Add minimum purchase amount on liquidations (#114)

* Implement the fix
* Fix existing test cases
* Add test cases for market patch

contracts/masset/liquidator/ILiquidator.sol

@@ -9,7 +9,8 @@ contract ILiquidator {
         address _bAsset,
         int128 _curvePosition,
         address[] calldata _uniswapPath,
-        uint256 _trancheAmount
+        uint256 _trancheAmount,
+        uint256 _minReturn
     )
         external;
 
@@ -18,7 +19,8 @@ contract ILiquidator {
         address _bAsset,
         int128 _curvePosition,
         address[] calldata _uniswapPath,
-        uint256 _trancheAmount
+        uint256 _trancheAmount,
+        uint256 _minReturn
     )
         external;
 

contracts/masset/liquidator/Liquidator.sol

@@ -9,6 +9,7 @@ import { InitializableModule } from "../../shared/InitializableModule.sol";
 import { ILiquidator } from "./ILiquidator.sol";
 import { MassetHelpers } from "../../masset/shared/MassetHelpers.sol";
 
+import { IBasicToken } from "../../shared/IBasicToken.sol";
 import { SafeMath } from "@openzeppelin/contracts/math/SafeMath.sol";
 import { SafeERC20 } from "@openzeppelin/contracts/token/ERC20/SafeERC20.sol";
 import { IERC20 } from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
@@ -40,6 +41,7 @@ contract Liquidator is
     uint256 private interval = 7 days;
 
     mapping(address => Liquidation) public liquidations;
+    mapping(address => uint256) public minReturn;
 
     struct Liquidation {
         address sellToken;
@@ -85,14 +87,16 @@ contract Liquidator is
     * @param _curvePosition Position of the bAsset in Curves MetaPool
     * @param _uniswapPath The Uniswap path as an array of addresses e.g. [COMP, WETH, DAI]
     * @param _trancheAmount The amount of bAsset units to buy in each weekly tranche
+    * @param _minReturn Minimum exact amount of bAsset to get for each (whole) sellToken unit
     */
     function createLiquidation(
         address _integration,
         address _sellToken,
         address _bAsset,
         int128 _curvePosition,
         address[] calldata _uniswapPath,
-        uint256 _trancheAmount
+        uint256 _trancheAmount,
+        uint256 _minReturn
     )
         external
         onlyGovernance
@@ -103,7 +107,8 @@ contract Liquidator is
             _integration != address(0) &&
             _sellToken != address(0) &&
             _bAsset != address(0) &&
-            _uniswapPath.length >= 2,
+            _uniswapPath.length >= 2 &&
+            _minReturn > 0,
             "Invalid inputs"
         );
         require(_validUniswapPath(_sellToken, _bAsset, _uniswapPath), "Invalid uniswap path");
@@ -116,6 +121,7 @@ contract Liquidator is
             lastTriggered: 0,
             trancheAmount: _trancheAmount
         });
+        minReturn[_integration] = _minReturn;
 
         emit LiquidationModified(_integration);
     }
@@ -127,13 +133,15 @@ contract Liquidator is
     * @param _curvePosition Position of the bAsset in Curves MetaPool
     * @param _uniswapPath The Uniswap path as an array of addresses e.g. [COMP, WETH, DAI]
     * @param _trancheAmount The amount of bAsset units to buy in each weekly tranche
+    * @param _minReturn Minimum exact amount of bAsset to get for each (whole) sellToken unit
     */
     function updateBasset(
         address _integration,
         address _bAsset,
         int128 _curvePosition,
         address[] calldata _uniswapPath,
-        uint256 _trancheAmount
+        uint256 _trancheAmount,
+        uint256 _minReturn
     )
         external
         onlyGovernance
@@ -142,14 +150,16 @@ contract Liquidator is
 
         address oldBasset = liquidation.bAsset;
         require(oldBasset != address(0), "Liquidation does not exist");
-
+
+        require(_minReturn > 0, "Must set some minimum value");
         require(_bAsset != address(0), "Invalid bAsset");
         require(_validUniswapPath(liquidation.sellToken, _bAsset, _uniswapPath), "Invalid uniswap path");
 
         liquidations[_integration].bAsset = _bAsset;
         liquidations[_integration].curvePosition = _curvePosition;
         liquidations[_integration].uniswapPath = _uniswapPath;
         liquidations[_integration].trancheAmount = _trancheAmount;
+        minReturn[_integration] = _minReturn;
 
         emit LiquidationModified(_integration);
     }
@@ -180,6 +190,8 @@ contract Liquidator is
         require(liquidation.bAsset != address(0), "Liquidation does not exist");
 
         delete liquidations[_integration];
+        delete minReturn[_integration];
+
         emit LiquidationEnded(_integration);
     }
 
