Can transaction fail but the calling contract will think it is successful?

Question

when making an ERC20 transfer it is a standard practice to wrap it in require statement so if it fails the tx will revert like this:

require(token.transfer(...));

What if I'm calling an external contract that doesn't return a boolean and the transaction fails.

Is it possible that the tx will fail(not revert) but the contract that called that function will think that it was successful?

Thanks. If the question isn't clear I'm happy to provide more context.

Answer 1

Yes it's definitely possible for the token to not follow standards (or be malicious). The general best practice is to get around this (if you need to allow any token in your protocol) is to force the return via safeTransfer

https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/utils/SafeERC20.sol.

bytes memory returndata = address(token).functionCall(data, "SafeERC20: low-level call failed");
if (returndata.length > 0) {
    // Return data is optional
    require(abi.decode(returndata, (bool)), "SafeERC20: ERC20 operation did not succeed");
}

But also know that tokens can be upgradeable (become malicious) or just be malicious from the start and return true even the transfer doesn't happen, so at some point you just have to let people decide if they want to interact with that token.