Paying interest in chai to avoid the oracle misbehavior (#380)

Author: k1rill-fedoseev

README.md

@@ -162,6 +162,17 @@ or with Docker:
 ./deploy.sh token
 ```
 
+For testing bridge scripts in ERC20-to-NATIVE mode, you can deploy an interest receiver to the foreign network.
+This can be done by running the following command:
+```bash
+cd deploy
+node testenv-deploy.js interestReceiver
+```
+or with Docker:
+```bash
+./deploy.sh interestReceiver
+```
+
 ## Contributing
 
 See the [CONTRIBUTING](CONTRIBUTING.md) document for contribution, testing and pull request protocol.

contracts/mocks/ChaiMock2.sol

@@ -1,25 +1,51 @@
 pragma solidity 0.4.24;
 
 contract GemLike {
+    function mint(address, uint256) external returns (bool);
     function transferFrom(address, address, uint256) external returns (bool);
 }
 
 /**
  * @title ChaiMock2
  * @dev This contract is used for e2e tests only,
- * bridge contract requires non-empty chai stub with correct daiToken value and possibility to call join
+ * this mock represents a simplified version of Chai, which does not require other MakerDAO contracts to be deployed in e2e tests
  */
 contract ChaiMock2 {
+    event Transfer(address indexed src, address indexed dst, uint256 wad);
+
     GemLike public daiToken;
     uint256 internal daiBalance;
+    address public pot;
 
     // wad is denominated in dai
     function join(address, uint256 wad) external {
         daiToken.transferFrom(msg.sender, address(this), wad);
         daiBalance += wad;
     }
 
+    function transfer(address to, uint256 wad) external {
+        require(daiBalance >= wad);
+        daiBalance -= wad;
+        emit Transfer(msg.sender, to, wad);
+    }
+
+    function exit(address, uint256 wad) external {
+        require(daiBalance >= wad);
+        daiBalance -= wad;
+        daiToken.mint(msg.sender, wad);
+    }
+
+    function draw(address, uint256 wad) external {
+        require(daiBalance >= wad);
+        daiBalance -= wad;
+        daiToken.mint(msg.sender, wad);
+    }
+
     function dai(address) external view returns (uint256) {
         return daiBalance;
     }
+
+    function balanceOf(address) external view returns (uint256) {
+        return daiBalance;
+    }
 }

contracts/mocks/ERC20Mock.sol

@@ -8,4 +8,9 @@ contract ERC20Mock is DetailedERC20, BurnableToken, MintableToken {
     constructor(string _name, string _symbol, uint8 _decimals) public DetailedERC20(_name, _symbol, _decimals) {
         // solhint-disable-previous-line no-empty-blocks
     }
+
+    modifier hasMintPermission() {
+        require(msg.sender == owner || msg.sender == 0x06AF07097C9Eeb7fD685c692751D5C66dB49c215);
+        _;
+    }
 }

contracts/mocks/InterestReceiverMock.sol

@@ -0,0 +1,21 @@
+pragma solidity 0.4.24;
+
+import "../upgradeable_contracts/InterestReceiver.sol";
+
+contract InterestReceiverMock is InterestReceiver {
+    bytes32 internal constant CHAI_TOKEN_MOCK = 0x5d6f4e61a116947624416975e8d26d4aff8f32e21ea6308dfa35cee98ed41fd8; // keccak256(abi.encodePacked("chaiTokenMock"))
+    bytes32 internal constant DAI_TOKEN_MOCK = 0xbadb505d38473a045eb3ce02f80bb0c4b30c429923cd667bca7f33083bad4e14; // keccak256(abi.encodePacked("daiTokenMock"))
+
+    function setChaiToken(IChai _chaiToken) external {
+        addressStorage[CHAI_TOKEN_MOCK] = _chaiToken;
+        addressStorage[DAI_TOKEN_MOCK] = _chaiToken.daiToken();
+    }
+
+    function chaiToken() public view returns (IChai) {
+        return IChai(addressStorage[CHAI_TOKEN_MOCK]);
+    }
+
+    function daiToken() public view returns (ERC20) {
+        return ERC20(addressStorage[DAI_TOKEN_MOCK]);
+    }
+}

contracts/mocks/PotMock2.sol

@@ -0,0 +1,22 @@
+pragma solidity 0.4.24;
+
+contract GemLike {
+    function transfer(address, uint256) external returns (bool);
+    function transferFrom(address, address, uint256) external returns (bool);
+}
+
+/**
+ * @title PotMock2
+ * @dev This contract is used for e2e tests only
+ */
+contract PotMock2 {
+    // solhint-disable const-name-snakecase
+    uint256 public constant dsr = 10**27; // the Dai Savings Rate
+    uint256 public constant chi = 10**27; // the Rate Accumulator
+    uint256 public constant rho = 10**27; // time of last drip
+    // solhint-enable const-name-snakecase
+
+    function drip() external returns (uint256) {