@@ -197,6 +209,9 @@ contract Liquidator is
     function triggerLiquidation(address _integration)
         external
     {
+        // solium-disable-next-line security/no-tx-origin
+        require(tx.origin == msg.sender, "Must be EOA");
+
         Liquidation memory liquidation = liquidations[_integration];
 
         address bAsset = liquidation.bAsset;
@@ -234,19 +249,24 @@ contract Liquidator is
         IERC20(sellToken).safeApprove(address(uniswap), 0);
         IERC20(sellToken).safeApprove(address(uniswap), sellAmount);
         // 3.2. Make the sale > https://uniswap.org/docs/v2/smart-contracts/router02/#swapexacttokensfortokens
+
+        // min amount out = sellAmount * priceFloor / 1e18
+        // e.g. 1e18 * 100e6 / 1e18 = 100e6
+        // e.g. 30e8 * 100e6 / 1e8 = 3000e6
+        // e.g. 30e18 * 100e18 / 1e18 = 3000e18
+        uint256 sellTokenDec = IBasicToken(sellToken).decimals();
+        uint256 minOut = sellAmount.mul(minReturn[_integration]).div(10 ** sellTokenDec);
+        require(minOut > 0, "Must have some price floor");
         uniswap.swapExactTokensForTokens(
             sellAmount,
-            0,
+            minOut,
             uniswapPath,
             address(this),
             block.timestamp.add(1800)
         );
-        uint256 bAssetBal = IERC20(bAsset).balanceOf(address(this));
 
         // 3.3. Trade on Curve
-        IERC20(bAsset).safeApprove(address(curve), 0);
-        IERC20(bAsset).safeApprove(address(curve), bAssetBal);
-        uint256 purchased = curve.exchange_underlying(liquidation.curvePosition, 0, bAssetBal, 0);
+        uint256 purchased = _sellOnCrv(bAsset, liquidation.curvePosition);
 
         // 4.0. Send to SavingsManager
         address savings = _savingsManager();
@@ -256,4 +276,15 @@ contract Liquidator is
 
         emit Liquidated(sellToken, mUSD, purchased, bAsset);
     }
+
+    function _sellOnCrv(address _bAsset, int128 _curvePosition) internal returns (uint256 purchased) {
+        uint256 bAssetBal = IERC20(_bAsset).balanceOf(address(this));
+
+        IERC20(_bAsset).safeApprove(address(curve), 0);
+        IERC20(_bAsset).safeApprove(address(curve), bAssetBal);
+        uint256 bAssetDec = IBasicToken(_bAsset).decimals();
+        // e.g. 100e6 * 95e16 / 1e6 = 100e18
+        uint256 minOutCrv = bAssetBal.mul(95e16).div(10 ** bAssetDec);
+        purchased = curve.exchange_underlying(_curvePosition, 0, bAssetBal, minOutCrv);
+    }
 }

contracts/z_mocks/shared/MockCurveMetaPool.sol

@@ -11,22 +11,30 @@ contract MockCurveMetaPool is ICurveMetaPool {
 
     address[] public coins;
     address mUSD;
+    // number of out per in (scaled)
+    uint256 ratio = 98e16;
+
 
     constructor(address[] memory _coins, address _mUSD) public {
         require(_coins[0] == _mUSD, "Coin 0 must be mUSD");
         coins = _coins;
         mUSD = _mUSD;
     }
 
+    function setRatio(uint256 _newRatio) external {
+        ratio = _newRatio;
+    }
+
     // takes dx i from sender, returns j
-    function exchange_underlying(int128 i, int128 j, uint256 dx, uint256 /*min_dy*/)
+    function exchange_underlying(int128 i, int128 j, uint256 dx, uint256 min_dy)
         external
         returns (uint256)
     {
         require(j == 0, "Output must be mUSD");
         address in_tok = coins[uint256(i)];
         uint256 decimals = IBasicToken(in_tok).decimals();
-        uint256 out_amt = dx * (10 ** (18 - decimals));
+        uint256 out_amt = dx * (10 ** (18 - decimals)) * ratio / 1e18;
+        require(out_amt >= min_dy, "CRV: Output amount not enough");
         IERC20(in_tok).transferFrom(msg.sender, address(this), dx);
         IERC20(mUSD).transfer(msg.sender, out_amt);
         return out_amt;

contracts/z_mocks/shared/MockTrigger.sol

@@ -0,0 +1,11 @@
+pragma solidity 0.5.16;
+
+import { ILiquidator } from "../../masset/liquidator/ILiquidator.sol";
+
+
+contract MockTrigger {
+
+    function trigger(ILiquidator _liq, address _integration) external {
+        _liq.triggerLiquidation(_integration);
+    }
+}

contracts/z_mocks/shared/MockUniswap.sol

@@ -10,11 +10,17 @@ import { IERC20 } from "@openzeppelin/contracts/token/ERC20/IERC20.sol";
 //   out token has 18 decimals
 contract MockUniswap is IUniswapV2Router02 {
 
+    // how many tokens to give out for 1 in
+    uint256 ratio = 106;
+
+    function setRatio(uint256 _outRatio) external {
+        ratio = _outRatio;
+    }
 
     // takes input from sender, produces output
     function swapExactTokensForTokens(
         uint amountIn,
-        uint /*amountOutMin*/,
+        uint amountOutMin,
         address[] calldata path,
         address /*to*/,
         uint /*deadline*/
@@ -28,7 +34,9 @@ contract MockUniswap is IUniswapV2Router02 {
         amounts[0] = amountIn;
         IERC20(path[0]).transferFrom(msg.sender, address(this), amountIn);
 
-        uint256 output = amountIn * 106;
+        uint256 output = amountIn * ratio;
+        require(output >= amountOutMin, "UNI: Output amount not enough");
+
         amounts[len-1] = output;
         IERC20(path[len-1]).transfer(msg.sender, output);
     }
@@ -41,7 +49,7 @@ contract MockUniswap is IUniswapV2Router02 {
         view
         returns (uint[] memory amounts)
     {
-        uint256 amountIn = amountOut / 106;
+        uint256 amountIn = amountOut / ratio;
         uint256 len = path.length;
         amounts = new uint[](len);
         amounts[0] = amountIn;