Proof of concept Python script for using a meta transaction to approve WCHI on Polygon

abi.json

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wchiMetaTxApproval.py

#!/usr/bin/env python3

# Example script for using a meta transaction on the Polygon WCHI contract
# to approve a spend.

from eth_account import Account
from eth_account.messages import encode_structured_data
from web3 import Web3
from web3.middleware import geth_poa_middleware

import json
import os

w3 = Web3 (Web3.HTTPProvider ("https://polygon-rpc.com"))
w3.middleware_onion.inject (geth_poa_middleware, layer=0)

# This loads a private key for the address that will actually
# send the transaction, i.e. needs MATIC to pay for gas.
relayerKey = os.getenv ("PRIVKEY")
relayer = Account.from_key (relayerKey)
print ("Relayer: %s" % relayer.address)

# This is a hardcoded private key to an example address, which
# we will use as the address that approves WCHI.
ownerKey = "0xc33067295fa18819d0a98ced3f87a006e7ea428a1e228a257edad8200970ef1a"
owner = Account.from_key (ownerKey)
print ("Owner: %s" % owner.address)

# WCHI address (on Polygon) and ABI.
wchiAddress = "0xE79feAAA457ad7899357E8E2065a3267aC9eE601"
with open ("abi.json", "rt") as f:
  abi = json.load (f)
wchi = w3.eth.contract (abi=abi, address=wchiAddress)

# Build the approval call that we will later send as meta transaction,
# and get the user's next nonce (for the meta transaction, not in general).
approval = wchi.functions.approve (relayer.address, 123).build_transaction ({
  "from": owner.address,
})
nonce = wchi.functions.getNonce (owner.address).call ()

# Set up the EIP-721 structured data we will be using to sign the
# meta transaction.
chainIdBytes = w3.to_bytes (w3.eth.chain_id)
msg = {
  "domain": {
    "name": wchi.functions.name ().call (),
    "version": wchi.functions.ERC712_VERSION ().call (),
    "verifyingContract": wchi.address,
    "salt": (b'\0' * (32 - len (chainIdBytes))) + chainIdBytes,
  },
  "primaryType": "MetaTransaction",
  "types": {
    "EIP712Domain": [
      {"name": "name", "type": "string"},
      {"name": "version", "type": "string"},
      {"name": "verifyingContract", "type": "address"},
      {"name": "salt", "type": "bytes32"},
    ],
    "MetaTransaction": [
      {"name": "nonce", "type": "uint256"},
      {"name": "from", "type": "address"},
      {"name": "functionSignature", "type": "bytes"},
    ],
  },
  "message": {
    "nonce": nonce,
    "from": owner.address,
    "functionSignature": w3.to_bytes (hexstr=approval["data"]),
  },
}

# Sign the data.
encoded = encode_structured_data (msg)
signed = owner.sign_message (encoded)

# Send the meta transaction with the relayer.
call = wchi.functions.executeMetaTransaction (
    approval["from"], approval["data"],
    w3.to_bytes (signed.r), w3.to_bytes (signed.s), signed.v)
gas = call.estimate_gas ({
  "from": relayer.address
})
tx = call.build_transaction ({
  "nonce": w3.eth.get_transaction_count (relayer.address),
  "gas": gas,
  "from": relayer.address,
  "maxFeePerGas": w3.to_wei (200, "gwei"),
  "maxPriorityFeePerGas": w3.to_wei (35, "gwei"),
})
signed = relayer.sign_transaction (tx)
print (w3.eth.send_raw_transaction (signed.rawTransaction.hex ()).hex ())