Catching custom errors

[Mudloop]

Hi,

I have a smart contract that has some custom errors :

error NotYourToken();
error NotCompleted();
error ArrayLengthMismatch();
...

I'm using ethers 5.6.9 umd.

I'm aware of this article : https://blog.ricmoo.com/highlights-ethers-js-may-2021-2826e858277d
But that doesn't seem to work for me.

The problem is that whenever a transaction fails in pre-check, it gives me an error like this :

Error: cannot estimate gas; transaction may fail or may require manual gas limit [ See: https://links.ethers.org/v5-errors-UNPREDICTABLE_GAS_LIMIT ] (reason="execution reverted", method="estimateGas", transaction={"from":"0x5Bd2010585E45172c25A1C9210776221Ada101AB","to":"0xFaF0Cd15E3a4DF1dE5167cC0a3c40EB2E439Bf00","data":"0xb1a8adfd00000000000000000000000000000000000000000000000000000000000000400000000000000000000000000000000000000000000000000000000000000080000000000000000000000000000000000000000000000000000000000000000100000000000000000000000000000000000000000000000000000000000000fb000000000000000000000000000000000000000000000000000000000000000100000000000000000000000000000000000000000000000000000000000000fb","accessList":null}, error={"code":-32603,"message":"execution reverted","data":{"originalError":{"code":3,"data":"0xa3df9bcf","message":"execution reverted"}}}, code=UNPREDICTABLE_GAS_LIMIT, version=providers/5.6.8)

It doesn't look like I can discern the actual reason it's failing from this.

If instead of using custom errors I switch to using string-based revert statement, I do get the correct error messages.

Anything I'm doing wrong?

Thanks!

[ricmoo]

That is a gas estimation error, which means the transaction is not calling a constant method. Only methods called using call can receive custom errors. If you would like to preflight a state-mutating method, you can use the contract.callStatic.METHOD_NAME(params) which will force using call, which on error will throw the custom error.

Let me know if that works. :)

[Mudloop]

Tried it, does the trick, thanks again!

[ricmoo]

That’s generally a problem with the eventually consistent nature of blockchain; that the preflight could pass and the tx could fail or vice versa. Even if the tx gets mined successfully, a re-org could occur and replace it with a failure (or vice versa).

There are advanced trace and debug API that can be used too, to get that data from a mined transaction too, but they require archive node access and the api available depend on the node you have access to.

[Mudloop]

That’s generally a problem with the eventually consistent nature of blockchain; that the preflight could pass and the tx could fail or vice versa. Even if the tx gets mined successfully, a re-org could occur and replace it with a failure (or vice versa).

Oh for sure, I'm aware of that, I just meant that this introduces another step where the problem can occur - there's now two preflights (the manual / callStatic one and the one the wallet does before sending the tx), and only one of them will get me the correct error. Not a big deal, just slightly less optional than when using string errors. I realize the tx can still fail either way.

I guess the reason for this is that the wallet providers don't properly deal with custom errors (yet) and don't pass that info on when they throw an error, right?

[ricmoo]

There isn’t really any preflight from the wallet. The error you were getting earlier was from the estimateGas call. I actually recommend hard coding the gas if it is something you can come up with a safe upper bound. :)

[Mudloop]

There isn’t really any preflight from the wallet. The error you were getting earlier was from the estimateGas call. I actually recommend hard coding the gas if it is something you can come up with a safe upper bound. :)

Makes sense, guess I misunderstood what was happening under the hood. Would it then be possible to add custom error handling to the estimateGas calls? Not really needed for my purposes since I was able to use callStatic, but I imagine it could be useful to have.

As for hardcoding gas, I might be able to in some cases, but not all. Will look into that, thanks!

[soxhog]

@Mudloop
I have found a solution to the error during gas estimation.
ethers 5.7.2

} catch (error: any) {
    const revertData = error.error.data.originalError.data;
    const decodedError = contract.interface.parseError(revertData);
    console.log("Custom Error:", decodedError.name);
}