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all: on-chain oracle checkpoint syncing (#19543)

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* all: implement simple checkpoint syncing

cmd, les, node: remove callback mechanism

cmd, node: remove callback definition

les: simplify the registrar

les: expose checkpoint rpc services in the light client

les, light: don't store untrusted receipt

cmd, contracts, les: discard stale checkpoint

cmd, contracts/registrar: loose restriction of registeration

cmd, contracts: add replay-protection

all: off-chain multi-signature contract

params: deploy checkpoint contract for rinkeby

cmd/registrar: add raw signing mode for registrar

cmd/registrar, contracts/registrar, les: fixed messages

* cmd/registrar, contracts/registrar: fix lints

* accounts/abi/bind, les: address comments

* cmd, contracts, les, light, params: minor checkpoint sync cleanups

* cmd, eth, les, light: move checkpoint config to config file

* cmd, eth, les, params: address comments

* eth, les, params: address comments

* cmd: polish up the checkpoint admin CLI

* cmd, contracts, params: deploy new version contract

* cmd/checkpoint-admin: add another flag for clef mode signing

* cmd, contracts, les: rename and regen checkpoint oracle with abigen
This commit is contained in:
gary rong
2019-06-28 15:34:02 +08:00
committed by Péter Szilágyi
parent 702f52fb99
commit f7cdea2bdc
49 changed files with 2861 additions and 383 deletions
Download Patch File Download Diff File Expand all files Collapse all files

