TOB-AJNA-2: global scalar (at index 8192) is never updated (#753)

* TOB-AJNA-2: global scalar (at index 8192) is never updated
- save an SLOAD in Deposits.treeSum as scalar is not updated (always 1)

* Changes after review: add F5 invariant

src/base/Pool.sol

@@ -780,6 +780,11 @@ abstract contract Pool is Clone, ReentrancyGuard, Multicall, IPool {
         return PoolCommons.utilization(emaState);
     }
 
+    /// @inheritdoc IPoolDerivedState
+    function depositScale(uint256 index_) external view override returns (uint256) {
+        return deposits.scaling[index_];
+    }
+
     /// @inheritdoc IPoolState
     function emasInfo() external view override returns (uint256, uint256, uint256, uint256) {
         return (

src/interfaces/pool/commons/IPoolDerivedState.sol

@@ -46,4 +46,13 @@ interface IPoolDerivedState {
      */
     function depositUtilization() external view returns (uint256);
 
+    /**
+     *  @notice Returns the scaling value of deposit at given index.
+     *  @param  index_  Deposit index.
+     *  @return Deposit scaling.
+     */
+    function depositScale(
+        uint256 index_
+    ) external view returns (uint256);
+
 }

src/libraries/internal/Deposits.sol

@@ -321,10 +321,8 @@ library Deposits {
     function treeSum(
         DepositsState storage deposits_
     ) internal view returns (uint256) {
-        // In a scaled Fenwick tree, sum is at the root node, but needs to be scaled
-        uint256 scaling = deposits_.scaling[SIZE];
-        // scaling == 0 means scale factor is actually 1
-        return (scaling != 0) ? Maths.wmul(scaling, deposits_.values[SIZE]) : deposits_.values[SIZE]; 
+        // In a scaled Fenwick tree, sum is at the root node and never scaled
+        return deposits_.values[SIZE];
     }
 
     /**

tests/INVARIANTS.md

@@ -49,6 +49,7 @@
 - **F2**: For any index `i`, the prefix sum up to and including `i` is the sum of values stored in indices `j<=i`
 - **F3**: For any index `i < MAX_FENWICK_INDEX`,  `findIndexOfSum(prefixSum(i)) > i`
 - **F4**: For any index i, there is zero deposit above i and below findIndexOfSum(prefixSum(i) + 1): `depositAtIndex(j) == 0 for i < j < findIndexOfSum(prefixSum(i)+1)`
+- **F5**: Global scalar is never updated (`DepositsState.scaling[8192]` is always 0)
 
 ## Exchange rate invariants ##
 - **R1**: Exchange rates are unchanged by pledging collateral

tests/forge/invariants/base/BasicInvariants.t.sol

@@ -43,6 +43,7 @@ abstract contract BasicInvariants is BaseInvariants {
         * F2: For any index i, the prefix sum up to and including i is the sum of values stored in indices j<=i
         * F3: For any index i < MAX_FENWICK_INDEX, findIndexOfSum(prefixSum(i)) > i
         * F4: For any index i, there is zero deposit above i and below findIndexOfSum(prefixSum(i) + 1): findIndexOfSum(prefixSum(i)) == findIndexOfSum(prefixSum(j) - deposits.valueAt(j)), where j is the next index from i with deposits != 0
+        * F5: Global scalar is never updated (`DepositsState.values[8192]` is always 0)
     ****************************************************************************************************************************************/
 
     // checks pool lps are equal to sum of all lender lps in a bucket 
@@ -352,6 +353,11 @@ abstract contract BasicInvariants is BaseInvariants {
         }
     }
 
+    // **F5**: Global scalar is never updated (`DepositsState.scaling[8192]` is always 0)
+    function invariant_fenwick_globalscalar_F5() public useCurrentTimestamp {
+        require(_pool.depositScale(8192) == 0, "F5: Global scalar was updated");
+    }
+
     function invariant_call_summary() public virtual useCurrentTimestamp {
         console.log("\nCall Summary\n");
         console.log("--Lender----------");