Support for a `custom` key in `TransactionRequest` and `CallOverrides`

[ly0va]

Currently, Signer.checkTransaction and contracts.populateTransaction reject custom override keys to prevent typos or unsupported options as seen here and here.

While the former method can be overridden by a derived class, there is no such option for the latter, as it is not a method of any class. Moreover, it is not exported, so there is no way to utilize the existing implementation without copy-pasting it.

The problem here is that when extending a Signer or a Wallet to be used to call contracts on networks that also use web3 API, but have additional options that must be specified to modify transaction calldata prior to sending, there is no way to pass those options into Signer.signTransaction or other methods that accept a TransactionRequest from the context of a contract, since contracts.populateTransaction used by the Contract class does not allow stray CallOverrides.

To give an example, I want to be able to do something like

// mylib.Wallet extends ethers.Wallet and overrides signTransaction and checkTransaction
// methods to be suitable for a custom network that needs some custom options to modify & sign transactions
const wallet = mylib.Wallet(key, provider); 

// contract uses wallet's overridden methods to build calldata
const contract = new ethers.Contract(address, abi, wallet);

// custom overrides are passed into wallet.signTransaction which will use them to modify the calldata
const tx = await contract.myFunction(
    ...args,    // regular arguments for the contract method
    {
        gasLimit: 30000,    // standard override
        custom: { customField: "abacaba" }  // custom override
    }
);

which is currently impossible due to checks in contracts.populateTransaction which I linked above.

Alternatively, the way to do this is by extending ethers.Contract using a lot of copy-pasted code to override Contract.populateTransacton, which is not pretty.
Adding a single custom field (or any other suitable name) would be a major help and will not compromise typo checks that are in place.

Maybe there is a much simpler way to do this which I am not seeing, in which case I would be very grateful for suggestions.
If not, then my suggested way is fairly straightforward to implement and I would gladly make a PR if this solution looks fine.

[ly0va]

To add one more argument for this change, if there is no way to specify custom options, then there is no point in CallOverrides and PopulatedTransaction being interfaces and not concrete types. Which, I guess, are made interfaces to provide more flexibility, which is then, unfortunately, denied.

[Madeindreams]

I'm also looking into adding custom argument to transactions.

But right now I'm experimenting. I'm trying to checkTransaction on a custom transaction. I am getting the address of the signer back but it's wrapped in an a promise that I can't await for.

const publicKey = wallet.checkTransaction(hashTx,this.signature)

output  { from: Promise { '0xAbC123....' } }

Even if I await for it inside a promise it always resolve to this. So I was wondering if that could be because I have custom attributes.

EDIT: Nope it has nothing to do with it. I have to await for
wallet.checkTransaction(hashTx,this.signature).from

[ricmoo]

This was added in 5.5.0, so I'll close this now. :)