Apply suggestions from code reviews

Fix comments
Check if the span batch is empty in LogContext()
Check chain ID of the protected tx in newSpanBatchTxs()
Write SpanBatchType explicitly when encoding span batch

op-node/rollup/derive/batch.go

@@ -72,7 +72,7 @@ func (b *BatchData) encodeTyped(buf *bytes.Buffer) error {
 		buf.WriteByte(SingularBatchType)
 		return rlp.Encode(buf, &b.SingularBatch)
 	case SpanBatchType:
-		buf.WriteByte(byte(b.BatchType))
+		buf.WriteByte(SpanBatchType)
 		return b.RawSpanBatch.encode(buf)
 	default:
 		return fmt.Errorf("unrecognized batch type: %d", b.BatchType)

op-node/rollup/derive/batch_tob_test.go

@@ -10,7 +10,7 @@ import (
 
 // FuzzBatchRoundTrip executes a fuzz test similar to TestBatchRoundTrip, which tests that arbitrary BatchData will be
 // encoded and decoded without loss of its original values.
-// Does not test span batch because fuzzer is not aware structure of span batch.
+// Does not test the span batch type because the fuzzer is not aware of the structure of a span batch.
 func FuzzBatchRoundTrip(f *testing.F) {
 	f.Fuzz(func(t *testing.T, fuzzedData []byte) {
 		// Create our fuzzer wrapper to generate complex values

op-node/rollup/derive/span_batch.go

@@ -22,8 +22,8 @@ import (
 // SpanBatchType := 1
 // spanBatch := SpanBatchType ++ prefix ++ payload
 // prefix := rel_timestamp ++ l1_origin_num ++ parent_check ++ l1_origin_check
-// payload := payload = block_count ++ origin_bits ++ block_tx_counts ++ txs
-// txs = contract_creation_bits ++ y_parity_bits ++ tx_sigs ++ tx_tos ++ tx_datas ++ tx_nonces ++ tx_gases
+// payload := block_count ++ origin_bits ++ block_tx_counts ++ txs
+// txs := contract_creation_bits ++ y_parity_bits ++ tx_sigs ++ tx_tos ++ tx_datas ++ tx_nonces ++ tx_gases
 
 var ErrTooBigSpanBatchFieldSize = errors.New("batch would cause field bytes to go over limit")
 
@@ -433,6 +433,9 @@ func (b *SpanBatch) GetTimestamp() uint64 {
 
 // LogContext creates a new log context that contains information of the batch
 func (b *SpanBatch) LogContext(log log.Logger) log.Logger {
+	if len(b.batches) == 0 {
+		return log.New("block_count", 0)
+	}
 	return log.New(
 		"batch_timestamp", b.batches[0].Timestamp,
 		"parent_check", hexutil.Encode(b.parentCheck),

op-node/rollup/derive/span_batch_txs.go

@@ -432,6 +432,9 @@ func newSpanBatchTxs(txs [][]byte, chainID *big.Int) (*spanBatchTxs, error) {
 		if err := tx.UnmarshalBinary(txs[idx]); err != nil {
 			return nil, errors.New("failed to decode tx")
 		}
+		if tx.Protected() && tx.ChainId().Cmp(chainID) != 0 {
+			return nil, fmt.Errorf("protected tx has chain ID %d, but expected chain ID %d", tx.ChainId(), chainID)
+		}
 		var txSig spanBatchSignature
 		v, r, s := tx.RawSignatureValues()
 		R, _ := uint256.FromBig(r)