Flash Loans – How to Fix Request Flashloan Fails: Gas Estimation ‘execution reverted: 27’

The given is the flashloan contract for uniswap v2. You can use execute function to do the flashloan in beetween different pools and dexes. It does a low level call to pair using swap method. You can just take different pairs and do flashloan. This works in ethereum, binanace smart chain, polygon, arbitrum etc. This is applicable in V2 dexes like Uniswap v2, shusiswap, pancakeswap etc. Here, execute method is where you put details and create flashloan swap, you only pay gas fee.

baseToken = base token of pair like ETH, BNB 

quoteToken = token to be taken flash loan of like USDT, USDC, DAI etc

pair0 = pair address from one dex

pair1 = pair address of other dex

baseAmountIn = flashloan amount

.

// SPDX-License-Identifier: MIT

pragma solidity ^ 0.8.7;

interface IERC20 {
    event Approval(
        address indexed owner,
        address indexed spender,
        uint256 value
    );
    event Transfer(address indexed from, address indexed to, uint256 value);

    function name() external view returns (string memory);

    function symbol() external view returns (string memory);

    function decimals() external view returns (uint8);

    function totalSupply() external view returns (uint256);

    function balanceOf(address owner) external view returns (uint256);

    function allowance(
        address owner,
        address spender
    ) external view returns (uint256);

    function approve(address spender, uint256 value) external returns (bool);

    function transfer(address to, uint256 value) external returns (bool);

    function transferFrom(
        address from,
        address to,
        uint256 value
    ) external returns (bool);
}

interface IPair {
    event Approval(
        address indexed owner,
        address indexed spender,
        uint256 value
    );
    event Transfer(address indexed from, address indexed to, uint256 value);

    function name() external pure returns (string memory);

    function symbol() external pure returns (string memory);

    function decimals() external pure returns (uint8);

    function totalSupply() external view returns (uint256);

    function balanceOf(address owner) external view returns (uint256);

    function allowance(
        address owner,
        address spender
    ) external view returns (uint256);

    function approve(address spender, uint256 value) external returns (bool);

    function transfer(address to, uint256 value) external returns (bool);

    function transferFrom(
        address from,
        address to,
        uint256 value
    ) external returns (bool);

    function DOMAIN_SEPARATOR() external view returns (bytes32);

    function PERMIT_TYPEHASH() external pure returns (bytes32);

    function nonces(address owner) external view returns (uint256);

    function permit(
        address owner,
        address spender,
        uint256 value,
        uint256 deadline,
        uint8 v,
        bytes32 r,
        bytes32 s
    ) external;

    event Mint(address indexed sender, uint256 amount0, uint256 amount1);
    event Burn(
        address indexed sender,
        uint256 amount0,
        uint256 amount1,
        address indexed to
    );
    event Swap(
        address indexed sender,
        uint256 amount0In,
        uint256 amount1In,
        uint256 amount0Out,
        uint256 amount1Out,
        address indexed to
    );
    event Sync(uint112 reserve0, uint112 reserve1);

    function MINIMUM_LIQUIDITY() external pure returns (uint256);

    function factory() external view returns (address);

    function token0() external view returns (address);

    function token1() external view returns (address);

    function getReserves()
        external
        view
        returns (uint112 reserve0, uint112 reserve1, uint32 blockTimestampLast);

    function price0CumulativeLast() external view returns (uint256);

    function price1CumulativeLast() external view returns (uint256);

    function kLast() external view returns (uint256);

    function mint(address to) external returns (uint256 liquidity);

    function burn(
        address to
    ) external returns (uint256 amount0, uint256 amount1);

    function swap(
        uint256 amount0Out,
        uint256 amount1Out,
        address to,
        bytes calldata data
    ) external;

    function skim(address to) external;

    function sync() external;

    function initialize(address, address) external;
}

library SafeMath {
    function add(uint256 x, uint256 y) internal pure returns (uint256 z) {
        require((z = x + y) >= x, "ds-math-add-overflow");
    }

    function sub(uint256 x, uint256 y) internal pure returns (uint256 z) {
        require((z = x - y) <= x, "ds-math-sub-underflow");
    }

    function mul(uint256 x, uint256 y) internal pure returns (uint256 z) {
        require(y == 0 || (z = x * y) / y == x, "ds-math-mul-overflow");
    }
}

library TransferHelper {
    function safeApprove(address token, address to, uint256 value) internal {
        // bytes4(keccak256(bytes('approve(address,uint256)')));
        (bool success, bytes memory data) = token.call(
            abi.encodeWithSelector(0x095ea7b3, to, value)
        );
        require(
            success && (data.length == 0 || abi.decode(data, (bool))),
            "APPROVE_FAILED"
        );
    }

    function safeTransfer(address token, address to, uint256 value) internal {
        // bytes4(keccak256(bytes('transfer(address,uint256)')));
        (bool success, bytes memory data) = token.call(
            abi.encodeWithSelector(0xa9059cbb, to, value)
        );
        require(
            success && (data.length == 0 || abi.decode(data, (bool))),
            "TRANSFER_FAILED"
        );
    }

    function safeTransferFrom(
        address token,
        address from,
        address to,
        uint256 value
    ) internal {
        // bytes4(keccak256(bytes('transferFrom(address,address,uint256)')));
        (bool success, bytes memory data) = token.call(
            abi.encodeWithSelector(0x23b872dd, from, to, value)
        );
        require(
            success && (data.length == 0 || abi.decode(data, (bool))),
            "TRANSFER_FROM_FAILED"
        );
    }

    function safeTransferETH(address to, uint256 value) internal {
        (bool success, ) = to.call{value: value}(new bytes(0));
        require(success, "TRANSFER_ETH_FAILED");
    }
}

struct CallbackData {
    address baseToken;
    address quoteToken;
    address pair0;
    address pair1;
    uint256 baseAmountIn;
    uint256 quoteAmount;
}

contract FlashLoanUniswapV2 {
    using SafeMath for uint256;
    address payable public owner;