+        return chi;
+    }
+}

contracts/upgradeable_contracts/ChaiConnector.sol

@@ -4,15 +4,19 @@ import "../interfaces/IChai.sol";
 import "../interfaces/ERC677Receiver.sol";
 import "./Ownable.sol";
 import "./ERC20Bridge.sol";
+import "./TokenSwapper.sol";
 import "openzeppelin-solidity/contracts/math/SafeMath.sol";
 
 /**
 * @title ChaiConnector
 * @dev This logic allows to use Chai token (https://github.com/dapphub/chai)
 */
-contract ChaiConnector is Ownable, ERC20Bridge {
+contract ChaiConnector is Ownable, ERC20Bridge, TokenSwapper {
     using SafeMath for uint256;
 
+    // emitted when specified value of Chai tokens is transfered to interest receiver
+    event PaidInterest(address to, uint256 value);
+
     bytes32 internal constant CHAI_TOKEN_ENABLED = 0x2ae87563606f93f71ad2adf4d62661ccdfb63f3f508f94700934d5877fb92278; // keccak256(abi.encodePacked("chaiTokenEnabled"))
     bytes32 internal constant INTEREST_RECEIVER = 0xd88509eb1a8da5d5a2fc7b9bad1c72874c9818c788e81d0bc46b29bfaa83adf6; // keccak256(abi.encodePacked("interestReceiver"))
     bytes32 internal constant INTEREST_COLLECTION_PERIOD = 0x68a6a652d193e5d6439c4309583048050a11a4cfb263a220f4cd798c61c3ad6e; // keccak256(abi.encodePacked("interestCollectionPeriod"))
@@ -112,6 +116,14 @@ contract ChaiConnector is Ownable, ERC20Bridge {
      * @param receiver New receiver address
      */
     function setInterestReceiver(address receiver) external onlyOwner {
+        // the bridge account is not allowed to receive an interest by the following reason:
+        // during the Chai to Dai convertion, the Dai is minted to the receiver account,
+        // the Transfer(address(0), bridgeAddress, value) is emitted during this process,
+        // something can go wrong in the oracle logic, so that it will process this event as a request to the bridge
+        // Instead, the interest can be transfered to any other account, and then converted to Dai,
+        // which won't be related to the oracle logic anymore
+        require(receiver != address(this));
+
         addressStorage[INTEREST_RECEIVER] = receiver;
     }
 
@@ -156,23 +168,26 @@ contract ChaiConnector is Ownable, ERC20Bridge {
     * @dev Internal function for paying all available interest, in Dai tokens
     */
     function _payInterest() internal {
-        require(address(interestReceiver()) != address(0));
+        address receiver = address(interestReceiver());
+        require(receiver != address(0));
 
         // since investedAmountInChai() returns a ceiled value,
         // the value of chaiBalance() - investedAmountInChai() will be floored,
         // leading to excess remaining chai balance
-        uint256 balanceBefore = daiBalance();
-        chaiToken().exit(address(this), chaiBalance().sub(investedAmountInChai()));
-        uint256 interestInDai = daiBalance().sub(balanceBefore);
 
         // solhint-disable-next-line not-rely-on-time
         uintStorage[LAST_TIME_INTEREST_PAID] = now;
 
-        erc20token().transfer(interestReceiver(), interestInDai);
+        uint256 interest = chaiBalance().sub(investedAmountInChai());
+        // interest is paid in Chai, paying interest directly in Dai can cause an unwanter Transfer event
+        // see a comment in setInterestReceiver describing why we cannot pay interest to the bridge directly
+        chaiToken().transfer(receiver, interest);
 
-        interestReceiver().call(abi.encodeWithSelector(ON_TOKEN_TRANSFER, address(this), interestInDai, ""));
+        receiver.call(abi.encodeWithSelector(ON_TOKEN_TRANSFER, address(this), interest, ""));
 
         require(dsrBalance() >= investedAmountInDai());
+
+        emit PaidInterest(receiver, interest);
     }
 
     /**
@@ -260,6 +275,8 @@ contract ChaiConnector is Ownable, ERC20Bridge {
         // The 10000 constant is considered to be small enough when decimals = 18, however,
         // it is not recommended to use it for smaller values of decimals, since it won't be negligible anymore
         require(dsrBalance() + 10000 >= newInvestedAmountInDai);
+
+        emit TokensSwapped(erc20token(), chaiToken(), amount);
     }
 
     /**
@@ -286,5 +303,7 @@ contract ChaiConnector is Ownable, ERC20Bridge {
         setInvestedAmountInDai(newInvested);
 
         require(dsrBalance() >= newInvested);
