op-node: Limit decompressed size in channel_out

This ensures that the channel creator respects the limits used in
derivation. This returns a sentinel error if the batch would make
the channel too large.

op-node/rollup/derive/channel_out.go

@@ -5,6 +5,7 @@ import (
 	"compress/zlib"
 	"crypto/rand"
 	"errors"
+	"fmt"
 	"io"
 
 	"github.com/ethereum-optimism/optimism/op-node/rollup"
@@ -14,15 +15,14 @@ import (
 )
 
 var ErrNotDepositTx = errors.New("first transaction in block is not a deposit tx")
+var ErrTooManyRLPBytes = errors.New("batch would cause RLP bytes to go over limit")
 
 type ChannelOut struct {
 	id ChannelID
 	// Frame ID of the next frame to emit. Increment after emitting
 	frame uint64
-	// How much we've pulled from the reader so far
-	offset uint64
-	// scratch for temporary buffering
-	scratch bytes.Buffer
+	// rlpLength is the uncompressed size of the channel. Must be less than MAX_RLP_BYTES_PER_CHANNEL
+	rlpLength int
 
 	// Compressor stage. Write input data to it
 	compress *zlib.Writer
@@ -38,9 +38,9 @@ func (co *ChannelOut) ID() ChannelID {
 
 func NewChannelOut() (*ChannelOut, error) {
 	c := &ChannelOut{
-		id:     ChannelID{}, // TODO: use GUID here instead of fully random data
-		frame:  0,
-		offset: 0,
+		id:        ChannelID{}, // TODO: use GUID here instead of fully random data
+		frame:     0,
+		rlpLength: 0,
 	}
 	_, err := rand.Read(c.id[:])
 	if err != nil {
@@ -59,9 +59,8 @@ func NewChannelOut() (*ChannelOut, error) {
 // TODO: reuse ChannelOut for performance
 func (co *ChannelOut) Reset() error {
 	co.frame = 0
-	co.offset = 0
+	co.rlpLength = 0
 	co.buf.Reset()
-	co.scratch.Reset()
 	co.compress.Reset(&co.buf)
 	co.closed = false
 	_, err := rand.Read(co.id[:])
@@ -71,11 +70,32 @@ func (co *ChannelOut) Reset() error {
 	return nil
 }
 
+// AddBlock adds a block to the channel. It returns an error
+// if there is a problem adding the block. The only sentinel
+// error that it returns is ErrTooManyRLPBytes. If this error
+// is returned, the channel should be closed and a new one
+// should be made.
 func (co *ChannelOut) AddBlock(block *types.Block) error {
 	if co.closed {
 		return errors.New("already closed")
 	}
-	return blockToBatch(block, co.compress)
+	batch, err := blockToBatch(block)
+	if err != nil {
+		return err
+	}
+	// We encode to a temporary buffer to determine the encoded length to
+	// ensure that the total size of all RLP elements is less than MAX_RLP_BYTES_PER_CHANNEL
+	var buf bytes.Buffer
+	if err := rlp.Encode(&buf, batch); err != nil {
+		return err
+	}
+	if co.rlpLength+buf.Len() > MaxRLPBytesPerChannel {
+		return ErrTooManyRLPBytes
+	}
+	co.rlpLength += buf.Len()
+
+	_, err = io.Copy(co.compress, &buf)
+	return err
 }
 
 // ReadyBytes returns the number of bytes that the channel out can immediately output into a frame.
@@ -141,35 +161,35 @@ func (co *ChannelOut) OutputFrame(w *bytes.Buffer, maxSize uint64) error {
 	}
 }
 
-// blockToBatch writes the raw block bytes (after batch encoding) to the writer
-func blockToBatch(block *types.Block, w io.Writer) error {
+// blockToBatch transforms a block into a batch object that can easily be RLP encoded.
+func blockToBatch(block *types.Block) (*BatchData, error) {
 	var opaqueTxs []hexutil.Bytes
-	for _, tx := range block.Transactions() {
+	for i, tx := range block.Transactions() {
 		if tx.Type() == types.DepositTxType {
 			continue
 		}
 		otx, err := tx.MarshalBinary()
 		if err != nil {
-			return err // TODO: wrap err
+			return nil, fmt.Errorf("could not encode tx %v in block %v: %w", i, tx.Hash(), err)
 		}
 		opaqueTxs = append(opaqueTxs, otx)
 	}
 	l1InfoTx := block.Transactions()[0]
 	if l1InfoTx.Type() != types.DepositTxType {
-		return ErrNotDepositTx
+		return nil, ErrNotDepositTx
 	}
 	l1Info, err := L1InfoDepositTxData(l1InfoTx.Data())
 	if err != nil {
-		return err // TODO: wrap err
+		return nil, fmt.Errorf("could not parse the L1 Info deposit: %w", err)
 	}
 
-	batch := &BatchData{BatchV1{
-		ParentHash:   block.ParentHash(),
-		EpochNum:     rollup.Epoch(l1Info.Number),
-		EpochHash:    l1Info.BlockHash,
-		Timestamp:    block.Time(),
-		Transactions: opaqueTxs,
-	},
-	}
-	return rlp.Encode(w, batch)
+	return &BatchData{
+		BatchV1{
+			ParentHash:   block.ParentHash(),
+			EpochNum:     rollup.Epoch(l1Info.Number),
+			EpochHash:    l1Info.BlockHash,
+			Timestamp:    block.Time(),
+			Transactions: opaqueTxs,
+		},
+	}, nil
 }