Solidity 0.6.x options: attempt/catch assertion

The attempt/catch syntax launched in 0.6.0 is arguably the most important leap in error dealing with capabilities in Solidity, since purpose strings for revert and require have been launched in v0.4.22. Each attempt and catch have been reserved key phrases since v0.5.9 and now we will use them to deal with failures in exterior perform calls with out rolling again the whole transaction (state adjustments within the referred to as perform are nonetheless rolled again, however the ones within the calling perform are usually not).

We’re shifting one step away from the purist “all-or-nothing” method in a transaction lifecycle, which falls in need of sensible behaviour we frequently need.

Dealing with exterior name failures

The attempt/catch assertion permits you to react on failed exterior calls and contract creation calls, so you can not use it for inside perform calls. Word that to wrap a public perform name throughout the similar contract with attempt/catch, it may be made exterior by calling the perform with this..

The instance beneath demonstrates how attempt/catch is utilized in a manufacturing facility sample the place contract creation would possibly fail. The next CharitySplitter contract requires a compulsory deal with property _owner in its constructor.

pragma solidity ^0.6.1;

contract CharitySplitter {
    deal with public proprietor;
    constructor (deal with _owner) public {
        require(_owner != deal with(0), "no-owner-provided");
        proprietor = _owner;
    }
}

There’s a manufacturing facility contract — CharitySplitterFactory which is used to create and handle situations of CharitySplitter. Within the manufacturing facility we will wrap the new CharitySplitter(charityOwner) in a attempt/catch as a failsafe for when that constructor would possibly fail due to an empty charityOwner being handed.

pragma solidity ^0.6.1;
import "./CharitySplitter.sol";
contract CharitySplitterFactory {
    mapping (deal with => CharitySplitter) public charitySplitters;
    uint public errorCount;
    occasion ErrorHandled(string purpose);
    occasion ErrorNotHandled(bytes purpose);
    perform createCharitySplitter(deal with charityOwner) public {
        attempt new CharitySplitter(charityOwner)
            returns (CharitySplitter newCharitySplitter)
        {
            charitySplitters[msg.sender] = newCharitySplitter;
        } catch {
            errorCount++;
        }
    }
}

Word that with attempt/catch, solely exceptions occurring contained in the exterior name itself are caught. Errors contained in the expression are usually not caught, for instance if the enter parameter for the new CharitySplitter is itself a part of an inside name, any errors it raises won’t be caught. Pattern demonstrating this behaviour is the modified createCharitySplitter perform. Right here the CharitySplitter constructor enter parameter is retrieved dynamically from one other perform — getCharityOwner. If that perform reverts, on this instance with “revert-required-for-testing”, that won’t be caught within the attempt/catch assertion.

perform createCharitySplitter(deal with _charityOwner) public {
    attempt new CharitySplitter(getCharityOwner(_charityOwner, false))
        returns (CharitySplitter newCharitySplitter)
    {
        charitySplitters[msg.sender] = newCharitySplitter;
    } catch (bytes reminiscence purpose) {
        ...
    }
}
perform getCharityOwner(deal with _charityOwner, bool _toPass)
        inside returns (deal with) {
    require(_toPass, "revert-required-for-testing");
    return _charityOwner;
}

Retrieving the error message

We will additional prolong the attempt/catch logic within the createCharitySplitter perform to retrieve the error message if one was emitted by a failing revert or require and emit it in an occasion. There are two methods to attain this:

1. Utilizing catch Error(string reminiscence purpose)

perform createCharitySplitter(deal with _charityOwner) public {
    attempt new CharitySplitter(_charityOwner) returns (CharitySplitter newCharitySplitter)
    {
        charitySplitters[msg.sender] = newCharitySplitter;
    }
    catch Error(string reminiscence purpose)
    {
        errorCount++;
        CharitySplitter newCharitySplitter = new
            CharitySplitter(msg.sender);
        charitySplitters[msg.sender] = newCharitySplitter;
        // Emitting the error in occasion
        emit ErrorHandled(purpose);
    }
    catch
    {
        errorCount++;
    }
}

Which emits the next occasion on a failed constructor require error:

CharitySplitterFactory.ErrorHandled(
    purpose: 'no-owner-provided' (sort: string)
)

2. Utilizing catch (bytes reminiscence purpose)

perform createCharitySplitter(deal with charityOwner) public {
    attempt new CharitySplitter(charityOwner)
        returns (CharitySplitter newCharitySplitter)
    {
        charitySplitters[msg.sender] = newCharitySplitter;
    }
    catch (bytes reminiscence purpose) {
        errorCount++;
        emit ErrorNotHandled(purpose);
    }
}

Which emits the next occasion on a failed constructor require error:

CharitySplitterFactory.ErrorNotHandled(
  purpose: hex'08c379a0000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000000116e6f2d6f776e65722d70726f7669646564000000000000000000000000000000' (sort: bytes)

The above two strategies for retrieving the error string produce the same outcome. The distinction is that the second methodology doesn’t ABI-decode the error string. The benefit of the second methodology is that it’s also executed if ABI decoding the error string fails or if no purpose was offered.

Future plans

There are plans to launch assist for error sorts which means we will declare errors in the same strategy to occasions permitting us to catch completely different sort of errors, for instance:

catch CustomErrorA(uint data1) { … }
catch CustomErrorB(uint[] reminiscence data2) { … }
catch {}

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