Add mock connector instances and patch overflow issue

contracts/savings/SavingsContract.sol

@@ -76,7 +76,7 @@ contract SavingsContract is
     // How often do we allow pokes
     uint256 constant private POKE_CADENCE = 4 hours;
     // Max APY generated on the capital in the connector
-    uint256 constant private MAX_APY = 2e18;
+    uint256 constant private MAX_APY = 4e18;
     uint256 constant private SECONDS_IN_YEAR = 365 days;
 
     // Add these constants to bytecode at deploytime
@@ -570,8 +570,6 @@ contract SavingsContract is
                 }
             }
             //     Else ideal == connectorBalance (e.g. 0), do nothing
-            // TODO - consider if this will actually work.. maybe check that new rawBalance is
-            // fully withdrawn?
             require(connector_.checkBalance() >= ideal, "Enforce system invariant");
 
             // 4i. Refresh exchange rate and emit event
@@ -606,7 +604,7 @@ contract SavingsContract is
         uint256 newExchangeRate = _calcExchangeRate(_realSum, _totalCredits);
         exchangeRate = newExchangeRate;
 
-        emit ExchangeRateUpdated(newExchangeRate, _realSum.sub(totalCredited));
+        emit ExchangeRateUpdated(newExchangeRate, _realSum > totalCredited ? _realSum.sub(totalCredited) : 0);
     }
 
     /**

contracts/z_mocks/savings/MockConnector.sol

@@ -5,8 +5,6 @@ import { IConnector } from "../../savings/peripheral/IConnector.sol";
 import { StableMath, SafeMath } from "../../shared/StableMath.sol";
 
 
-// Turn this into a real mock by issuing shares on deposit that go up in value
-
 contract MockConnector is IConnector {
 
     using StableMath for uint256;
@@ -46,10 +44,5 @@ contract MockConnector is IConnector {
 
     function checkBalance() external view returns (uint256) {
         return deposited;
-        // return StableMath.max(deposited, )
     }
-
-    // function _bumpSharePrice() internal private {
-    //     sharePrice = sharePrice.mul(1001).div(1000);
-    // }
 }

contracts/z_mocks/savings/MockLendingConnector.sol

@@ -0,0 +1,67 @@
+pragma solidity 0.5.16;
+
+import { IERC20, ERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
+import { IConnector } from "../../savings/peripheral/IConnector.sol";
+import { StableMath, SafeMath } from "../../shared/StableMath.sol";
+
+
+contract MockLendingConnector is IConnector {
+
+    using StableMath for uint256;
+    using SafeMath for uint256;
+
+    address save;
+    address mUSD;
+
+    uint256 lastValue;
+    uint256 lastAccrual;
+    uint256 constant perSecond = 31709791983;
+
+    constructor(
+        address _save,
+        address _mUSD
+    ) public {
+        save = _save;
+        mUSD = _mUSD;
+    }
+
+    modifier onlySave() {
+        require(save == msg.sender, "Only SAVE can call this");
+        _;
+    }
+
+    modifier _accrueValue() {
+        uint256 currentTime = block.timestamp;
+        if(lastAccrual != 0){
+            uint256 timeDelta = currentTime.sub(lastAccrual);
+            uint256 interest = timeDelta.mul(perSecond);
+            uint256 newValue = lastValue.mulTruncate(interest);
+            lastValue += newValue;
+        }
+        lastAccrual = currentTime;
+        _;
+    }
+
+    function poke() external _accrueValue {
+
+    }
+
+    function deposit(uint256 _amount) external _accrueValue onlySave {
+        IERC20(mUSD).transferFrom(save, address(this), _amount);
+        lastValue += _amount;
+    }
+
+    function withdraw(uint256 _amount) external _accrueValue onlySave {
+        IERC20(mUSD).transfer(save, _amount);
+        lastValue -= _amount;
+    }
+
+    function withdrawAll() external _accrueValue onlySave {
+        IERC20(mUSD).transfer(save, lastValue);
+        lastValue -= lastValue;
+    }
+
+    function checkBalance() external view returns (uint256) {
+        return lastValue;
+    }
+}

contracts/z_mocks/savings/MockVaultConnector.sol

@@ -0,0 +1,101 @@
+pragma solidity 0.5.16;
+
+import { IERC20, ERC20 } from "@openzeppelin/contracts/token/ERC20/ERC20.sol";
+import { IConnector } from "../../savings/peripheral/IConnector.sol";
+import { StableMath, SafeMath } from "../../shared/StableMath.sol";
+
+
+// Use this as a template for any volatile vault implementations, to ensure
+// connector invariant is held
+contract MockVaultConnector is IConnector {
+
+    using StableMath for uint256;
+    using SafeMath for uint256;
+
+    address save;
+    address mUSD;
+
+    uint256 trackedB;
+    uint256 realB;
+    uint256 lastAccrual;
+    uint256 constant perSecond = 31709791983;
+
+    constructor(
+        address _save,
+        address _mUSD
+    ) public {
+        save = _save;
+        mUSD = _mUSD;
+    }
+
+    modifier onlySave() {
+        require(save == msg.sender, "Only SAVE can call this");
+        _;
+    }
+
+    // realBalance
+    // trackedBalance
+
+    // step 1: deposit 100
+    //  - log trackedB amount
+    // step 2: check Balance
+    //  - get real balance
+    //  - trackedB > realB ? trackedB : realB
+    // step 3: realB goes to 100.1
+    // step 4: withdraw 10
+    //  - checkBalance must be >= 90.1 afterwards
+    //  - trackedB = 90.1
+    //  - trackedB > realB ? trackedB : realB
+    // stpe 5: withdraw 10
+    //  - checkedBalance must be >= 80.1 after
+    //  - trackedB = 80.1
+
+    modifier _accrueValue() {
+        _;
+        uint256 currentTime = block.timestamp;
+        if(lastAccrual != 0){
+            uint256 timeDelta = currentTime.sub(lastAccrual);
+            uint256 interest = timeDelta.mul(perSecond);
+            uint256 newValue = realB.mulTruncate(interest);
+            realB = newValue;
+        }
+        lastAccrual = currentTime;
+    }
+
+    function poke() external _accrueValue {
+
+    }
+
+    function deposit(uint256 _amount) external _accrueValue onlySave {
+        uint256 checkedB = _checkBalanceExt();
+        trackedB = checkedB.add(_amount);
+
+        IERC20(mUSD).transferFrom(save, address(this), _amount);
+        realB += _amount.mul(995).div(1000);
+    }
+
+    function withdraw(uint256 _amount) external _accrueValue onlySave {
+        uint256 checkedB = _checkBalanceExt();
+        trackedB = checkedB.sub(_amount);
+
+        IERC20(mUSD).transfer(save, _amount);
+        realB -= _amount.mul(1005).div(1000);
+    }
+
+    function withdrawAll() external _accrueValue onlySave {
+        trackedB = 0;
+
+        IERC20(mUSD).transfer(save, realB);
+        realB -= realB;
+    }
+
+    function checkBalance() external view returns (uint256) {
+        return _checkBalanceExt();
+    }
+
+    // a call to checkBalance followed by a deposit/withdraw then another checkbalance
+    // will always yield a sideways or increase
+    function _checkBalanceExt() internal view returns (uint256) {
+        return trackedB > realB ? trackedB : realB;
+    }
+}