166
cmd/checkpoint-admin/common.go Normal file
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// Copyright 2018 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/>.
package main
import (
"io/ioutil"
"strconv"
"strings"
"github.com/ethereum/go-ethereum/accounts/keystore"
"github.com/ethereum/go-ethereum/cmd/utils"
"github.com/ethereum/go-ethereum/common"
"github.com/ethereum/go-ethereum/console"
"github.com/ethereum/go-ethereum/contracts/checkpointoracle"
"github.com/ethereum/go-ethereum/ethclient"
"github.com/ethereum/go-ethereum/params"
"github.com/ethereum/go-ethereum/rpc"
"gopkg.in/urfave/cli.v1"
)
// newClient creates a client with specified remote URL.
func newClient(ctx *cli.Context) *ethclient.Client {
client, err := ethclient.Dial(ctx.GlobalString(nodeURLFlag.Name))
if err != nil {
utils.Fatalf("Failed to connect to Ethereum node: %v", err)
}
return client
}
// newRPCClient creates a rpc client with specified node URL.
func newRPCClient(url string) *rpc.Client {
client, err := rpc.Dial(url)
if err != nil {
utils.Fatalf("Failed to connect to Ethereum node: %v", err)
}
return client
}
// getContractAddr retrieves the register contract address through
// rpc request.
func getContractAddr(client *rpc.Client) common.Address {
var addr string
if err := client.Call(&addr, "les_getCheckpointContractAddress"); err != nil {
utils.Fatalf("Failed to fetch checkpoint oracle address: %v", err)
}
return common.HexToAddress(addr)
}
// getCheckpoint retrieves the specified checkpoint or the latest one
// through rpc request.
func getCheckpoint(ctx *cli.Context, client *rpc.Client) *params.TrustedCheckpoint {
var checkpoint *params.TrustedCheckpoint
if ctx.GlobalIsSet(indexFlag.Name) {
var result [3]string
index := uint64(ctx.GlobalInt64(indexFlag.Name))
if err := client.Call(&result, "les_getCheckpoint", index); err != nil {
utils.Fatalf("Failed to get local checkpoint %v, please ensure the les API is exposed", err)
}
checkpoint = &params.TrustedCheckpoint{
SectionIndex: index,
SectionHead: common.HexToHash(result[0]),
CHTRoot: common.HexToHash(result[1]),
BloomRoot: common.HexToHash(result[2]),
}
} else {
var result [4]string
err := client.Call(&result, "les_latestCheckpoint")
if err != nil {
utils.Fatalf("Failed to get local checkpoint %v, please ensure the les API is exposed", err)
}
index, err := strconv.ParseUint(result[0], 0, 64)
if err != nil {
utils.Fatalf("Failed to parse checkpoint index %v", err)
}
checkpoint = &params.TrustedCheckpoint{
SectionIndex: index,
SectionHead: common.HexToHash(result[1]),
CHTRoot: common.HexToHash(result[2]),
BloomRoot: common.HexToHash(result[3]),
}
}
return checkpoint
}
// newContract creates a registrar contract instance with specified
// contract address or the default contracts for mainnet or testnet.
func newContract(client *rpc.Client) (common.Address, *checkpointoracle.CheckpointOracle) {
addr := getContractAddr(client)
if addr == (common.Address{}) {
utils.Fatalf("No specified registrar contract address")
}
contract, err := checkpointoracle.NewCheckpointOracle(addr, ethclient.NewClient(client))
if err != nil {
utils.Fatalf("Failed to setup registrar contract %s: %v", addr, err)
}
return addr, contract
}
// promptPassphrase prompts the user for a passphrase.
// Set confirmation to true to require the user to confirm the passphrase.
func promptPassphrase(confirmation bool) string {
passphrase, err := console.Stdin.PromptPassword("Passphrase: ")
if err != nil {
utils.Fatalf("Failed to read passphrase: %v", err)
}
if confirmation {
confirm, err := console.Stdin.PromptPassword("Repeat passphrase: ")
if err != nil {
utils.Fatalf("Failed to read passphrase confirmation: %v", err)
}
if passphrase != confirm {
utils.Fatalf("Passphrases do not match")
}
}
return passphrase
}
// getPassphrase obtains a passphrase given by the user. It first checks the
// --password command line flag and ultimately prompts the user for a
// passphrase.
func getPassphrase(ctx *cli.Context) string {
passphraseFile := ctx.String(utils.PasswordFileFlag.Name)
if passphraseFile != "" {
content, err := ioutil.ReadFile(passphraseFile)
if err != nil {
utils.Fatalf("Failed to read passphrase file '%s': %v",
passphraseFile, err)
}
return strings.TrimRight(string(content), "\r\n")
}
// Otherwise prompt the user for the passphrase.
return promptPassphrase(false)
}
// getKey retrieves the user key through specified key file.
func getKey(ctx *cli.Context) *keystore.Key {
// Read key from file.
keyFile := ctx.GlobalString(keyFileFlag.Name)
keyJson, err := ioutil.ReadFile(keyFile)
if err != nil {
utils.Fatalf("Failed to read the keyfile at '%s': %v", keyFile, err)
}
// Decrypt key with passphrase.
passphrase := getPassphrase(ctx)
key, err := keystore.DecryptKey(keyJson, passphrase)
if err != nil {
utils.Fatalf("Failed to decrypt user key '%s': %v", keyFile, err)
}
return key
}