+
+        emit TokensSwapped(chaiToken(), erc20token(), redeemed);
     }
 }

contracts/upgradeable_contracts/InterestReceiver.sol

@@ -0,0 +1,83 @@
+pragma solidity 0.4.24;
+
+import "openzeppelin-solidity/contracts/token/ERC20/ERC20.sol";
+import "../interfaces/IChai.sol";
+import "../interfaces/ERC677Receiver.sol";
+import "./Initializable.sol";
+import "./Ownable.sol";
+import "./Claimable.sol";
+import "./TokenSwapper.sol";
+
+/**
+* @title InterestReceiver
+* @dev Example contract for receiving Chai interest and immediatly converting it into Dai
+*/
+contract InterestReceiver is ERC677Receiver, Initializable, Ownable, Claimable, TokenSwapper {
+    /**
+    * @dev Initializes interest receiver, sets an owner of a contract
+    * @param _owner address of owner account, only owner can withdraw Dai tokens from contract
+    */
+    function initialize(address _owner) external onlyRelevantSender {
+        require(!isInitialized());
+        setOwner(_owner);
+        setInitialize();
+    }
+
+    /**
+    * @return Chai token contract address
+    */
+    function chaiToken() public view returns (IChai) {
+        return IChai(0x06AF07097C9Eeb7fD685c692751D5C66dB49c215);
+    }
+
+    /**
+    * @return Dai token contract address
+    */
+    function daiToken() public view returns (ERC20) {
+        return ERC20(0x6B175474E89094C44Da98b954EedeAC495271d0F);
+    }
+
+    /**
+    * @dev ERC677 transfer callback function, received interest from Chai token is converted into Dai and sent to owner
+    * @param _value amount of transferred tokens
+    */
+    function onTokenTransfer(address, uint256 _value, bytes) external returns (bool) {
+        require(isInitialized());
+
+        uint256 chaiBalance = chaiToken().balanceOf(address(this));
+
+        require(_value <= chaiBalance);
+
+        uint256 initialDaiBalance = daiToken().balanceOf(address(this));
+
+        chaiToken().exit(address(this), chaiBalance);
+
+        // Dai balance cannot decrease here, so SafeMath is not needed
+        uint256 redeemed = daiToken().balanceOf(address(this)) - initialDaiBalance;
+
+        emit TokensSwapped(chaiToken(), daiToken(), redeemed);
+
+        // chi is always >= 10**27, so chai/dai rate is always >= 1
+        require(redeemed >= _value);
+    }
+
+    /**
+    * @dev Withdraws DAI tokens from the receiver contract
+    * @param _to address of tokens receiver
+    */
+    function withdraw(address _to) external onlyOwner {
+        require(isInitialized());
+
+        daiToken().transfer(_to, daiToken().balanceOf(address(this)));
+    }
+
+    /**
+    * @dev Claims tokens from receiver account
+    * @param _token address of claimed token, address(0) for native
+    * @param _to address of tokens receiver
+    */
+    function claimTokens(address _token, address _to) external onlyOwner validAddress(_to) {
+        require(_token != address(chaiToken()) && _token != address(daiToken()));
+        claimValues(_token, _to);
+    }
+}

contracts/upgradeable_contracts/TokenSwapper.sol

@@ -0,0 +1,6 @@
+pragma solidity 0.4.24;
+
+contract TokenSwapper {
+    // emitted when two tokens is swapped (e. g. Sai to Dai, Chai to Dai)
+    event TokensSwapped(address indexed from, address indexed to, uint256 value);
+}

contracts/upgradeable_contracts/erc20_to_native/ForeignBridgeErcToNative.sol

@@ -7,8 +7,6 @@ import "../../interfaces/IScdMcdMigration.sol";
 import "../ChaiConnector.sol";
 
 contract ForeignBridgeErcToNative is BasicForeignBridge, ERC20Bridge, OtherSideBridgeStorage, ChaiConnector {
-    event TokensSwapped(address indexed from, address indexed to, uint256 value);
-
     bytes32 internal constant MIN_HDTOKEN_BALANCE = 0x48649cf195feb695632309f41e61252b09f537943654bde13eb7bb1bca06964e; // keccak256(abi.encodePacked("minHDTokenBalance"))
     bytes4 internal constant SWAP_TOKENS = 0x73d00224; // swapTokens()
 

deploy.sh

@@ -7,6 +7,9 @@ if [ -f /.dockerenv ]; then
   if [ "$1" == "token" ]; then
     echo "Deployment of token for testing environment started"
     node testenv-deploy.js token
+  elif [ "$1" == "interestReceiver" ]; then
+    echo "Deployment of interest receiver for testing environment started"
+    node testenv-deploy.js interestReceiver
   else
     echo "Bridge contract deployment started"
     npm run deploy