[Ethereum] way to capture the require / revert failure reason

revert-opcodesolidityweb3.pyweb3js

In my contract, I have a bunch of checking at the start of a public function. It looks something like this:

require ( prx < 1e18, "Value prx too high")
require ( prx != 0, "Value prx cannot be zero")
if (!validAmt(amt)) { revert("Value amt invalid"); }

Obviously I would like the user to see the failure and the reason text, is this possible in web3.js, or perhaps web3.py?

I know I can capture events, but I don't want to log events on these kind of failures.

Best Answer

In your web3 code, you can use a try/catch block, and parse the thrown error:

try {
  const receipt = await web3.methods.foo().send(...);
  ...
} catch (e) {
  //parse e here, the reason will be inside
}

Related Solutions

[Ethereum] How to receive revert() reason for past transactions

In solidity 0.4.22 the require and revert reason were added. As can be seen here, they are abi-encoded as if it were a call to a function "Error(string)".

This blog post gives an example: an eth_call to a function

function myFunction(uint256 input) public view returns (uint256) {
    require(input >= 5, "myFunction only accepts arguments which are greather than or equal to 5");
    return input * input - 25;
}

with an invalid input argument (less than 5 in this example), will return

0x08c379a0000000000000000000000000000000000000000000000000000000000000002000000000000000000000000000000000000000000000000000000000000000476d7946756e6374696f6e206f6e6c79206163636570747320617267756d656e747320776869636820617265206772656174686572207468616e206f7220657175616c20746f203500000000000000000000000000000000000000000000000000

which is

0x08c379a0                                                       // Function selector
0000000000000000000000000000000000000000000000000000000000000020 // Offset of string return value
0000000000000000000000000000000000000000000000000000000000000047 // Length of string return value (the revert reason)
6d7946756e6374696f6e206f6e6c79206163636570747320617267756d656e74 // first 32 bytes of the revert reason
7320776869636820617265206772656174686572207468616e206f7220657175 // next 32 bytes of the revert reason
616c20746f203500000000000000000000000000000000000000000000000000 // last 7 bytes of the revert reason

So decoding the returned string will give you the revert reason.

With Web3j this could be done like:

public Optional<String> getRevertReason(EthCall ethCall) {
    String errorMethodId = "0x08c379a0"; // Numeric.toHexString(Hash.sha3("Error(string)".getBytes())).substring(0, 10)
    List<TypeReference<Type>> revertReasonTypes = Collections.singletonList(TypeReference.create((Class<Type>) AbiTypes.getType("string")));

    if (!ethCall.hasError() && ethCall.getValue() != null && ethCall.getValue().startsWith(errorMethodId)) {
        String encodedRevertReason = ethCall.getValue().substring(errorMethodId.length());
        List<Type> decoded = FunctionReturnDecoder.decode(encodedRevertReason, revertReasonTypes);
        Utf8String decodedRevertReason = (Utf8String) decoded.get(0);
        return Optional.of(decodedRevertReason.getValue());
    }
    return Optional.empty();
}

Require vs Revert – When to Use Require vs Custom Revert Errors in Smart Contracts

Custom errors are more gas efficient than using require with a string explanation. So ideally you'd always use this over require.

Best Answer

Related Solutions

[Ethereum] How to receive revert() reason for past transactions

Require vs Revert – When to Use Require vs Custom Revert Errors in Smart Contracts

Related Topic