Solidity 0.6.x options: attempt/catch assertion

The try/catch syntax introduced in 0.6.0 is arguably the most important leap in error dealing with capabilities in Solidity, since motive strings for revert and require had been launched in v0.4.22. Each attempt and catch have been reserved key phrases since v0.5.9 and now we are able to use them to deal with failures in exterior perform calls with out rolling again the whole transaction (state adjustments within the known 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” strategy in a transaction lifecycle, which falls in need of sensible behaviour we regularly 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. Observe that to wrap a public perform name inside the identical 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 handle property _owner in its constructor.

pragma solidity ^0.6.1;

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

There’s a manufacturing facility contract — CharitySplitterFactory which is used to create and handle cases of CharitySplitter. Within the manufacturing facility we are able to 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 (handle => CharitySplitter) public charitySplitters;
    uint public errorCount;
    occasion ErrorHandled(string motive);
    occasion ErrorNotHandled(bytes motive);
    perform createCharitySplitter(handle charityOwner) public {
        attempt new CharitySplitter(charityOwner)
            returns (CharitySplitter newCharitySplitter)
        {
            charitySplitters[msg.sender] = newCharitySplitter;
        } catch {
            errorCount++;
        }
    }
}

Observe that with attempt/catch, solely exceptions taking place 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 is not going to 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(handle _charityOwner) public {
    attempt new CharitySplitter(getCharityOwner(_charityOwner, false))
        returns (CharitySplitter newCharitySplitter)
    {
        charitySplitters[msg.sender] = newCharitySplitter;
    } catch (bytes reminiscence motive) {
        ...
    }
}
perform getCharityOwner(handle _charityOwner, bool _toPass)
        inside returns (handle) {
    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 realize this:

1. Utilizing catch Error(string reminiscence motive)

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

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

CharitySplitterFactory.ErrorHandled(
    motive: 'no-owner-provided' (kind: string)
)

2. Utilizing catch (bytes reminiscence motive)

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

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

CharitySplitterFactory.ErrorNotHandled(
  motive: hex'08c379a0000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000000116e6f2d6f776e65722d70726f7669646564000000000000000000000000000000' (kind: bytes)

The above two strategies for retrieving the error string produce an identical end result. The distinction is that the second technique doesn’t ABI-decode the error string. The benefit of the second technique is that it’s also executed if ABI decoding the error string fails or if no motive was offered.

Future plans

There are plans to launch help for error varieties which means we can declare errors in an identical solution to occasions permitting us to catch totally different kind of errors, for instance:

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