Support contract error reason strings

• py-evm Version: 0.2.0a18
• OS: linux
• Python Version (python --version): 3.6

What is wrong?

Currently in the case of revert opcodes evm.exceptions.Revert is raised. Since solidity 0.4.22, revert can pass reason string. See solidity doc.

references: https://github.com/ethereum/EIPs/issues/838 https://github.com/ethereum/solidity/pull/3364#discussion_r180455883

How can it be fixed

One obstacle is how py-evm does contract calls, by running a transaction in a temporary state with gas price set to zero. Normally calls don’t have a gas set, so gas estimation is triggered, but then fails, masking the (would be) revert return value. Somewhat unrelated to the core issue of returning the reason string.

solidity code:

pragma solidity ^0.4.22;

contract DoRevert {
    function do_revert(bool _revert) public returns (string message) {
        if (_revert) {
            revert("ribbert, ribbert");
        }   
        return "No ribbert";
    }   
}

web3 with eth-tester provider py-evm backend

contract.functions.do_revert(True).call()

exception:

../../../web3/contract.py:1110: in call
    **self.kwargs
../../../web3/contract.py:1364: in call_contract_function
    return_data = web3.eth.call(call_transaction, block_identifier=block_id)
../../../venv/lib/python3.6/site-packages/eth_utils/functional.py:22: in inner
    return callback(fn(*args, **kwargs))
../../../web3/eth.py:294: in call
    [transaction, block_identifier],
../../../web3/manager.py:107: in request_blocking
    response = self._make_request(method, params)
../../../web3/manager.py:90: in _make_request
    return request_func(method, params)
../../../web3/middleware/gas_price_strategy.py:18: in middleware
    return make_request(method, params)
cytoolz/functoolz.pyx:232: in cytoolz.functoolz.curry.__call__
    ???
../../../web3/middleware/formatting.py:48: in apply_formatters
    response = make_request(method, formatted_params)
../../../web3/middleware/attrdict.py:18: in middleware
    response = make_request(method, params)
cytoolz/functoolz.pyx:232: in cytoolz.functoolz.curry.__call__
    ???
../../../web3/middleware/formatting.py:48: in apply_formatters
    response = make_request(method, formatted_params)
../../../web3/middleware/normalize_errors.py:9: in middleware
    result = make_request(method, params)
cytoolz/functoolz.pyx:232: in cytoolz.functoolz.curry.__call__
    ???
../../../web3/middleware/formatting.py:48: in apply_formatters
    response = make_request(method, formatted_params)
cytoolz/functoolz.pyx:232: in cytoolz.functoolz.curry.__call__
    ???
../../../web3/middleware/formatting.py:48: in apply_formatters
    response = make_request(method, formatted_params)
../../../web3/providers/eth_tester/middleware.py:309: in middleware
    fill_default_gas,
cytoolz/functoolz.pyx:589: in cytoolz.functoolz.pipe
    ???
cytoolz/functoolz.pyx:565: in cytoolz.functoolz.c_pipe
    ???
cytoolz/functoolz.pyx:232: in cytoolz.functoolz.curry.__call__
    ???
../../../web3/providers/eth_tester/middleware.py:295: in fill_default
    guess_val = guess_func(web3, transaction)
../../../web3/providers/eth_tester/middleware.py:287: in guess_gas
    return web3.eth.estimateGas(transaction) * 2
../../../web3/eth.py:304: in estimateGas
    [transaction],
../../../web3/manager.py:107: in request_blocking
    response = self._make_request(method, params)
../../../web3/manager.py:90: in _make_request
    return request_func(method, params)
../../../web3/middleware/gas_price_strategy.py:18: in middleware
    return make_request(method, params)
cytoolz/functoolz.pyx:232: in cytoolz.functoolz.curry.__call__
    ???
../../../web3/middleware/formatting.py:48: in apply_formatters
    response = make_request(method, formatted_params)
../../../web3/middleware/attrdict.py:18: in middleware
    response = make_request(method, params)
cytoolz/functoolz.pyx:232: in cytoolz.functoolz.curry.__call__
    ???
../../../web3/middleware/formatting.py:48: in apply_formatters
    response = make_request(method, formatted_params)
../../../web3/middleware/normalize_errors.py:9: in middleware
    result = make_request(method, params)
cytoolz/functoolz.pyx:232: in cytoolz.functoolz.curry.__call__
    ???
../../../web3/middleware/formatting.py:48: in apply_formatters
    response = make_request(method, formatted_params)
cytoolz/functoolz.pyx:232: in cytoolz.functoolz.curry.__call__
    ???
../../../web3/middleware/formatting.py:48: in apply_formatters
    response = make_request(method, formatted_params)
../../../web3/providers/eth_tester/middleware.py:320: in middleware
    return make_request(method, [filled_transaction] + params[1:])
../../../web3/middleware/fixture.py:12: in middleware
    return make_request(method, params)
cytoolz/functoolz.pyx:232: in cytoolz.functoolz.curry.__call__
    ???
../../../web3/middleware/formatting.py:48: in apply_formatters
    response = make_request(method, formatted_params)
../../../web3/providers/eth_tester/main.py:46: in make_request
    response = delegator(self.ethereum_tester, params)
cytoolz/functoolz.pyx:232: in cytoolz.functoolz.curry.__call__
    ???
../../../web3/providers/eth_tester/defaults.py:36: in call_eth_tester
    return getattr(eth_tester, fn_name)(*fn_args, **fn_kwargs)
../../../venv/lib/python3.6/site-packages/eth_tester/main.py:466: in estimate_gas
    raw_gas_estimate = self.backend.estimate_gas(raw_transaction)

[pipermerriam]

I think you’ve got this in the correct location (eth-tester). Once you get the PR open I’ve got some minor feedback, but overall it looks like a solid approach.

[jacqueswww]

@carver may I try and tackle this one now? As I am anyways working on adding reason string support to vyper (https://github.com/ethereum/vyper/issues/889) 😃