How can I decode a low-level return data from a reverting call?

Question

Take the following low-level call written in Solidity:

(bool success, bytes memory response) =
          address(proxy).call(abi.encodeWithSelector(plugins.reverter.withReasonString.selector));

I know that this call will revert because I wrote the plugins.reverter contract (this is a test):

contract TargetReverter {
    function withReasonString() external pure {
        revert("You shall not pass");
    }
}

I want to ABI decode the response into a string, but I cannot make it work. The following code does not work:

abi.decode(response, (string));

I imagine that this is because response starts with a 4-byte function selector:

0x08c379a000000000000000000000000000000000000000000000000000000000000000200000000000000000000000000000000000000000000000000000000000000012596f75207368616c6c206e6f7420706173730000000000000000000000000000

How can I turn the bytes above into a string?

Answer 1

Just found another solution that is much more gas efficient, as it uses a native Error function to parse the response:

{
    (bool success, bytes memory response) = address(proxy).call(abi.encodeWithSelector(plugins.reverter.withReasonString.selector));

    /// Signature for "Error(string)"
    bytes4 errorSignature = 0x08c379a0;

    /// Make sure the response is from a standard revert()
    if (!success && bytes4(response) == errorSignature) {
        (bool success2, bytes memory response2) = address(this).call(response);

        string errorText = abi.decode(response2, (string));

        return errorText;
    }
}

And following this we define the function that can parse the revert() encoded data

function Error(string memory text) public pure returns(string memory) {
    return text;
}

Answer 2

abi.decode() needs to receive ABI-encoded data without signatures, but revert() returns a standard Error(string), resulting in ABI-encoded signature+data.

So, you just need to remove the signature before decoding:

if (!success) {
    for (uint256 i; i < response.length-4;) {
        response[i] = response[i+4];
        unchecked{ ++i; }
    }
    
    string memory errorMessage = abi.decode(response, (string));

    return errorMessage;
}