beacon/engine, rpc: add PremarshaledJSON for execution payload envelopes

Author: jrhea

beacon/engine/premarshal.go

@@ -0,0 +1,173 @@
+// Copyright 2026 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package engine
+
+import (
+	"encoding/hex"
+	"encoding/json"
+	"slices"
+
+	"github.com/ethereum/go-ethereum/common/hexutil"
+)
+
+// estimateBlobsBundleSize returns a rough estimate of the JSON size for a BlobsBundle.
+func estimateBlobsBundleSize(b *BlobsBundle) int {
+	size := 80 // JSON structure overhead
+	for _, blob := range b.Blobs {
+		size += len(blob)*2 + 6
+	}
+	for _, c := range b.Commitments {
+		size += len(c)*2 + 6
+	}
+	for _, p := range b.Proofs {
+		size += len(p)*2 + 6
+	}
+	return size
+}
+
+// marshalBlobsBundle writes BlobsBundle as JSON and appends it to buf.
+func marshalBlobsBundle(buf []byte, b *BlobsBundle) []byte {
+	buf = append(buf, `{"commitments":`...)
+	buf = marshalHexBytesArray(buf, b.Commitments)
+
+	buf = append(buf, `,"proofs":`...)
+	buf = marshalHexBytesArray(buf, b.Proofs)
+
+	buf = append(buf, `,"blobs":`...)
+	buf = marshalHexBytesArray(buf, b.Blobs)
+
+	buf = append(buf, '}')
+	return buf
+}
+
+// marshalHexBytesArray writes an array of hex-encoded byte slices to buf.
+func marshalHexBytesArray(buf []byte, items []hexutil.Bytes) []byte {
+	buf = append(buf, '[')
+	for i, item := range items {
+		if i > 0 {
+			buf = append(buf, ',')
+		}
+		buf = writeHexBytes(buf, item)
+	}
+	buf = append(buf, ']')
+	return buf
+}
+
+// writeHexBytes writes a hex-encoded byte slice as a JSON string ("0x...") to buf.
+// NOTE: This function avoids allocations by pre-allocating the buffer space needed;
+// otherwise, we would use hexutil.Encode() and append the result to the buffer.
+// hexutil.Encode() uses 64% more memory than writing to buffer directly.
+func writeHexBytes(buf []byte, data []byte) []byte {
+	buf = append(buf, '"', '0', 'x')
+	buf = slices.Grow(buf, len(data)*2+1)
+	cur := len(buf)
+	buf = buf[:cur+len(data)*2]
+	hex.Encode(buf[cur:], data)
+	buf = append(buf, '"')
+	return buf
+}
+
+// PremarshaledJSON implements rpc.JSONPremarshaled. It returns pre-serialized
+// JSON by delegating small fields to their existing MarshalJSON methods and
+// hand-rolling only the BlobsBundle.
+func (e ExecutionPayloadEnvelope) PremarshaledJSON() ([]byte, error) {
+	// Marshal the execution payload using its gencodec MarshalJSON.
+	payload, err := e.ExecutionPayload.MarshalJSON()
+	if err != nil {
+		return nil, err
+	}
+
+	// Marshal the block value.
+	blockValue, err := json.Marshal((*hexutil.Big)(e.BlockValue))
+	if err != nil {
+		return nil, err
+	}
+
+	// Marshal the execution requests.
+	var requests []byte
+	if e.Requests != nil {
+		hexRequests := make([]hexutil.Bytes, len(e.Requests))
+		for i, req := range e.Requests {
+			hexRequests[i] = req
+		}
+		requests, err = json.Marshal(hexRequests)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	// Marshal the override.
+	override, err := json.Marshal(e.Override)
+	if err != nil {
+		return nil, err
+	}
+
+	// Marshal the witness.
+	var witness []byte
+	if e.Witness != nil {
+		witness, err = json.Marshal(e.Witness)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	// Estimate buffer size.
+	size := len(payload) + len(blockValue) + len(requests) + len(override) + len(witness)
+	if e.BlobsBundle != nil {
+		size += estimateBlobsBundleSize(e.BlobsBundle)
+	}
+	size += 128 // JSON bloat (keys, braces, commas, etc.)
+	buf := make([]byte, 0, size)
+
+	// Write the execution payload to the buffer
+	buf = append(buf, `{"executionPayload":`...)
+	buf = append(buf, payload...)
+
+	// Write the block value to the buffer
+	buf = append(buf, `,"blockValue":`...)
+	buf = append(buf, blockValue...)
+
+	// Write the blobs bundle to the buffer
+	buf = append(buf, `,"blobsBundle":`...)
+	if e.BlobsBundle != nil {
+		buf = marshalBlobsBundle(buf, e.BlobsBundle)
+	} else {
+		buf = append(buf, "null"...)
+	}
+
+	// Write the execution requests to the buffer
+	buf = append(buf, `,"executionRequests":`...)
+	if requests != nil {
+		buf = append(buf, requests...)
+	} else {
+		buf = append(buf, "null"...)
+	}
+
+	// Write the override to the buffer
+	buf = append(buf, `,"shouldOverrideBuilder":`...)
+	buf = append(buf, override...)
+
+	// Write the witness to the buffer if present
+	if witness != nil {
+		buf = append(buf, `,"witness":`...)
+		buf = append(buf, witness...)
+	}
+
+	// Close the envelope
+	buf = append(buf, '}')
+	return buf, nil
+}