    /**
     * Modifier to make a function callable only by owner addresses.
     */
    modifier onlyOwner() {
        require(owner == msg.sender, "UNAUTHORIZED");
        _;
    }

    constructor() {
        owner = payable(msg.sender);
    }

    function execute(
        address baseToken,
        address quoteToken,
        address pair0,
        address pair1,
        uint256 baseAmountIn
    ) public {
        // calculate the swapping amounts
        uint256 quoteAmount;
        uint256 baseAmountOut;
        {
            (uint256 r0base, uint256 r0quote) = getReserves(pair0,baseToken,quoteToken);
            (uint256 r1quote, uint256 r1base) = getReserves(pair1,quoteToken,baseToken);
            quoteAmount = getAmountOut(baseAmountIn, r0base, r0quote);
            baseAmountOut = getAmountOut(quoteAmount, r1quote, r1base);
        }

        require(baseAmountOut > baseAmountIn, "UNPROFITABLE");
        uint256 balanceBefore = IERC20(baseToken).balanceOf(address(this));

        // flash swap to get base token from pair1 & repay quote token in callback
        uint256 amount0Out;
        uint256 amount1Out;
        {
            (address token0, ) = sortTokens(baseToken, quoteToken);
            (amount0Out, amount1Out) = quoteToken == token0 ? (uint256(0), baseAmountOut) : (baseAmountOut, uint256(0));
        }

        CallbackData memory callbackData;
        callbackData.baseToken = baseToken;
        callbackData.quoteToken = quoteToken;
        callbackData.pair0 = pair0;
        callbackData.pair1 = pair1;
        callbackData.baseAmountIn = baseAmountIn;
        callbackData.quoteAmount = quoteAmount;
        IPair(pair1).swap(
            amount0Out,
            amount1Out,
            address(this),
            abi.encode(callbackData)
        );

        // validate the profit
        uint256 balanceAfter = IERC20(baseToken).balanceOf(address(this));
        require(balanceAfter > balanceBefore, "SOMETHING_IS_WRONG");

        // transfer the profit to owner
        TransferHelper.safeTransfer(baseToken,owner,balanceAfter - balanceBefore);
    }

    function uniswapV2Call(
        address,
        uint256,
        uint256,
        bytes calldata data
    ) external {
        _callback(data);
    }

    function _callback(bytes calldata data) internal {
        // decode data
        CallbackData memory cb = abi.decode(data, (CallbackData));
        // swap base for quote in pair0 and repay pair1 in quote token
        TransferHelper.safeTransfer(cb.baseToken, cb.pair0, cb.baseAmountIn);
        (address token0, ) = sortTokens(cb.baseToken, cb.quoteToken);
        (uint256 amount0Out, uint256 amount1Out) = cb.baseToken == token0 ? (uint256(0), cb.quoteAmount) : (cb.quoteAmount, uint256(0));
        IPair(cb.pair0).swap(amount0Out, amount1Out, cb.pair1, new bytes(0));
    }

    // **** LIBRARY FUNCTIONS ****

    // returns sorted token addresses, used to handle return values from pairs sorted in this order
    function sortTokens(
        address tokenA,
        address tokenB
    ) internal pure returns (address token0, address token1) {
        require(tokenA != tokenB, "IDENTICAL_ADDRESSES");
        (token0, token1) = tokenA < tokenB ? (tokenA, tokenB) : (tokenB, tokenA);
        require(token0 != address(0), "ZERO_ADDRESS");
    }

    // fetches and sorts the reserves for a pair
    function getReserves(
        address pairAddress,
        address tokenA,
        address tokenB
    ) internal view returns (uint256 reserveA, uint256 reserveB) {
        (address token0, ) = sortTokens(tokenA, tokenB);
        (uint256 reserve0, uint256 reserve1, ) = IPair(pairAddress).getReserves();
        (reserveA, reserveB) = tokenA == token0 ? (reserve0, reserve1) : (reserve1, reserve0);
    }

    // given an input amount of an asset and pair reserves, returns the maximum output amount of the other asset
    function getAmountOut(
        uint256 amountIn,
        uint256 reserveIn,
        uint256 reserveOut
    ) internal pure returns (uint256 amountOut) {
        require(amountIn > 0, "INSUFFICIENT_INPUT_AMOUNT");
        require(reserveIn > 0 && reserveOut > 0, "INSUFFICIENT_LIQUIDITY");
        uint256 amountInWithFee = amountIn.mul(997);
        uint256 numerator = amountInWithFee.mul(reserveOut);
        uint256 denominator = reserveIn.mul(1000).add(amountInWithFee);
        amountOut = numerator / denominator;
    }

    // given an output amount of an asset and pair reserves, returns a required input amount of the other asset
    function getAmountIn(
        uint256 amountOut,
        uint256 reserveIn,
        uint256 reserveOut
    ) internal pure returns (uint256 amountIn) {
        require(amountOut > 0, "INSUFFICIENT_OUTPUT_AMOUNT");
        require(reserveIn > 0 && reserveOut > 0, "INSUFFICIENT_LIQUIDITY");
        uint256 numerator = reserveIn.mul(amountOut).mul(1000);
        uint256 denominator = reserveOut.sub(amountOut).mul(997);
        amountIn = (numerator / denominator).add(1);
    }
}