Merge pull request #683 from aave/fix/675-return-backing-amount

fix: Return real backing amount when backUnbacked

contracts/interfaces/IPool.sol

@@ -211,7 +211,7 @@ interface IPool {
   event MintedToTreasury(address indexed reserve, uint256 amountMinted);
 
   /**
-   * @dev Mints an `amount` of aTokens to the `onBehalfOf`
+   * @notice Mints an `amount` of aTokens to the `onBehalfOf`
    * @param asset The address of the underlying asset to mint
    * @param amount The amount to mint
    * @param onBehalfOf The address that will receive the aTokens
@@ -226,16 +226,17 @@ interface IPool {
   ) external;
 
   /**
-   * @dev Back the current unbacked underlying with `amount` and pay `fee`.
+   * @notice Back the current unbacked underlying with `amount` and pay `fee`.
    * @param asset The address of the underlying asset to back
    * @param amount The amount to back
    * @param fee The amount paid in fees
+   * @return The backed amount
    **/
   function backUnbacked(
     address asset,
     uint256 amount,
     uint256 fee
-  ) external;
+  ) external returns (uint256);
 
   /**
    * @notice Supplies an `amount` of underlying asset into the reserve, receiving in return overlying aTokens.

contracts/protocol/libraries/logic/BridgeLogic.sol

@@ -100,20 +100,22 @@ library BridgeLogic {
 
   /**
    * @notice Back the current unbacked with `amount` and pay `fee`.
+   * @dev It is not possible to back more than the existing unbacked amount of the reserve
    * @dev Emits the `BackUnbacked` event
    * @param reserve The reserve to back unbacked for
    * @param asset The address of the underlying asset to repay
    * @param amount The amount to back
    * @param fee The amount paid in fees
    * @param protocolFeeBps The fraction of fees in basis points paid to the protocol
+   * @return The backed amount
    **/
   function executeBackUnbacked(
     DataTypes.ReserveData storage reserve,
     address asset,
     uint256 amount,
     uint256 fee,
     uint256 protocolFeeBps
-  ) external {
+  ) external returns (uint256) {
     DataTypes.ReserveCache memory reserveCache = reserve.cache();
 
     reserve.updateState(reserveCache);
@@ -137,5 +139,7 @@ library BridgeLogic {
     IERC20(asset).safeTransferFrom(msg.sender, reserveCache.aTokenAddress, added);
 
     emit BackUnbacked(asset, msg.sender, backingAmount, fee);
+
+    return backingAmount;
   }
 }

contracts/protocol/pool/Pool.sol

@@ -136,8 +136,9 @@ contract Pool is VersionedInitializable, PoolStorage, IPool {
     address asset,
     uint256 amount,
     uint256 fee
-  ) external virtual override onlyBridge {
-    BridgeLogic.executeBackUnbacked(_reserves[asset], asset, amount, fee, _bridgeProtocolFee);
+  ) external virtual override onlyBridge returns (uint256) {
+    return
+      BridgeLogic.executeBackUnbacked(_reserves[asset], asset, amount, fee, _bridgeProtocolFee);
   }
 
   /// @inheritdoc IPool