add_liquidity will revert in some cases

[hats-bug-reporter]

Github username: @MGF15
Twitter username: --
Submission hash (on-chain): 0xb0878f3292d893808b8ba4f6fe35c740369c807b459292c1589209f2c2f4239c
Severity: low

Description:
Description
the add_liquidity function that allow user to deposit coins into the pool, however in some cases the function will revert when transfer from the sender when an ERC-20 that revert on zero amount transfer like eg LEND because no check if amount in 0

for (uint256 i = 0; i < N_COINS; i++) {
            uint256 amount = amounts[i];
            address coin = coins[i];
            transfer_in(coin, amount); // @audit miss 0 amount check 
        }

Attachments

1. Proof of Concept (PoC) File

function add_liquidity(uint256[N_COINS] memory amounts, uint256 min_mint_amount) external payable nonReentrant {
        //Amounts is amounts of c-tokens
        require(!is_killed, "Killed");
        if (!support_ROSE) {
            require(msg.value == 0, "Inconsistent quantity"); // Avoid sending ROSE by mistake.
        }
        uint256[N_COINS] memory fees;
        uint256 _fee = (fee * N_COINS) / (4 * (N_COINS - 1));
        uint256 _admin_fee = admin_fee;
        uint256 amp = get_A();

        uint256 token_supply = token.totalSupply();
        //Initial invariant
        uint256 D0;
        uint256[N_COINS] memory old_balances = balances;
        if (token_supply > 0) {
            D0 = get_D_mem(old_balances, amp);
        }
        uint256[N_COINS] memory new_balances = [old_balances[0], old_balances[1]];

        for (uint256 i = 0; i < N_COINS; i++) {
            if (token_supply == 0) {
                require(amounts[i] > 0, "Initial deposit requires all coins");
            }
            // balances store amounts of c-tokens
            new_balances[i] = old_balances[i] + amounts[i];
        }

        // Invariant after change
        uint256 D1 = get_D_mem(new_balances, amp);
        require(D1 > D0, "D1 must be greater than D0");

        // We need to recalculate the invariant accounting for fees
        // to calculate fair user's share
        uint256 D2 = D1;
        if (token_supply > 0) {
            // Only account for fees if we are not the first to deposit
            for (uint256 i = 0; i < N_COINS; i++) {
                uint256 ideal_balance = (D1 * old_balances[i]) / D0;
                uint256 difference;
                if (ideal_balance > new_balances[i]) {
                    difference = ideal_balance - new_balances[i];
                } else {
                    difference = new_balances[i] - ideal_balance;
                }

                fees[i] = (_fee * difference) / FEE_DENOMINATOR;
                balances[i] = new_balances[i] - ((fees[i] * _admin_fee) / FEE_DENOMINATOR);
                new_balances[i] -= fees[i];
            }
            D2 = get_D_mem(new_balances, amp);
        } else {
            balances = new_balances;
        }

        // Calculate, how much pool tokens to mint
        uint256 mint_amount;
        if (token_supply == 0) {
            mint_amount = D1; // Take the dust if there was any
        } else {
            mint_amount = (token_supply * (D2 - D0)) / D0;
        }
        require(mint_amount >= min_mint_amount, "Slippage screwed you");

        // Take coins from the sender
        for (uint256 i = 0; i < N_COINS; i++) {
            uint256 amount = amounts[i];
            address coin = coins[i];
            transfer_in(coin, amount); // @audit miss 0 amount check 
        }

        // Mint pool tokens
        token.mint(msg.sender, mint_amount);

        emit AddLiquidity(msg.sender, amounts, fees, D1, token_supply + mint_amount);
    }

[omega-audits]

If the user deposits 0 there is no deposit regardless, and special tokens like these are usually not considered regardless.