cmd/fetchpayload: add payload-building utility (#33919)

This PR adds a cmd tool fetchpayload which connects to a
node and gets all the information in order to create a serialized
payload that can then be passed to the zkvm.

cmd/fetchpayload/main.go

@@ -0,0 +1,177 @@
+// Copyright 2026 The go-ethereum Authors
+// This file is part of go-ethereum.
+//
+// go-ethereum is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// go-ethereum 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 General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with go-ethereum. If not, see <http://www.gnu.org/licenses/>.
+
+// fetchpayload queries an Ethereum node over RPC, fetches a block and its
+// execution witness, and writes the combined Payload (ChainID + Block +
+// Witness) to disk in the format consumed by cmd/keeper.
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"math/big"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+
+	"github.com/ethereum/go-ethereum/common/hexutil"
+	"github.com/ethereum/go-ethereum/core/stateless"
+	"github.com/ethereum/go-ethereum/core/types"
+	"github.com/ethereum/go-ethereum/ethclient"
+	"github.com/ethereum/go-ethereum/rlp"
+	"github.com/ethereum/go-ethereum/rpc"
+)
+
+// Payload is duplicated from cmd/keeper/main.go (package main, not importable).
+type Payload struct {
+	ChainID uint64
+	Block   *types.Block
+	Witness *stateless.Witness
+}
+
+func main() {
+	var (
+		rpcURL   = flag.String("rpc", "http://localhost:8545", "RPC endpoint URL")
+		blockArg = flag.String("block", "latest", `Block number: decimal, 0x-hex, or "latest"`)
+		format   = flag.String("format", "rlp", "Comma-separated output formats: rlp, hex, json")
+		outDir   = flag.String("out", "", "Output directory (default: current directory)")
+	)
+	flag.Parse()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	// Parse block number (nil means "latest" in ethclient).
+	blockNum, err := parseBlockNumber(*blockArg)
+	if err != nil {
+		fatal("invalid block number %q: %v", *blockArg, err)
+	}
+
+	// Connect to the node.
+	client, err := ethclient.DialContext(ctx, *rpcURL)
+	if err != nil {
+		fatal("failed to connect to %s: %v", *rpcURL, err)
+	}
+	defer client.Close()
+
+	chainID, err := client.ChainID(ctx)
+	if err != nil {
+		fatal("failed to get chain ID: %v", err)
+	}
+
+	// Fetch the block first so we have a concrete number for the witness call,
+	// avoiding a race where "latest" advances between the two RPCs.
+	block, err := client.BlockByNumber(ctx, blockNum)
+	if err != nil {
+		fatal("failed to fetch block: %v", err)
+	}
+	fmt.Printf("Fetched block %d (%#x)\n", block.NumberU64(), block.Hash())
+
+	// Fetch the execution witness via the debug namespace.
+	var extWitness stateless.ExtWitness
+	err = client.Client().CallContext(ctx, &extWitness, "debug_executionWitness", rpc.BlockNumber(block.NumberU64()))
+	if err != nil {
+		fatal("failed to fetch execution witness: %v", err)
+	}
+
+	witness := new(stateless.Witness)
+	err = witness.FromExtWitness(&extWitness)
+	if err != nil {
+		fatal("failed to convert witness: %v", err)
+	}
+
+	payload := Payload{
+		ChainID: chainID.Uint64(),
+		Block:   block,
+		Witness: witness,
+	}
+
+	// Encode payload as RLP (shared by "rlp" and "hex" formats).
+	rlpBytes, err := rlp.EncodeToBytes(payload)
+	if err != nil {
+		fatal("failed to RLP-encode payload: %v", err)
+	}
+
+	// Write one output file per requested format.
+	blockHex := fmt.Sprintf("%x", block.NumberU64())
+	for f := range strings.SplitSeq(*format, ",") {
+		f = strings.TrimSpace(f)
+		outPath := filepath.Join(*outDir, fmt.Sprintf("%s_payload.%s", blockHex, f))
+
+		var data []byte
+		switch f {
+		case "rlp":
+			data = rlpBytes
+		case "hex":
+			data = []byte(hexutil.Encode(rlpBytes))
+		case "json":
+			data, err = marshalJSONPayload(chainID, block, &extWitness)
+			if err != nil {
+				fatal("failed to JSON-encode payload: %v", err)
+			}
+		default:
+			fatal("unknown format %q (valid: rlp, hex, json)", f)
+		}
+
+		if err := os.WriteFile(outPath, data, 0644); err != nil {
+			fatal("failed to write %s: %v", outPath, err)
+		}
+		fmt.Printf("Wrote %s (%d bytes)\n", outPath, len(data))
+	}
+}
+
+// parseBlockNumber converts a CLI string to *big.Int.
+// Returns nil for "latest" (ethclient convention for the head block).
+func parseBlockNumber(s string) (*big.Int, error) {
+	if strings.EqualFold(s, "latest") {
+		return nil, nil
+	}
+	n := new(big.Int)
+	if strings.HasPrefix(s, "0x") || strings.HasPrefix(s, "0X") {
+		if _, ok := n.SetString(s[2:], 16); !ok {
+			return nil, fmt.Errorf("invalid hex number")
+		}
+		return n, nil
+	}
+	if _, ok := n.SetString(s, 10); !ok {
+		return nil, fmt.Errorf("invalid decimal number")
+	}
+	return n, nil
+}
+
+// jsonPayload is a JSON-friendly representation of Payload. It uses ExtWitness
+// instead of the internal Witness (which has no JSON marshaling).
+type jsonPayload struct {
+	ChainID uint64                `json:"chainId"`
+	Block   *types.Block          `json:"block"`
+	Witness *stateless.ExtWitness `json:"witness"`
+}
+
+func marshalJSONPayload(chainID *big.Int, block *types.Block, ext *stateless.ExtWitness) ([]byte, error) {
+	return json.MarshalIndent(jsonPayload{
+		ChainID: chainID.Uint64(),
+		Block:   block,
+		Witness: ext,
+	}, "", "  ")
+}
+
+func fatal(format string, args ...any) {
+	fmt.Fprintf(os.Stderr, format+"\n", args...)
+	os.Exit(1)
+}

core/stateless/encoding.go

@@ -40,8 +40,8 @@ func (w *Witness) ToExtWitness() *ExtWitness {
 	return ext
 }
 
-// fromExtWitness converts the consensus witness format into our internal one.
-func (w *Witness) fromExtWitness(ext *ExtWitness) error {
+// FromExtWitness converts the consensus witness format into our internal one.
+func (w *Witness) FromExtWitness(ext *ExtWitness) error {
 	w.Headers = ext.Headers
 
 	w.Codes = make(map[string]struct{}, len(ext.Codes))
@@ -66,7 +66,7 @@ func (w *Witness) DecodeRLP(s *rlp.Stream) error {
 	if err := s.Decode(&ext); err != nil {
 		return err
 	}
-	return w.fromExtWitness(&ext)
+	return w.FromExtWitness(&ext)
 }
 
 // ExtWitness is a witness RLP encoding for transferring across clients.