How ink! handles DispatchError from ChainExtension

Question

ChainExtension will return Result<RetVal, DispatchError>. (As you can see in the bottom pircutre)

• When ChainExtension implementation returns Ok(RetVal::Converging(1)) and handle that status code 1 on ink! contract side like below, then Extrinsic itself will be shown as success event even if dispatch failed.

impl ink_env::chain_extension::FromStatusCode for ContractErrorCode {
    fn from_status_code(status_code: u32) -> Result<(), Self> {
        match status_code {
            0 => Ok(()),
            1 => Err(Self::Failed),
            _ => panic!("encountered unknown status code"),
        }
    }
}

(What I did for experiments was ignore Error(DispatchError) and return Ok(RetVal::Converging(1)) instead so that ink! can handle status code 1.)

• When it returns DispatchError (Extrinsic itself fail), ink! smart contract cannot handle it and return its failure reason with #[message] functions' returned value. for e.g. Result<(), ContractError> (here ContractError reflects DispatchError's reason and perhaps we'd like to show it on UI).

The question is What is the appropriate way for ink! smart contract messages to handle DispatchError (from ChainExtension) and return it as its returned value?

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Answer 1

The question is What is the appropriate way for ink! smart contract messages to handle DispatchError (from ChainExtension) and return it as its returned value?

How I understand your question is that you want for the frontend to have the information that an error occurred while calling a contract. And specifically that it was a DispatchError in a chain extension.

The problem that you face when returning RetVal::Diverging from a chain extension's fn call is that Substrate will emit an event ExtrinsicFailed(pallet_contracts::ContractReverted) without the information that it was a DispatchError.

What you can do is to throw the DispatchError immediately in your fn call. This will then result in an ExtrinsicFailed event being emitted on-chain, containing the info that it was a dispatch error.

So in the code from your screenshot it would be:

let result =
  RmrkExtension::execute_func::<E>(func_id_matcher, env)?;