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cmd/checkpoint-admin/exec.go Normal file
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// Copyright 2018 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/>.
package main
import (
"bytes"
"context"
"encoding/binary"
"fmt"
"math/big"
"strings"
"time"
"github.com/ethereum/go-ethereum/accounts"
"github.com/ethereum/go-ethereum/accounts/abi/bind"
"github.com/ethereum/go-ethereum/cmd/utils"
"github.com/ethereum/go-ethereum/common"
"github.com/ethereum/go-ethereum/common/hexutil"
"github.com/ethereum/go-ethereum/contracts/checkpointoracle"
"github.com/ethereum/go-ethereum/contracts/checkpointoracle/contract"
"github.com/ethereum/go-ethereum/crypto"
"github.com/ethereum/go-ethereum/ethclient"
"github.com/ethereum/go-ethereum/log"
"github.com/ethereum/go-ethereum/params"
"github.com/ethereum/go-ethereum/rpc"
"gopkg.in/urfave/cli.v1"
)
var commandDeploy = cli.Command{
Name: "deploy",
Usage: "Deploy a new checkpoint oracle contract",
Flags: []cli.Flag{
nodeURLFlag,
clefURLFlag,
signersFlag,
thresholdFlag,
keyFileFlag,
utils.PasswordFileFlag,
},
Action: utils.MigrateFlags(deploy),
}
var commandSign = cli.Command{
Name: "sign",
Usage: "Sign the checkpoint with the specified key",
Flags: []cli.Flag{
nodeURLFlag,
clefURLFlag,
indexFlag,
hashFlag,
oracleFlag,
keyFileFlag,
signerFlag,
utils.PasswordFileFlag,
},
Action: utils.MigrateFlags(sign),
}
var commandPublish = cli.Command{
Name: "publish",
Usage: "Publish a checkpoint into the oracle",
Flags: []cli.Flag{
nodeURLFlag,
indexFlag,
signaturesFlag,
keyFileFlag,
utils.PasswordFileFlag,
},
Action: utils.MigrateFlags(publish),
}
// deploy deploys the checkpoint registrar contract.
//
// Note the network where the contract is deployed depends on
// the network where the connected node is located.
func deploy(ctx *cli.Context) error {
// Gather all the addresses that should be permitted to sign
var addrs []common.Address
for _, account := range strings.Split(ctx.String(signersFlag.Name), ",") {
if trimmed := strings.TrimSpace(account); !common.IsHexAddress(trimmed) {
utils.Fatalf("Invalid account in --signers: '%s'", trimmed)
}
addrs = append(addrs, common.HexToAddress(account))
}
// Retrieve and validate the signing threshold
needed := ctx.Int(thresholdFlag.Name)
if needed == 0 || needed > len(addrs) {
utils.Fatalf("Invalid signature threshold %d", needed)
}
// Print a summary to ensure the user understands what they're signing
fmt.Printf("Deploying new checkpoint oracle:\n\n")
for i, addr := range addrs {
fmt.Printf("Admin %d => %s\n", i+1, addr.Hex())
}
fmt.Printf("\nSignatures needed to publish: %d\n", needed)
// Retrieve the private key, create an abigen transactor and an RPC client
transactor := bind.NewKeyedTransactor(getKey(ctx).PrivateKey)
client := newClient(ctx)
// Deploy the checkpoint oracle
oracle, tx, _, err := contract.DeployCheckpointOracle(transactor, client, addrs, big.NewInt(int64(params.CheckpointFrequency)),
big.NewInt(int64(params.CheckpointProcessConfirmations)), big.NewInt(int64(needed)))
if err != nil {
utils.Fatalf("Failed to deploy checkpoint oracle %v", err)
}
log.Info("Deployed checkpoint oracle", "address", oracle, "tx", tx.Hash().Hex())
return nil
}
// sign creates the signature for specific checkpoint
// with local key. Only contract admins have the permission to
// sign checkpoint.
func sign(ctx *cli.Context) error {
var (
offline bool // The indicator whether we sign checkpoint by offline.
chash common.Hash
cindex uint64
address common.Address
node *rpc.Client
oracle *checkpointoracle.CheckpointOracle
)
if !ctx.GlobalIsSet(nodeURLFlag.Name) {
// Offline mode signing
offline = true
if !ctx.IsSet(hashFlag.Name) {
utils.Fatalf("Please specify the checkpoint hash (--hash) to sign in offline mode")
}
chash = common.HexToHash(ctx.String(hashFlag.Name))
if !ctx.IsSet(indexFlag.Name) {
utils.Fatalf("Please specify checkpoint index (--index) to sign in offline mode")
}
cindex = ctx.Uint64(indexFlag.Name)
if !ctx.IsSet(oracleFlag.Name) {
utils.Fatalf("Please specify oracle address (--oracle) to sign in offline mode")
}
address = common.HexToAddress(ctx.String(oracleFlag.Name))
} else {
// Interactive mode signing, retrieve the data from the remote node
node = newRPCClient(ctx.GlobalString(nodeURLFlag.Name))
checkpoint := getCheckpoint(ctx, node)
chash = checkpoint.Hash()
cindex = checkpoint.SectionIndex
address = getContractAddr(node)
// Check the validity of checkpoint
reqCtx, cancelFn := context.WithTimeout(context.Background(), 10*time.Second)
defer cancelFn()
head, err := ethclient.NewClient(node).HeaderByNumber(reqCtx, nil)
if err != nil {
return err
}
num := head.Number.Uint64()
if num < ((cindex+1)*params.CheckpointFrequency + params.CheckpointProcessConfirmations) {
utils.Fatalf("Invalid future checkpoint")
}
_, oracle = newContract(node)
latest, _, h, err := oracle.Contract().GetLatestCheckpoint(nil)
if err != nil {
