Add market borrow caps (#65)

Co-authored-by: Aryeh Greenberg <aryeh@aryehgreenberg.com>

contracts/Comptroller.sol

@@ -11,9 +11,9 @@ import "./Governance/Comp.sol";
 
 /**
  * @title Compound's Comptroller Contract
- * @author Compound
+ * @author Compound (modified by Arr00)
  */
-contract Comptroller is ComptrollerV3Storage, ComptrollerInterface, ComptrollerErrorReporter, Exponential {
+contract Comptroller is ComptrollerV4Storage, ComptrollerInterface, ComptrollerErrorReporter, Exponential {
     /// @notice Emitted when an admin supports a market
     event MarketListed(CToken cToken);
 
@@ -62,6 +62,12 @@ contract Comptroller is ComptrollerV3Storage, ComptrollerInterface, ComptrollerE
     /// @notice Emitted when COMP is distributed to a borrower
     event DistributedBorrowerComp(CToken indexed cToken, address indexed borrower, uint compDelta, uint compBorrowIndex);
 
+    /// @notice Emitted when borrow cap for a cToken is changed
+    event NewBorrowCap(CToken indexed cToken, uint newBorrowCap);
+
+    /// @notice Emitted when borrow cap guardian is changed
+    event NewBorrowCapGuardian(address oldBorrowCapGuardian, address newBorrowCapGuardian);
+
     /// @notice The threshold above which the flywheel transfers COMP, in wei
     uint public constant compClaimThreshold = 0.001e18;
 
@@ -365,6 +371,16 @@ contract Comptroller is ComptrollerV3Storage, ComptrollerInterface, ComptrollerE
             return uint(Error.PRICE_ERROR);
         }
 
+
+        uint borrowCap = borrowCaps[cToken];
+        // Borrow cap of 0 corresponds to unlimited borrowing
+        if (borrowCap != 0) {
+            uint totalBorrows = CToken(cToken).totalBorrows();
+            (MathError mathErr, uint nextTotalBorrows) = addUInt(totalBorrows, borrowAmount);
+            require(mathErr == MathError.NO_ERROR, "total borrows overflow");
+            require(nextTotalBorrows < borrowCap, "market borrow cap reached");
+        }
+
         (Error err, , uint shortfall) = getHypotheticalAccountLiquidityInternal(borrower, CToken(cToken), 0, borrowAmount);
         if (err != Error.NO_ERROR) {
             return uint(err);
@@ -1021,6 +1037,44 @@ contract Comptroller is ComptrollerV3Storage, ComptrollerInterface, ComptrollerE
         allMarkets.push(CToken(cToken));
     }
 
+
+    /**
+      * @notice Set the given borrow caps for the given cToken markets. Borrowing that brings total borrows to or above borrow cap will revert.
+      * @dev Admin or borrowCapGuardian function to set the borrow caps. A borrow cap of 0 corresponds to unlimited borrowing.
+      * @param cTokens The addresses of the markets (tokens) to change the borrow caps for
+      * @param newBorrowCaps The new borrow cap values in underlying to be set. A value of 0 corresponds to unlimited borrowing.
+      */
+    function _setMarketBorrowCaps(CToken[] calldata cTokens, uint[] calldata newBorrowCaps) external {
+    	require(msg.sender == admin || msg.sender == borrowCapGuardian, "only admin or borrow cap guardian can set borrow caps"); 
+
+        uint numMarkets = cTokens.length;
+        uint numBorrowCaps = newBorrowCaps.length;
+
+        require(numMarkets != 0 && numMarkets == numBorrowCaps, "invalid input");
+
+        for(uint i = 0; i < numMarkets; i++) {
+            borrowCaps[address(cTokens[i])] = newBorrowCaps[i];
+            emit NewBorrowCap(cTokens[i], newBorrowCaps[i]);
+        }
+    }
+
+    /**
+     * @notice Admin function to change the Borrow Cap Guardian
+     * @param newBorrowCapGuardian The address of the new Borrow Cap Guardian
+     */
+    function _setBorrowCapGuardian(address newBorrowCapGuardian) external {
+        require(msg.sender == admin, "only admin can set borrow cap guardian");
+
+        // Save current value for inclusion in log
+        address oldBorrowCapGuardian = borrowCapGuardian;
+
+        // Store borrowCapGuardian with value newBorrowCapGuardian
+        borrowCapGuardian = newBorrowCapGuardian;
+
+        // Emit NewBorrowCapGuardian(OldBorrowCapGuardian, NewBorrowCapGuardian)
+        emit NewBorrowCapGuardian(oldBorrowCapGuardian, newBorrowCapGuardian);
+    }
+
     /**
      * @notice Admin function to change the Pause Guardian
      * @param newPauseGuardian The address of the new Pause Guardian