transferFrom fails even though allowance granted?

Question

Please see attached Tenderly stack trace:

https://dashboard.tenderly.co/shared/simulation/86e27f67-6f7e-4122-83da-d43f455fb57a

I'm not sure why transferfrom is failing even through I have granted uint256 unlimited allowance to the contract withdrawing my USDT.

Contract code (Vyper)

@external
def swap(
    _from: address,
    _to: address,
    _amount: uint256,
    _minout: uint256,
    _order_id: Bytes[10],
    _tofiat: bool,
    _totopup: bool,
) -> uint256:

    sanctioned: bool = Chainanalysis(0x40C57923924B5c5c5455c48D93317139ADDaC8fb).isSanctioned(msg.sender)
    assert sanctioned != True, "sanctioned"
    assert self.paused == False, "paused"

    if (_totopup):
        assert _to == USDC_eth, "USDC"
    
    if (_tofiat):
        assert _to == USDC_eth, "USDC"

    
    ERC20(_from).transferFrom(msg.sender,self,_amount)
    ERC20(_from).approve(UNISWAP, _amount)
    balance_before: uint256 = ERC20(_to).balanceOf(self)
    result: Bytes[32] = raw_call(
        UNISWAP,
        concat(
            method_id("exactInputSingle((address,address,uint24,address,uint256,uint256,uint256,uint160))"),
            convert(_from, bytes32),
            convert(_to, bytes32),
            convert(3000, bytes32), #pool fee
            convert(self, bytes32), #recipient
            convert(block.timestamp, bytes32), #deadline
            convert(_amount, bytes32),
            convert(_minout, bytes32),
            convert(0, bytes32), #sqrtPriceLimitX96
        ),
        max_outsize=32
    )

    output: uint256 = convert(result, uint256)
    balance_after: uint256 = ERC20(_to).balanceOf(self)
    assert balance_after - balance_before == output, "not balanced"
    assert output >= _minout, "too little"
    if (_totopup):
        fees: uint256 = output * self.fee_topup/10000
        ERC20(_to).transfer(profits,fees)
        log TopedUp(msg.sender,_amount,output,_order_id)
        return output
    elif(_tofiat):
        fees: uint256 = output * self.fee_fiat/10000
        ERC20(_to).transfer(profits,fees)
        if (self.topartner):
            ERC20(USDC_eth).transfer(self.partner_wallet,_amount-fees)
        log Success(msg.sender,_amount,output,_order_id)
        return output
    else:
        fees: uint256 = output * self.fee_swap/10000
        if (_to == USDC_eth):
            ERC20(_to).transfer(profits,fees)
            ERC20(_to).transfer(msg.sender,output-fees)
        else:
            ERC20(_to).transfer(msg.sender,output-fees)
            new_output: uint256 = self.to_usdc(_to,fees)
            ERC20(USDC_eth).transfer(profits,new_output)
        log Success(msg.sender,_amount,output,_order_id)
        return output

05/11/2023 updated:

Based on feedback provided by @Vas Trek have updated this to check if allowance is already set, if it is, to zero allowance and to reset it.

Issue still persists.

allowance: uint256 = ERC20(_from).allowance(self,UNISWAP) 
if (allowance == 0): 
ERC20(_from).approve(UNISWAP, _amount) 
else: 
ERC20(_from).approve(UNISWAP, 0) ERC20(_from).approve(UNISWAP,_amount) 

Kind regards,

Answer 1

I think it's failing on ERC20(_from).approve(UNISWAP, _amount)

two possible reasons:

a) UNISWAP address is zero

b) (most probable) UNISWAP was already approved to take USDTs from this contract.

The second option is USDT token specific. USDT's approve function includes this statement: require(!((_value != 0) && (allowed[msg.sender][_spender] != 0)));

It means that if the approval was already granted and you want to reset it, you must run approve(address, 0) before making new approve call.