return err
}
if cindex < latest {
utils.Fatalf("Checkpoint is too old")
}
if cindex == latest && (latest != 0 || h.Uint64() != 0) {
utils.Fatalf("Stale checkpoint, latest registered %d, given %d", latest, cindex)
}
}
var (
signature string
signer string
)
// isAdmin checks whether the specified signer is admin.
isAdmin := func(addr common.Address) error {
signers, err := oracle.Contract().GetAllAdmin(nil)
if err != nil {
return err
}
for _, s := range signers {
if s == addr {
return nil
}
}
return fmt.Errorf("signer %v is not the admin", addr.Hex())
}
// Print to the user the data thy are about to sign
fmt.Printf("Oracle => %s\n", address.Hex())
fmt.Printf("Index %4d => %s\n", cindex, chash.Hex())
switch {
case ctx.GlobalIsSet(clefURLFlag.Name):
// Sign checkpoint in clef mode.
signer = ctx.String(signerFlag.Name)
if !offline {
if err := isAdmin(common.HexToAddress(signer)); err != nil {
return err
}
}
clef := newRPCClient(ctx.GlobalString(clefURLFlag.Name))
p := make(map[string]string)
buf := make([]byte, 8)
binary.BigEndian.PutUint64(buf, cindex)
p["address"] = address.Hex()
p["message"] = hexutil.Encode(append(buf, chash.Bytes()...))
if err := clef.Call(&signature, "account_signData", accounts.MimetypeDataWithValidator, signer, p); err != nil {
utils.Fatalf("Failed to sign checkpoint, err %v", err)
}
case ctx.GlobalIsSet(keyFileFlag.Name):
// Sign checkpoint in raw private key file mode.
key := getKey(ctx)
signer = key.Address.Hex()
if !offline {
if err := isAdmin(key.Address); err != nil {
return err
}
}
sig, err := crypto.Sign(sighash(cindex, address, chash), key.PrivateKey)
if err != nil {
utils.Fatalf("Failed to sign checkpoint, err %v", err)
}
sig[64] += 27 // Transform V from 0/1 to 27/28 according to the yellow paper
signature = common.Bytes2Hex(sig)
default:
utils.Fatalf("Please specify clef URL or private key file path to sign checkpoint")
}
fmt.Printf("Signer => %s\n", signer)
fmt.Printf("Signature => %s\n", signature)
return nil
}
// sighash calculates the hash of the data to sign for the checkpoint oracle.
func sighash(index uint64, oracle common.Address, hash common.Hash) []byte {
buf := make([]byte, 8)
binary.BigEndian.PutUint64(buf, index)
data := append([]byte{0x19, 0x00}, append(oracle[:], append(buf, hash[:]...)...)...)
return crypto.Keccak256(data)
}
// ecrecover calculates the sender address from a sighash and signature combo.
func ecrecover(sighash []byte, sig []byte) common.Address {
sig[64] -= 27
defer func() { sig[64] += 27 }()
signer, err := crypto.SigToPub(sighash, sig)
if err != nil {
utils.Fatalf("Failed to recover sender from signature %x: %v", sig, err)
}
return crypto.PubkeyToAddress(*signer)
}
// publish registers the specified checkpoint which generated by connected node
// with a authorised private key.
func publish(ctx *cli.Context) error {
// Print the checkpoint oracle's current status to make sure we're interacting
// with the correct network and contract.
status(ctx)
// Gather the signatures from the CLI
var sigs [][]byte
for _, sig := range strings.Split(ctx.String(signaturesFlag.Name), ",") {
trimmed := strings.TrimPrefix(strings.TrimSpace(sig), "0x")
if len(trimmed) != 130 {
utils.Fatalf("Invalid signature in --signature: '%s'", trimmed)
} else {
sigs = append(sigs, common.Hex2Bytes(trimmed))
}
}
// Retrieve the checkpoint we want to sign to sort the signatures
var (
client = newRPCClient(ctx.GlobalString(nodeURLFlag.Name))
addr, oracle = newContract(client)
checkpoint = getCheckpoint(ctx, client)
sighash = sighash(checkpoint.SectionIndex, addr, checkpoint.Hash())
)
for i := 0; i < len(sigs); i++ {
for j := i + 1; j < len(sigs); j++ {
signerA := ecrecover(sighash, sigs[i])
signerB := ecrecover(sighash, sigs[j])
if bytes.Compare(signerA.Bytes(), signerB.Bytes()) > 0 {
sigs[i], sigs[j] = sigs[j], sigs[i]
}
}
}
// Retrieve recent header info to protect replay attack
reqCtx, cancelFn := context.WithTimeout(context.Background(), 10*time.Second)
defer cancelFn()
head, err := ethclient.NewClient(client).HeaderByNumber(reqCtx, nil)
if err != nil {
return err
}
num := head.Number.Uint64()
recent, err := ethclient.NewClient(client).HeaderByNumber(reqCtx, big.NewInt(int64(num-128)))
if err != nil {
return err
}
// Print a summary of the operation that's going to be performed
fmt.Printf("Publishing %d => %s:\n\n", checkpoint.SectionIndex, checkpoint.Hash().Hex())
for i, sig := range sigs {
fmt.Printf("Signer %d => %s\n", i+1, ecrecover(sighash, sig).Hex())
}
fmt.Println()
fmt.Printf("Sentry number => %d\nSentry hash => %s\n", recent.Number, recent.Hash().Hex())
// Publish the checkpoint into the oracle
tx, err := oracle.RegisterCheckpoint(getKey(ctx).PrivateKey, checkpoint.SectionIndex, checkpoint.Hash().Bytes(), recent.Number, recent.Hash(), sigs)
if err != nil {
utils.Fatalf("Register contract failed %v", err)
}
log.Info("Successfully registered checkpoint", "tx", tx.Hash().Hex())
return nil
}

