fix: Prevent the ration between batch shares and debt to be too low

Author: bingen

contracts/src/Dependencies/Constants.sol

@@ -28,6 +28,11 @@ uint128 constant MIN_INTEREST_RATE_CHANGE_PERIOD = 1 seconds; // prevents more t
 
 uint256 constant REDEMPTION_FEE_FLOOR = _1pct / 2; // 0.5%
 
+// For the shares ratio to decrease 1e9 (1e18/MIN_BATCH_SHARES_RATIO),
+// at an average annual rate of 10%, it would take more than 217 years (log(1e9)/log(1.1))
+// at an average annual rate of 50%, it would take more than 51 years (log(1e9)/log(1.5))
+uint256 constant MIN_BATCH_SHARES_RATIO = 1e9;
+
 // Half-life of 12h. 12h = 720 min
 // (1/2) = d^720 => d = (1/2)^(1/720)
 uint256 constant REDEMPTION_MINUTE_DECAY_FACTOR = 999037758833783000;

contracts/src/TroveManager.sol

@@ -166,6 +166,7 @@ contract TroveManager is LiquityBase, ITroveManager, ITroveEvents {
     error NotShutDown();
     error NotEnoughBoldBalance();
     error MinCollNotReached(uint256 _coll);
+    error BatchSharesRatioTooLow();
 
     // --- Events ---
 
@@ -1784,6 +1785,12 @@ contract TroveManager is LiquityBase, ITroveManager, ITroveEvents {
                 if (_batchDebt == 0) {
                     batchDebtSharesDelta = debtIncrease;
                 } else {
+                    // For the shares ratio to decrease 1e9 (1e18/MIN_BATCH_SHARES_RATIO),
+                    // at an average annual rate of 10%, it would take more than 217 years (log(1e9)/log(1.1))
+                    // at an average annual rate of 50%, it would take more than 51 years (log(1e9)/log(1.5))
+                    // When that happens, no more debt can be manually added to the batch, so batch should be migrated to a new one
+                    _requireAtLeastMinSharesRatio(currentBatchDebtShares, _batchDebt);
+
                     batchDebtSharesDelta = currentBatchDebtShares * debtIncrease / _batchDebt;
                 }
 
@@ -1825,6 +1832,12 @@ contract TroveManager is LiquityBase, ITroveManager, ITroveEvents {
         }
     }
 
+    function _requireAtLeastMinSharesRatio(uint256 _currentBatchDebtShares, uint256 _batchDebt) internal pure {
+        if (_currentBatchDebtShares * DECIMAL_PRECISION / _batchDebt < MIN_BATCH_SHARES_RATIO) {
+            revert BatchSharesRatioTooLow();
+        }
+    }
+
     function onRemoveFromBatch(
         uint256 _troveId,
         uint256 _newTroveColl, // entire, with redistribution

contracts/src/test/rebasingBatchShares.t.sol

@@ -98,8 +98,11 @@ contract RebasingBatchShares is DevTestSetup {
 
         // === 4: Free Loans === //
         uint256 debtB4 = borrowerOperations.getEntireSystemDebt();
-        // We can now open a new Trove
-        uint256 anotherATroveId = openTroveAndJoinBatchManager(A, 100 ether, MIN_DEBT, B, MIN_ANNUAL_INTEREST_RATE);
+        // We shouldn’t be able to open a new Trove now
+        uint256 anotherATroveId = openTroveExpectRevert(A, 100 ether, MIN_DEBT, B);
+        assertEq(anotherATroveId, 0);
+
+        /*
         LatestTroveData memory anotherATrove = troveManager.getLatestTroveData(anotherATroveId);
         uint256 aDebt = anotherATrove.entireDebt;
 
@@ -115,6 +118,7 @@ contract RebasingBatchShares is DevTestSetup {
 
         assertGt(debtAfter, debtB4, "Debt should have increased");
         assertLt(balB4, balAfter, "Something should have benn paid");
+        */
     }
 
     uint128 subTractor = 1;
@@ -140,6 +144,7 @@ contract RebasingBatchShares is DevTestSetup {
         // Log ratio
         uint256 batchSharesRatio = batchShares * DECIMAL_PRECISION / batchDebt;
         console2.log("shares / batch ratio: ", batchSharesRatio);
+        console2.log("Ratio too low?        ", batchSharesRatio < MIN_BATCH_SHARES_RATIO);
 
         // Trove
         /*
@@ -149,6 +154,7 @@ contract RebasingBatchShares is DevTestSetup {
         console2.log("Trove Shares:         ", troveBatchShares);
         uint256 troveSharesRatio = troveBatchShares * DECIMAL_PRECISION / troveData.entireDebt;
         console2.log("Trove ratio:          ", troveSharesRatio);
+        console2.log("Ratio too low?        ", troveSharesRatio < MIN_BATCH_SHARES_RATIO);
         */
     }
 
@@ -198,4 +204,30 @@ contract RebasingBatchShares is DevTestSetup {
 
         return allBatchDebtShares;
     }
+
+    function openTroveExpectRevert(address _troveOwner, uint256 _coll, uint256 _debt, address _batchAddress)
+        internal
+        returns (uint256)
+    {
+        IBorrowerOperations.OpenTroveAndJoinInterestBatchManagerParams memory params = IBorrowerOperations
+            .OpenTroveAndJoinInterestBatchManagerParams({
+            owner: _troveOwner,
+            ownerIndex: 0,
+            collAmount: _coll,
+            boldAmount: _debt,
+            upperHint: 0,
+            lowerHint: 0,
+            interestBatchManager: _batchAddress,
+            maxUpfrontFee: 1e24,
+            addManager: address(0),
+            removeManager: address(0),
+            receiver: address(0)
+        });
+        vm.startPrank(_troveOwner);
+        vm.expectRevert(TroveManager.BatchSharesRatioTooLow.selector);
+        uint256 troveId = borrowerOperations.openTroveAndJoinInterestBatchManager(params);
+        vm.stopPrank();
+
+        return troveId;
+    }
 }