Prevent invariant overflow (#1690)

* Add check to prevent invariant overflow * Update pkg/pool-stable/contracts/ComposableStablePoolProtocolFees.sol Co-authored-by: Tom French <[email protected]> Co-authored-by: Tom French <[email protected]>
balancer · Nov 25, 2022 · a5f3958 · a5f3958
1 parent 93af2fd
commit a5f3958
Showing 1 changed file with 25 additions and 8 deletions.
diff --git a/pkg/pool-stable/contracts/ComposableStablePoolProtocolFees.sol b/pkg/pool-stable/contracts/ComposableStablePoolProtocolFees.sol
@@ -258,15 +258,32 @@ abstract contract ComposableStablePoolProtocolFees is
 
         uint256 postJoinExitInvariant = StableMath._calculateInvariant(currentAmp, balances);
 
-        uint256 protocolFeeAmount = InvariantGrowthProtocolSwapFees.calcDueProtocolFees(
-            postJoinExitInvariant.divDown(preJoinExitInvariant),
-            preJoinExitSupply,
-            postJoinExitSupply,
-            getProtocolFeePercentageCache(ProtocolFeeType.SWAP)
-        );
+        // Compute the portion of the invariant increase due to fees
+        uint256 supplyGrowthRatio = postJoinExitSupply.divDown(preJoinExitSupply);
+        uint256 feelessInvariant = preJoinExitInvariant.mulDown(supplyGrowthRatio);
+
+        // The postJoinExitInvariant should always be greater than the feelessInvariant (since the invariant and total
+        // supply move proportionally outside of fees, which the postJoinInvariant includes and the feelessInvariant
+        // does not). However, in the unexpected case in which due to rounding errors this is not true, we simply skip
+        // further computation of protocol fees.
+        if (postJoinExitInvariant > feelessInvariant) {
+            uint256 invariantDeltaFromFees = postJoinExitInvariant - feelessInvariant;
+
+            // To convert to a percentage of pool ownership, multiply by the rate,
+            // then normalize against the final invariant
+            uint256 protocolOwnershipPercentage = Math.divDown(
+                Math.mul(invariantDeltaFromFees, getProtocolFeePercentageCache(ProtocolFeeType.SWAP)),
+                postJoinExitInvariant
+            );
 
-        if (protocolFeeAmount > 0) {
-            _payProtocolFees(protocolFeeAmount);
+            if (protocolOwnershipPercentage > 0) {
+                uint256 protocolFeeAmount = ProtocolFees.bptForPoolOwnershipPercentage(
+                    postJoinExitSupply,
+                    protocolOwnershipPercentage
+                );
+
+                _payProtocolFees(protocolFeeAmount);
+            }
         }
 
         _updatePostJoinExit(currentAmp, postJoinExitInvariant);