Unexpected Mutations

Today was one of those days.

I’m writing a web3 application on top of web3.js. If you don’t know, web3 is one of several libraries out there that help Javascript developers connect to and manipulate Ethereum blockchains.

I’m a competent developer, so I’ve written an abstraction layer to simplify interaction with my smart contracts. I call the abstraction layer’s nice clean API, and the abstraction layer deals with the web3 library.

There are two basic ways ways to invoke a smart contract function. A call returns a value from the blockchain, and a send alters data on the blockchain without returning a value. For either invocation, web3 takes an options object as an argument. This object sets the sender address, some stuff related to gas fees, and the amount of money you’re sending if that’s what your transaction is doing.

Because of call/send segregation, you often find yourself invoking several functions in series in order to accomplish something useful.

Most of those series are pretty short. But as my application has progressed, I’ve reached a point where I need to do quite a bit of stuff on the front end to get the application into a state where I can see what I am working on. So I decided to create an abstraction layer for my abstraction layer that mocks the actions a user takes on the front end. Now I can write scripts that save me a ton of pointing and clicking.

Sweet.

Or so I thought… until the scripts started failing for absolutely no obvious reason. Under the hood, the scripts are doing the exact same things both my unit tests and my actual front end are doing, error free. It took me two hours to figure out what these function invocations in my scripts have in common that they do not have in common anywhere else. The answer is at once funny and irritating as hell.

What they have in common is the options object.

At the top of each script, I set…

const options = { from: web3.eth.accounts.wallet[accountIndex].address };

…and then I use that options object in each contract function invocation in my script, like this:

// get farm
const farm = new Farm(web3, farmAddress);
const games = await farm.getGames(options);

// get game
const game = new Game(web3, games[gameIndex]);
const { minLockup, minDeposit } = await game.getParams(options); // Defined below.

// deploy deposit
const deposit = new Deposit(web3);
await deposit.deploy(options); //

Each of the asynchronous lines above is actually a call to my web3 wrapper. Here are a couple defined:

Farm.getGames = async (options) => {
  return await this.contract.methods.getGames().call(options);
};

Game.getParams = async (options) =>
  this.constructor.decodeParams(
    await this.contract.methods.getParams().call(options)
  );

Not a lot going on here… but this code was working just fine on my front end and in my unit tests, and causing an unexplained revert when I deployed the Deposit contract from my script.

Once I realized my script was sharing the options object between the web3 invocations, I console logged it after each one. After farm.getGames, the options object still had the expected value:

{
  from: '0xD28D1f59...';
}

But after game.getParams it looked like this:

{
  from: '0xd28d1f59...',
  data: '0x5e615a6b',
  gasPrice: undefined,
  gas: undefined,
  to: '0xb97da33a...'
}

… which, when I passed it to deposit.deploy, was causing the reversion.

Hence the title of this post. Apparently, the web3 contract methods call function is mutating the options object you pass to it. I never noticed that before, because until today I wasn’t reusing the options object.

That is… weird. Hard to imagine it is by design. But it’s also easy to fix, once you know about it. Here’s how I altered my abstraction layer:

Farm.getGames = async (options) => {
  return await this.contract.methods.getGames().call({ ...options });
};

Game.getParams = async (options) =>
  this.constructor.decodeParams(
    await this.contract.methods.getParams().call({ ...options })
  );

Now I’m passing a shallow copy of the options object to web3, which it can mutate all it wants, and I don’t care.

Problem solved!

Update: I raised an issue on the web3 GitHub project. Interested to see how that goes.

Closure: They fixed it! I love it when the system works. 😁