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cmd/checkpoint-admin/main.go Normal file
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// Copyright 2018 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/>.
// checkpoint-admin is a utility that can be used to query checkpoint information
// and register stable checkpoints into an oracle contract.
package main
import (
"fmt"
"os"
"github.com/ethereum/go-ethereum/cmd/utils"
"github.com/ethereum/go-ethereum/common/fdlimit"
"github.com/ethereum/go-ethereum/log"
"gopkg.in/urfave/cli.v1"
)
const (
commandHelperTemplate = `{{.Name}}{{if .Subcommands}} command{{end}}{{if .Flags}} [command options]{{end}} [arguments...]
{{if .Description}}{{.Description}}
{{end}}{{if .Subcommands}}
SUBCOMMANDS:
{{range .Subcommands}}{{.Name}}{{with .ShortName}}, {{.}}{{end}}{{ "\t" }}{{.Usage}}
{{end}}{{end}}{{if .Flags}}
OPTIONS:
{{range $.Flags}}{{"\t"}}{{.}}
{{end}}
{{end}}`
)
var (
// Git SHA1 commit hash of the release (set via linker flags)
gitCommit = ""
gitDate = ""
)
var app *cli.App
func init() {
app = utils.NewApp(gitCommit, gitDate, "ethereum checkpoint helper tool")
app.Commands = []cli.Command{
commandStatus,
commandDeploy,
commandSign,
commandPublish,
}
app.Flags = []cli.Flag{
oracleFlag,
keyFileFlag,
nodeURLFlag,
clefURLFlag,
utils.PasswordFileFlag,
}
cli.CommandHelpTemplate = commandHelperTemplate
}
// Commonly used command line flags.
var (
indexFlag = cli.Int64Flag{
Name: "index",
Usage: "Checkpoint index (query latest from remote node if not specified)",
}
hashFlag = cli.StringFlag{
Name: "hash",
Usage: "Checkpoint hash (query latest from remote node if not specified)",
}
oracleFlag = cli.StringFlag{
Name: "oracle",
Usage: "Checkpoint oracle address (query from remote node if not specified)",
}
thresholdFlag = cli.Int64Flag{
Name: "threshold",
Usage: "Minimal number of signatures required to approve a checkpoint",
}
keyFileFlag = cli.StringFlag{
Name: "keyfile",
Usage: "The private key file (keyfile signature is not recommended)",
}
nodeURLFlag = cli.StringFlag{
Name: "rpc",
Value: "http://localhost:8545",
Usage: "The rpc endpoint of a local or remote geth node",
}
clefURLFlag = cli.StringFlag{
Name: "clef",
Value: "http://localhost:8550",
Usage: "The rpc endpoint of clef",
}
signerFlag = cli.StringFlag{
Name: "signer",
Usage: "Signer address for clef mode signing",
}
signersFlag = cli.StringFlag{
Name: "signers",
Usage: "Comma separated accounts of trusted checkpoint signers",
}
signaturesFlag = cli.StringFlag{
Name: "signatures",
Usage: "Comma separated checkpoint signatures to submit",
}
)
func main() {
log.Root().SetHandler(log.LvlFilterHandler(log.LvlInfo, log.StreamHandler(os.Stderr, log.TerminalFormat(true))))
fdlimit.Raise(2048)
if err := app.Run(os.Args); err != nil {
fmt.Fprintln(os.Stderr, err)
os.Exit(1)
}
}

