fix: adapt repay rounding

Author: QGarchery

src/PositionsManager.sol

@@ -59,7 +59,7 @@ contract PositionsManager is IPositionsManager, PositionsManagerInternal {
 
         Types.SupplyRepayVars memory vars = _executeSupply(underlying, amount, from, onBehalf, maxIterations, indexes);
 
-        _pool.repayToPool(underlying, market.variableDebtToken, vars.toRepay);
+        _pool.repayToPool(underlying, market.variableDebtToken, vars.toRepay, indexes.borrow.poolIndex);
         _pool.supplyToPool(underlying, vars.toSupply, indexes.supply.poolIndex);
 
         return amount;
@@ -143,7 +143,7 @@ contract PositionsManager is IPositionsManager, PositionsManagerInternal {
         Types.SupplyRepayVars memory vars =
             _executeRepay(underlying, amount, repayer, onBehalf, _defaultIterations.repay, indexes);
 
-        _pool.repayToPool(underlying, market.variableDebtToken, vars.toRepay);
+        _pool.repayToPool(underlying, market.variableDebtToken, vars.toRepay, indexes.borrow.poolIndex);
         _pool.supplyToPool(underlying, vars.toSupply, indexes.supply.poolIndex);
 
         return amount;
@@ -259,7 +259,12 @@ contract PositionsManager is IPositionsManager, PositionsManagerInternal {
             underlyingCollateral, vars.seized, borrower, liquidator, collateralIndexes.supply.poolIndex
         );
 
-        _pool.repayToPool(underlyingBorrowed, _market[underlyingBorrowed].variableDebtToken, repayVars.toRepay);
+        _pool.repayToPool(
+            underlyingBorrowed,
+            _market[underlyingBorrowed].variableDebtToken,
+            repayVars.toRepay,
+            borrowIndexes.borrow.poolIndex
+        );
         _pool.supplyToPool(underlyingBorrowed, repayVars.toSupply, borrowIndexes.supply.poolIndex);
         _pool.withdrawFromPool(underlyingCollateral, _market[underlyingCollateral].aToken, vars.seized);
 

src/libraries/PoolLib.sol

@@ -35,8 +35,13 @@ library PoolLib {
 
     /// @notice Repays `amount` of `underlying` to `pool`.
     /// @dev If the debt has been fully repaid already, the function will return early.
-    function repayToPool(IPool pool, address underlying, address variableDebtToken, uint256 amount) internal {
-        if (amount == 0 || IVariableDebtToken(variableDebtToken).scaledBalanceOf(address(this)) == 0) return;
+    function repayToPool(IPool pool, address underlying, address variableDebtToken, uint256 amount, uint256 index)
+        internal
+    {
+        if (amount.rayDivDown(index) == 0 || IVariableDebtToken(variableDebtToken).scaledBalanceOf(address(this)) == 0)
+        {
+            return;
+        }
 
         pool.repay(underlying, amount, Constants.VARIABLE_INTEREST_MODE, address(this)); // Reverts if debt is 0.
     }

test/integration/TestIntegrationPoolLib.sol

@@ -80,7 +80,7 @@ contract TestIntegrationPoolLibRepay is TestIntegrationPoolLib {
         uint256 balanceBefore = ERC20(dai).balanceOf(address(this));
         uint256 vBalanceBefore = ERC20(vDai).balanceOf(address(this));
 
-        pool.repayToPool(dai, vDai, amount / 4);
+        pool.repayToPool(dai, vDai, amount / 4, pool.getReserveNormalizedVariableDebt(dai));
 
         assertEq(ERC20(dai).balanceOf(address(this)) + amount / 4, balanceBefore, "balance");
         assertApproxEqAbs(ERC20(vDai).balanceOf(address(this)) + amount / 4, vBalanceBefore, 2, "vBalance");