decodeFunctionResult: get uint256 output from a transaction receipt using ethers v5

[bitcazador]

Describe the bug
Incapable of decoding uint256 output from a public scope function (txdata properly mined and waited). <mycontract>.interface.decodeFunctionResult( functionFragment, txdata ).
In can perfectly distinguish in the txdata the function identifier and the result in _hex, but ethers is incapable of extracting the uint256 result output.

Reproduction steps
I attach a simple and reproducible contract, with two internal variables to play, _str_1 and _val_1. I have to problem no extract the result from view scope function. But I cannot extract the uint256 _val_1 (although I can distinguish the result in the txdata). For example, if set _val_1 = 1 in the constructor, and introduce the uint256 input_ = 2 inside fun_sum256, then performs 1+2 =3 inside, and the produced input txdata is correct (sighash= 0x78081f40), with _hex value "2", so the input txdata is correct:
"0x78081f400000000000000000000000000000000000000000000000000000000000000002"
but what about the produced output uint256 3?

simple contract1.sol

pragma solidity ^0.8.0;
// SPDX-License-Identifier: MIT
contract Contract1  {

  string public _str_1;
  uint256 public _val_1;

  constructor(string memory str_in1, uint256 in_val_1 ) {
    _str_1 = str_in1;
    _val_1 = in_val_1;
  } //endconstructor

  function get_str() external view returns (string memory) {
    return _str_1;
  } //endfun get_str

  function set_str(string memory str_in1) external returns (string memory) {
    _str_1 = str_in1;
    return _str_1;
  } //endfun set_str

  function fun_sum256(uint256 in_val_2) public returns (uint256) {
    _val_1 += in_val_2;
    return _val_1;
  }

} //endcon

script js:

// create the factory and deploy the contract1
  const fact_0        = new ethers.ContractFactory(Contract1['abi'], Contract1['bytecode'], signers[0]);
  const in_str_1_1    = 'hola';
  const in_val_1_2    = ethers.BigNumber.from(1);
  const con_0         = await fact_0.deploy(in_str_1_1, in_val_1_2);
  await con_0.deployTransaction.wait(); // wait to be mined, in order to be safe to interact with
  console.log(`successfully deployed: ${JSON.stringify(con_0)}`);


  // interaction {get,set}
  const ins_1         = con_0;                // instance. get directly the one deployed
  const str_1         = await ins_1.get_str(); // get
  await ins_1.set_str('superhola');           // set
  const str_2         = await ins_1.get_str();
  console.log(`get_function: success. valor: ${str_1}`);
  console.log(`set_function: success. valor: ${str_2}`);


  // try read tx_receipt and get uint256 output
  const in_val_2_1    = ethers.BigNumber.from(2);
  const tx_2          = await ins_1.fun_sum256(in_val_2_1);
  const tx_2_post     = await tx_2.wait();

  // const obj_iface     = new ethers.utils.Interface(Contract1.abi);
  // const resp_1        = await obj_iface.decodeFunctionResult("fun_sum256", tx_2.data);
  const resp_2        = await ins_1.interface.decodeFunctionResult(ins_1.interface.functions["fun_sum256(uint256)"], tx_2.data); //also tried from Contract1

error:

