prefix check.
 func decodeTxAndExecResult(buf []byte) (uint32, *transaction.Transaction, *state.AppExecResult, error) {
-	if len(buf) >= 6 && buf[5] == transaction.DummyVersion {
+	if len(buf) == 1+4 { // conflict record stub.
 		return 0, nil, nil, storage.ErrKeyNotFound
 	}
 	r := io.NewBinReaderFromBuf(buf)
@@ -605,7 +605,7 @@ func (dao *Simple) DeleteHeaderHashes(since uint32, batchSize int) {
 }
 
 // GetTransaction returns Transaction and its height by the given hash
-// if it exists in the store. It does not return dummy transactions.
+// if it exists in the store. It does not return conflict record stubs.
 func (dao *Simple) GetTransaction(hash util.Uint256) (*transaction.Transaction, uint32, error) {
 	key := dao.makeExecutableKey(hash)
 	b, err := dao.Store.Get(key)
@@ -844,8 +844,9 @@ func (dao *Simple) StoreAsCurrentBlock(block *block.Block) {
 	dao.Store.Put(dao.mkKeyPrefix(storage.SYSCurrentBlock), buf.Bytes())
 }
 
-// StoreAsTransaction stores the given TX as DataTransaction. It also stores transactions
-// the given tx has conflicts with as DataTransaction with dummy version. It can reuse the given
+// StoreAsTransaction stores the given TX as DataTransaction. It also stores conflict records
+// (hashes of transactions the given tx has conflicts with) as DataTransaction with value containing
+// only five bytes: 1-byte [storage.ExecTransaction] executable prefix + 4-bytes-LE block index. It can reuse the given
 // buffer for the purpose of value serialization.
 func (dao *Simple) StoreAsTransaction(tx *transaction.Transaction, index uint32, aer *state.AppExecResult) error {
 	key := dao.makeExecutableKey(tx.Hash())

pkg/core/transaction/transaction.go

@@ -26,8 +26,6 @@ const (
 	// MaxAttributes is maximum number of attributes including signers that can be contained
 	// within a transaction. It is set to be 16.
 	MaxAttributes = 16
-	// DummyVersion represents reserved transaction version for trimmed transactions.
-	DummyVersion = 255
 )
 
 // ErrInvalidWitnessNum returns when the number of witnesses does not match signers.
@@ -408,7 +406,7 @@ var (
 
 // isValid checks whether decoded/unmarshalled transaction has all fields valid.
 func (t *Transaction) isValid() error {
-	if t.Version > 0 && t.Version != DummyVersion {
+	if t.Version > 0 {
 		return ErrInvalidVersion
 	}
 	if t.SystemFee < 0 {