Merge branch 'fix/56' into 'master'

Resolve "Validate divison in the aToken/variable debt token to ensure it's not rounded down to 0"

Closes #56

See merge request aave-tech/protocol-v2!64

contracts/lendingpool/LendingPoolCollateralManager.sol

@@ -239,12 +239,15 @@ contract LendingPoolCollateralManager is VersionedInitializable, LendingPoolStor
         principalReserve.variableBorrowIndex
       );
     } else {
+      //if the user does not have variable debt, no need to try to burn variable
+      //debt tokens
+      if (vars.userVariableDebt > 0) {
         IVariableDebtToken(principalReserve.variableDebtTokenAddress).burn(
           user,
           vars.userVariableDebt,
           principalReserve.variableBorrowIndex
         );
-
+      }
       IStableDebtToken(principalReserve.stableDebtTokenAddress).burn(
         user,
         vars.actualAmountToLiquidate.sub(vars.userVariableDebt)

contracts/libraries/helpers/Errors.sol

@@ -45,10 +45,12 @@ library Errors {
   string public constant INVALID_EQUAL_ASSETS_TO_SWAP = '56';
   string public constant NO_MORE_RESERVES_ALLOWED = '59';
 
-  // require error messages - aToken
+  // require error messages - aToken - DebtTokens
   string public constant CALLER_MUST_BE_LENDING_POOL = '28'; // 'The caller of this function must be a lending pool'
   string public constant CANNOT_GIVE_ALLOWANCE_TO_HIMSELF = '30'; // 'User cannot give allowance to himself'
   string public constant TRANSFER_AMOUNT_NOT_GT_0 = '31'; // 'Transferred amount needs to be greater than zero'
+  string public constant INVALID_MINT_AMOUNT = '53'; //invalid amount to mint
+  string public constant INVALID_BURN_AMOUNT = '54'; //invalid amount to burn
 
   // require error messages - ReserveLogic
   string public constant RESERVE_ALREADY_INITIALIZED = '34'; // 'Reserve has already been initialized'

contracts/tokenization/AToken.sol

@@ -101,7 +101,9 @@ contract AToken is VersionedInitializable, IncentivizedERC20, IAToken {
     uint256 amount,
     uint256 index
   ) external override onlyLendingPool {
-    _burn(user, amount.rayDiv(index));
+    uint256 amountScaled = amount.rayDiv(index);
+    require(amountScaled != 0, Errors.INVALID_BURN_AMOUNT);
+    _burn(user, amountScaled);
 
     //transfers the underlying to the target
     IERC20(UNDERLYING_ASSET_ADDRESS).safeTransfer(receiverOfUnderlying, amount);
@@ -116,22 +118,41 @@ contract AToken is VersionedInitializable, IncentivizedERC20, IAToken {
    * only lending pools can call this function
    * @param user the address receiving the minted tokens
    * @param amount the amount of tokens to mint
+   * @param index the the last index of the reserve
    */
   function mint(
     address user,
     uint256 amount,
     uint256 index
   ) external override onlyLendingPool {
-    //mint an equivalent amount of tokens to cover the new deposit
-    _mint(user, amount.rayDiv(index));
+    uint256 amountScaled = amount.rayDiv(index);
+    require(amountScaled != 0, Errors.INVALID_MINT_AMOUNT);
+    _mint(user, amountScaled);
 
     //transfer event to track balances
     emit Transfer(address(0), user, amount);
     emit Mint(user, amount, index);
   }
 
+
+  /**
+   * @dev mints aTokens to reserve treasury
+   * only lending pools can call this function
+   * @param amount the amount of tokens to mint to the treasury
+   * @param index the the last index of the reserve
+   */
   function mintToTreasury(uint256 amount, uint256 index) external override onlyLendingPool {
-    _mint(RESERVE_TREASURY_ADDRESS, amount.div(index));
+
+    if(amount == 0){
+      return;
+    }
+
+    //compared to the normal mint, we don't check for rounding errors.
+    //the amount to mint can easily be very small since is a fraction of the interest
+    //accrued. in that case, the treasury will experience a (very small) loss, but it
+    //wont cause potentially valid transactions to fail.
+
+    _mint(RESERVE_TREASURY_ADDRESS, amount.rayDiv(index));
 
     //transfer event to track balances
     emit Transfer(address(0), RESERVE_TREASURY_ADDRESS, amount);

contracts/tokenization/VariableDebtToken.sol

@@ -7,6 +7,7 @@ import {SafeMath} from '@openzeppelin/contracts/math/SafeMath.sol';
 import {DebtTokenBase} from './base/DebtTokenBase.sol';
 import {WadRayMath} from '../libraries/math/WadRayMath.sol';
 import {IVariableDebtToken} from './interfaces/IVariableDebtToken.sol';
+import {Errors} from '../libraries/helpers/Errors.sol';
 
 /**
  * @title contract VariableDebtToken
@@ -60,7 +61,10 @@ contract VariableDebtToken is DebtTokenBase, IVariableDebtToken {
     uint256 index
   ) external override onlyLendingPool {
     
-    _mint(user, amount.rayDiv(index));
+    uint256 amountScaled = amount.rayDiv(index);
+    require(amountScaled != 0, Errors.INVALID_MINT_AMOUNT);
+
+    _mint(user, amountScaled);
 
     emit Transfer(address(0), user, amount);
     emit Mint(user, amount, index);
@@ -76,7 +80,11 @@ contract VariableDebtToken is DebtTokenBase, IVariableDebtToken {
     uint256 amount,
     uint256 index
   ) external override onlyLendingPool {
-    _burn(user, amount.rayDiv(index));
+
+    uint256 amountScaled = amount.rayDiv(index);
+    require(amountScaled != 0, Errors.INVALID_BURN_AMOUNT);
+
+    _burn(user, amountScaled);
 
     emit Transfer(user, address(0), amount);
     emit Burn(user, amount, index);