Error: no matching error (argument="sighash", value="0x78081f40", code=INVALID_ARGUMENT, version=abi/5.5.0)
    at Logger.makeError (...\node_modules\@ethersproject\logger\src.ts\index.ts:225:28)
    at Logger.throwError (...\node_modules\@ethersproject\logger\src.ts\index.ts:237:20)
    at Logger.throwArgumentError (...\node_modules\@ethersproject\logger\src.ts\index.ts:241:21)
    at Interface.getError (...\node_modules\@ethersproject\abi\src.ts\interface.ts:269:20)
    at Interface.decodeFunctionResult (...\node_modules\@ethersproject\abi\src.ts\interface.ts:413:44)
    at main (...\scripts\pruebas1\xtest_contract1_0.js:58:47)
    at processTicksAndRejections (internal/process/task_queues.js:95:5) {
  reason: 'no matching error',
  code: 'INVALID_ARGUMENT',
  argument: 'sighash',
  value: '0x78081f40'
}
Error: call revert exception (method="fun_sum256(uint256)", errorArgs=null, errorName=null, errorSignature=null, reason=null, code=CALL_EXCEPTION, version=abi/5.5.0)
    at Logger.makeError (...\node_modules\@ethersproject\logger\src.ts\index.ts:225:28)
    at Logger.throwError (...\node_modules\@ethersproject\logger\src.ts\index.ts:237:20)
    at Interface.decodeFunctionResult (...\node_modules\@ethersproject\abi\src.ts\interface.ts:425:23)
    at main (...\my-nft\scripts\pruebas1\xtest_contract1_0.js:58:47)
    at processTicksAndRejections (internal/process/task_queues.js:95:5) {
  reason: null,
  code: 'CALL_EXCEPTION',
  method: 'fun_sum256(uint256)',
  errorArgs: null,
  errorName: null,
  errorSignature: null

Environment:
Node.js v14.17.4,
ethers v5.5.2,
ganache-cli. v6.12.2 (ganache-core: 2.13.2)
I can distinguish the txdata is right, but the decode fails. The function identifier is also right, and it is also observable in the txdata perfectly set.

Search Terms
Decoding; txdata, decodeFunctionResult, transaction receipt, nodejs

[ricmoo]

The decodeFuncrionResult method is for results. You likely want the decodeFunctionData method, which operates on transaction data.

[bitcazador]

decodeFunctionData works fine for checking the input tx BigNumber { _hex: '0x02', _isBigNumber: true }. But I am interested in the result, in the output BigNumber { _hex: '0x03', _isBigNumber: true }.
I kindly ask you to describe when it is more appropriated decodeFunctionResult, because I thought it was for extracting the output of a function inside an smart contract. So, is decodeFunctionResult working properly? Thank you for your response and regards.

[ricmoo]

The encodeFunctIonData and decodeFunctionData work on the data you pass along in as data to a transaction, either using sendTransaction or call. The encodeFunctionResult and decodeFunctionResult work on the data return sed from a call.

Make sense? (Moving this to discussions)

[bitcazador]

I am sorry to say bad news, but what I read from decodeFunctionData was the uint256 input I provided.

What I read from decodeFunctionData was the tx_input 2, not the output result 3. I am sorry to insist that decodeFunctionResult would be the appropriate one according to ethers docs, but it doesn't work.

IMHO it's definitely a bug, I am really sorry for what that means.. In the meanwhile I will move back to v.4. Otherwise could you provide a simple reproducible script that works? Thank you again anyway.

[ricmoo]

Can you provide 3-4 lines of sample code that causes the error? Something I can paste into https://playground.ethers.org ?

If you are passing in tx.data, you want decodeFunctionData. If you are passing in await provider.call(tx) you want decodeFunctionResult. If you just have a transaction you want to parse, you can use interface.parseTransaction(tx).

[bitcazador]

Oh I see.. Then if I understood properly, it is for call functions that are only for view-functions, so for read-only functions (no tx, no change of the state, no gas cost). Then.. how Remix-ide is able to retrieve the uint256 value output result from a non-view function (only public)? For me it's a great mistery beyond my small knowledge :(

That was the small example and script I made for reproducing the issue. If I have something better I'll let you know. I see now with more detail that in ethers v5 docs decodeFunctionResult it's meant for view functions according to the examples. So.. I don't know how is able remix-ide to do it for non-view functions. As I said.. it's a mistery for me.

[ricmoo]

Remix is probably running its own injected provider, which simulates the entire blockchain in JavaScript internally. Or they are using trace_ or debug_ API’s on an archive node. this isn’t something the average backend can do though; you need a custom service or advanced middleware (like Ganache forking) to implement this.

If you need the return data from a transaction, the only generic solution is to use events to emit the result you need.

[bitcazador]

Yes, you are right. Emit is the solution. Thank you for your appreciate help.