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cmd/checkpoint-admin/status.go Normal file
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// Copyright 2018 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/>.
package main
import (
"fmt"
"github.com/ethereum/go-ethereum/cmd/utils"
"github.com/ethereum/go-ethereum/common"
"gopkg.in/urfave/cli.v1"
)
var commandStatus = cli.Command{
Name: "status",
Usage: "Fetches the signers and checkpoint status of the oracle contract",
Flags: []cli.Flag{
nodeURLFlag,
},
Action: utils.MigrateFlags(status),
}
// status fetches the admin list of specified registrar contract.
func status(ctx *cli.Context) error {
// Create a wrapper around the checkpoint oracle contract
addr, oracle := newContract(newRPCClient(ctx.GlobalString(nodeURLFlag.Name)))
fmt.Printf("Oracle => %s\n", addr.Hex())
fmt.Println()
// Retrieve the list of authorized signers (admins)
admins, err := oracle.Contract().GetAllAdmin(nil)
if err != nil {
return err
}
for i, admin := range admins {
fmt.Printf("Admin %d => %s\n", i+1, admin.Hex())
}
fmt.Println()
// Retrieve the latest checkpoint
index, checkpoint, height, err := oracle.Contract().GetLatestCheckpoint(nil)
if err != nil {
return err
}
fmt.Printf("Checkpoint (published at #%d) %d => %s\n", height, index, common.Hash(checkpoint).Hex())
return nil
}