Merge pull request #17 from euler-xyz/cantina-331

Cantina 331: EulerSavingsRate: Lack of early interest accruals due to precision loss
euler-xyz · Jul 30, 2024 · 47188d2 · 47188d2
2 parents 6d8747e + b25a795
commit 47188d2
Show file tree

Hide file tree

Showing 2 changed files with 47 additions and 8 deletions.
diff --git a/src/Synths/EulerSavingsRate.sol b/src/Synths/EulerSavingsRate.sol
@@ -142,6 +142,7 @@ contract EulerSavingsRate is EVCUtil, ERC4626 {
         uint256 maxGulp = type(uint168).max - esrSlotCache.interestLeft;
         if (toGulp > maxGulp) toGulp = maxGulp; // cap interest, allowing the vault to function
 
+        esrSlotCache.lastInterestUpdate = uint40(block.timestamp);
         esrSlotCache.interestSmearEnd = uint40(block.timestamp + INTEREST_SMEAR);
         esrSlotCache.interestLeft += uint168(toGulp); // toGulp <= maxGulp <= max uint168
 
@@ -157,15 +158,17 @@ contract EulerSavingsRate is EVCUtil, ERC4626 {
         ESRSlot memory esrSlotCache = esrSlot;
         uint256 accruedInterest = interestAccruedFromCache(esrSlotCache);
 
-        // it's safe to down-cast because the accrued interest is a fraction of interest left
-        esrSlotCache.interestLeft -= uint168(accruedInterest);
-        esrSlotCache.lastInterestUpdate = uint40(block.timestamp);
-        // write esrSlotCache back to storage in a single SSTORE
-        esrSlot = esrSlotCache;
-        // Move interest accrued to totalAssets
-        _totalAssets = _totalAssets + accruedInterest;
+        if (accruedInterest > 0) {
+            // it's safe to down-cast because the accrued interest is a fraction of interest left
+            esrSlotCache.interestLeft -= uint168(accruedInterest);
+            esrSlotCache.lastInterestUpdate = uint40(block.timestamp);
+            // write esrSlotCache back to storage in a single SSTORE
+            esrSlot = esrSlotCache;
+            // Move interest accrued to totalAssets
+            _totalAssets = _totalAssets + accruedInterest;
 
-        emit InterestUpdated(accruedInterest, esrSlotCache.interestLeft);
+            emit InterestUpdated(accruedInterest, esrSlotCache.interestLeft);
+        }
 
         return esrSlotCache;
     }

diff --git a/test/unit/esr/ESR.Fuzz.t.sol b/test/unit/esr/ESR.Fuzz.t.sol
@@ -68,6 +68,42 @@ contract ESRFuzzTest is ESRTest {
         }
     }
 
+    function testFuzz_conditionalAccruedInterestUpdate(uint32 interestAmount) public {
+        // min deposit requirement before gulp
+        doDeposit(user, 1e7);
+
+        // mint some interest to be distributed
+        asset.mint(address(esr), interestAmount);
+
+        uint256 balance = asset.balanceOf(address(esr));
+        uint256 totalAssets = esr.totalAssets();
+
+        esr.gulp();
+        skip(1);
+
+        if (interestAmount < esr.INTEREST_SMEAR()) {
+            assertEq(esr.totalAssets(), totalAssets);
+            assertEq(esr.totalAssets() + interestAmount, balance);
+        } else {
+            uint256 accruedInterest = interestAmount / esr.INTEREST_SMEAR();
+            assertEq(esr.totalAssets() + interestAmount - accruedInterest, balance);
+            vm.expectEmit();
+            emit EulerSavingsRate.InterestUpdated(accruedInterest, interestAmount - accruedInterest);
+        }
+
+        vm.recordLogs();
+        esr.gulp();
+
+        Vm.Log[] memory logs = vm.getRecordedLogs();
+        if (interestAmount < esr.INTEREST_SMEAR()) {
+            assertEq(logs.length, 1);
+            assertNotEq(logs[0].topics[0], EulerSavingsRate.InterestUpdated.selector);
+        } else {
+            assertEq(logs.length, 2);
+            assertEq(logs[0].topics[0], EulerSavingsRate.InterestUpdated.selector);
+        }
+    }
+
     // fuzz test that any deposits added are added to the totalAssetsDeposited
     function testFuzz_deposit(uint256 depositAmount, uint256 depositAmount2) public {
         depositAmount = bound(depositAmount, 0, type(uint112).max);