Guide: Check For VRFCoordinator's Callback Using Etherscan API (Lesson 7)

[ZeroEkkusu]

Gm, fellow devs

In Lesson 7, Patrick used time.sleep(60) in his tests to wait for the Chainlink Oracle's response with randomness. However, as reported here, the response can often take much longer and cause tests to fail.

I've written a simple helpful_script to get the job done using the Etherscan API
The other way is to listen for events on the node, if you've done that, make a PR to my repo!

from brownie import chain, config, network
import requests
import time
import web3

def wait_for_randomness(lottery):
    # Keeps checking for a fulfillRandomness callback using the block explorer's API, and returns the randomness

    # Initial frequency, in seconds
    sleep_time = 120
    # Last checked block num
    from_block = len(chain)
    print("Waiting For Data...\n")
    i = 1

    # Until randomness received
    while(True):
        print(f"Check #{i} in {sleep_time} secs\n")
        # Wait
        time.sleep(sleep_time)
        # Get last mined block num
        to_block = len(chain)

        # Check if randomness received
        # 🔗 See https://docs.etherscan.io/api-endpoints/logs
        response = requests.get(
            config["networks"][network.show_active()]["explorer_api"],
            params={
                "module": "logs",
                "action": "getLogs",
                "fromBlock": from_block,
                "toBlock": to_block,
                "address": lottery.address,
                "topic0": web3.Web3.keccak(text='RandomnessReceived(uint256)').hex(),
                "apikey": config["api_keys"]["etherscan"],
            },
            headers={'User-Agent': 'Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.93 Safari/537.36'}).json()
        # Return randomness if received
        if response['status'] == "1":
            return int(response['result'][0]['data'], 16)

        # Half sleep time if longer than 15 seconds
        if(sleep_time > 15):
            sleep_time = int(round(sleep_time/2))

        from_block = to_block

        i += 1

Here's how to set it up:

• In your Lottery.sol, define

event RandomnessReceived(uint256 _randomness);

and emit it at the end of your fulfillRandomness() function

// Emit an event for testing purposes
emit RandomnessReceived(_randomness);

• In your brownie-config.yaml, add

api_keys:
  # Reference the API key env
  etherscan: ${ETHERSCAN_TOKEN}

and, under networks > rinkeby

explorer_api: 'https://api-rinkeby.etherscan.io/api'

That's it!

Now, you can do your integration testing as follows

from scripts.deploy_lottery import deploy_lottery
from scripts.helpful_scripts import LOCAL_BLOCKCHAIN_ENVIRONMENTS, fund_with_link, get_account, wait_for_randomness
from brownie import network
import pytest


def test_can_pick_winner():
    # A basic integration test

    # Only on an actual mainnet/testnet
    if network.show_active() in LOCAL_BLOCKCHAIN_ENVIRONMENTS:
        pytest.skip()
    # Deploy a Lottery
    lottery = deploy_lottery()
    # Get the actual accounts that will participate in the lottery
    account = get_account(env="key1")
    account2 = get_account(env="key2")
    # Call the startLottery function
    lottery.startLottery({"from": account})
    # These will be the entries
    entries = [account, account2, account, account2]
    # Call the enter function for each entry
    for a in entries:
        lottery.enter({"from": a, "value": lottery.getEntranceFee()})
    # Fund the contract with LINK
    fund_with_link(lottery)
    # Call the endLottery function
    lottery.endLottery({"from": account})
    # Wait for and get the randomness
    randomness = wait_for_randomness(lottery)
    # Assert that the right account has been picked as the winner
    assert lottery.recentWinner() == entries[randomness % len(entries)].address
    # Assert that the funds have been transfered
    assert lottery.balance() == 0

The get_account(env="key1") part is a modification of the get_account() function that I've made, but you don't have to do it like that. If you're interested, see my repo.

Thanks for coming to my ted talk

[cromewar]

you are amazing for providing this workaround @ZeroEkkusu

[freddie71010]

This is pretty cool @ZeroEkkusu , I just integrated your code into my project too! Thanks!

[ZeroEkkusu]

There's a better way, I saw some Patrick's code recently where he checked for events in a native way. But can't remember where.

[n4n0b1t3]

This is awesome! Great content, thank you for sharing, and please keep us updated, when you find the "native way".

[freddie71010]

Coming back to this topic as Chainlink VRF has been updated to V2. I am having trouble getting this to work properly. I can't seem to figure out how to convert the hex into the actual English readable text and vice versa. Any ideas if i'm even using the right values in the code snippet below?

"topic0": Web3.keccak(text='ReceiveRandomNumber(uint256[] numReceived)').hex(),
OR
"topic0": "0x2b9b68a0f2880244fa2999d92504cb5dc5933b0ab58e5bede6671e51de8b74f2",

TX: https://rinkeby.etherscan.io/address/0xBF176c97CCcdfa7B9D790664083a95A9b71CFFD9#events