Ordering of _transfer and _approve in ERC20 transferFrom?

dmdv · December 17, 2020, 9:35am

Hi

I cannot see any logic behind transfer.

It calls transfer
It call approve

OpenZeppelin/openzeppelin-contracts/blob/051d340171a93a3d401aaaea46b4b62fa81e5d7c/contracts/token/ERC20/ERC20.sol#L152


 * Emits an {Approval} event indicating the updated allowance. This is not
 * required by the EIP. See the note at the beginning of {ERC20}.
 *
 * Requirements:
 *
 * - `sender` and `recipient` cannot be the zero address.
 * - `sender` must have a balance of at least `amount`.
 * - the caller must have allowance for ``sender``'s tokens of at least
 * `amount`.
 */
function transferFrom(address sender, address recipient, uint256 amount) public virtual override returns (bool) {
    _transfer(sender, recipient, amount);
    _approve(sender, _msgSender(), _allowances[sender][_msgSender()].sub(amount, "ERC20: transfer amount exceeds allowance"));
    return true;
}
/**
 * @dev Atomically increases the allowance granted to `spender` by the caller.
 *
 * This is an alternative to {approve} that can be used as a mitigation for
 * problems described in {IERC20-approve}.

Shouldn’t this be the other way?

Approve (and fail if there’s not enough allowance)
Transfer

abcoathup · December 17, 2020, 8:37pm

Hi @dmdv,

Welcome to the community

In the case of ERC20 , the order is irrelevant because _transfer and _approve are independent and never call any other contract: they are executed atomically.

However , this is very relevant for users that intent to override _transfer or _approve with their own implementations: we should be clear about the order in which these things happen.

See: https://github.com/OpenZeppelin/openzeppelin-contracts/issues/2030#issuecomment-568487500

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Ordering of _transfer and _approve in ERC20 transferFrom?

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