removeLiquidity function in a smart contract, getting execution reverted due to ds-math-sub-underflow

Question

i'm trying to create a smart contract which removes user's lp share from the pool doing these steps:

1. calculates the current user's balance of LP tokens for a particular pair
2. removes their liquidity from the pool and sends all the assets back to them

contract UniswapLiquidity {
    address UniswapV2Router = 0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D;
    address UniswapV2Factory = 0x5C69bEe701ef814a2B6a3EDD4B1652CB9cc5aA6f;
    address WETH = 0xB4FBF271143F4FBf7B91A5ded31805e42b2208d6;
    
    event Logs(string message, uint value);

    function removeLiquidity(
        address _tokenA,
        address _tokenB
    ) external returns (address pair) {
        //get pair address
        pair = IUniswapV2Factory(UniswapV2Factory).getPair(_tokenA, _tokenB);
        
        uint liquidity = IERC20(pair).balanceOf(msg.sender);

        //approve router to remove our liquidity token
        require(IERC20(pair).approve(address(this), liquidity));
        require(IERC20(pair).approve(UniswapV2Router, liquidity));

        (uint amountA, uint amountB) = IUniswapV2Router02(UniswapV2Router)
            .removeLiquidity(
                _tokenA,
                _tokenB,
                liquidity,
                1001, // 0, 1 or a calculation dont make any sense
                1001,
                msg.sender,
                block.timestamp + 20 seconds
            );

        emit Logs("amount A gotten", amountA);
        emit Logs("amount B gotten", amountB);
        emit Logs("Liquidity token burnt", liquidity);
    }
}

but im getting execution reverted due to ds-math-sub-underflow, though i tried different solutions, like extra approves and attempting to remove only 10% of the share using safemath, it didnt work out

Answer 1

As you mention in your comment, it's likely failing when the router tries to transfer the LP tokens from your contract to the pair contract. In order for this to work, the user must set their own approval of the LP tokens to your smart contract prior to calling your contract. Then you use transferFrom to take the LP tokens from the user into your UniswapLiquidity contract. Then set the approval (as you are already doing) from the UniswapLiquidity contract to the router contract.

The require(IERC20(pair).approve(address(this), liquidity)); line can also be removed as the contract is just approving to itself, which isn't necessary