feat(flash-loan): add flash loan

src/Blue.sol

@@ -3,6 +3,8 @@ pragma solidity 0.8.20;
 
 import {IIrm} from "src/interfaces/IIrm.sol";
 import {IERC20} from "src/interfaces/IERC20.sol";
+import {IFlashLender} from "src/interfaces/IFlashLender.sol";
+import {IFlashBorrower} from "src/interfaces/IFlashBorrower.sol";
 
 import {Errors} from "./libraries/Errors.sol";
 import {SharesMath} from "src/libraries/SharesMath.sol";
@@ -14,7 +16,7 @@ uint256 constant WAD = 1e18;
 uint256 constant MAX_FEE = 0.2e18;
 uint256 constant ALPHA = 0.5e18;
 
-contract Blue {
+contract Blue is IFlashLender {
     using SharesMath for uint256;
     using FixedPointMathLib for uint256;
     using SafeTransferLib for IERC20;
@@ -245,6 +247,17 @@ contract Blue {
         market.borrowableAsset.safeTransferFrom(msg.sender, address(this), repaid);
     }
 
+    // Flash Loans.
+
+    /// @inheritdoc IFlashLender
+    function flashLoan(IFlashBorrower receiver, address token, uint256 amount, bytes calldata data) external {
+        IERC20(token).safeTransfer(address(receiver), amount);
+
+        receiver.onFlashLoan(msg.sender, token, amount, data);
+
+        IERC20(token).safeTransferFrom(address(receiver), address(this), amount);
+    }
+
     // Position management.
 
     function setApproval(address manager, bool isAllowed) external {

src/interfaces/IERC20.sol

@@ -3,4 +3,6 @@ pragma solidity >=0.5.0;
 
 /// @dev Empty because we only call functions in assembly. It prevents calling
 /// transfer (transferFrom) instead of safeTransfer (safeTransferFrom).
-interface IERC20 {}
+interface IERC20 {
+    function balanceOf(address owner) external view returns (uint256);
+}

src/interfaces/IFlashBorrower.sol

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.5.0;
+
+interface IFlashBorrower {
+    /**
+     * @dev Receives a flash loan.
+     * @param initiator The initiator of the loan.
+     * @param token The token lent.
+     * @param amount The amount of tokens lent.
+     * @param data Arbitrary data structure, intended to contain user-defined parameters.
+     */
+    function onFlashLoan(address initiator, address token, uint256 amount, bytes calldata data) external;
+}

src/interfaces/IFlashLender.sol

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.5.0;
+
+import {IFlashBorrower} from "src/interfaces/IFlashBorrower.sol";
+
+interface IFlashLender {
+    /**
+     * @dev Initiate a flash loan.
+     * @param receiver The receiver of the tokens in the loan, and the receiver of the callback.
+     * @param token The token lent.
+     * @param amount The amount of tokens lent.
+     * @param data Arbitrary data structure, intended to contain user-defined parameters.
+     */
+    function flashLoan(IFlashBorrower receiver, address token, uint256 amount, bytes calldata data) external;
+}

src/libraries/Errors.sol

@@ -25,4 +25,6 @@ library Errors {
     string internal constant INSUFFICIENT_LIQUIDITY = "insufficient liquidity";
 
     string internal constant HEALTHY_POSITION = "position is healthy";
+
+    string internal constant INVALID_SUCCESS_HASH = "invalid success hash";
 }

src/mocks/FlashBorrowerMock.sol

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity ^0.8.0;
+
+import {IFlashLender} from "src/interfaces/IFlashLender.sol";
+import {IFlashBorrower} from "src/interfaces/IFlashBorrower.sol";
+
+import {ERC20, SafeTransferLib} from "solmate/utils/SafeTransferLib.sol";
+
+contract FlashBorrowerMock is IFlashBorrower {
+    using SafeTransferLib for ERC20;
+
+    IFlashLender private immutable _LENDER;
+
+    constructor(IFlashLender lender) {
+        _LENDER = lender;
+    }
+
+    /* EXTERNAL */
+
+    /// @inheritdoc IFlashBorrower
+    function onFlashLoan(address, address token, uint256 amount, bytes calldata) external {
+        require(msg.sender == address(_LENDER), "invalid lender");
+
+        ERC20(token).safeApprove(address(_LENDER), amount);
+    }
+}

test/forge/Blue.t.sol

@@ -8,6 +8,7 @@ import "src/Blue.sol";
 import {ERC20Mock as ERC20} from "src/mocks/ERC20Mock.sol";
 import {OracleMock as Oracle} from "src/mocks/OracleMock.sol";
 import {IrmMock as Irm} from "src/mocks/IrmMock.sol";
+import {FlashBorrowerMock} from "src/mocks/FlashBorrowerMock.sol";
 
 contract BlueTest is Test {
     using FixedPointMathLib for uint256;
@@ -25,6 +26,7 @@ contract BlueTest is Test {
     Irm private irm;
     Market public market;
     Id public id;
+    FlashBorrowerMock internal flashBorrower;
 
     function setUp() public {
         // Create Blue.
@@ -35,6 +37,7 @@ contract BlueTest is Test {
         collateralAsset = new ERC20("collateral", "C", 18);
         borrowableOracle = new Oracle();
         collateralOracle = new Oracle();
+        flashBorrower = new FlashBorrowerMock(blue);
 
         irm = new Irm(blue);
         market = Market(
@@ -685,6 +688,17 @@ contract BlueTest is Test {
 
         vm.stopPrank();
     }
+
+    function testFlashLoan(uint256 amount) public {
+        amount = bound(amount, 1, 2 ** 64);
+
+        borrowableAsset.setBalance(address(this), amount);
+        blue.supply(market, amount, address(this));
+
+        blue.flashLoan(flashBorrower, address(borrowableAsset), amount, bytes(""));
+
+        assertEq(borrowableAsset.balanceOf(address(blue)), amount, "balanceOf");
+    }
 }
 
 function neq(Market memory a, Market memory b) pure returns (bool) {

test/hardhat/Blue.spec.ts

@@ -5,6 +5,7 @@ import { expect } from "chai";
 import { BigNumber, constants, utils } from "ethers";
 import hre from "hardhat";
 import { Blue, OracleMock, ERC20Mock, IrmMock } from "types";
+import { FlashBorrowerMock } from "types/src/mocks/FlashBorrowerMock";
 
 const closePositions = false;
 const initBalance = constants.MaxUint256.div(2);
@@ -45,6 +46,7 @@ describe("Blue", () => {
   let borrowableOracle: OracleMock;
   let collateralOracle: OracleMock;
   let irm: IrmMock;
+  let flashBorrower: FlashBorrowerMock;
 
   let market: Market;
   let id: Buffer;
@@ -110,6 +112,10 @@ describe("Blue", () => {
 
     await borrowable.setBalance(liquidator.address, initBalance);
     await borrowable.connect(liquidator).approve(blue.address, constants.MaxUint256);
+
+    const FlashBorrowerFactory = await hre.ethers.getContractFactory("FlashBorrowerMock", admin);
+
+    flashBorrower = await FlashBorrowerFactory.deploy(blue.address);
   });
 
   it("should simulate gas cost [main]", async () => {
@@ -185,4 +191,13 @@ describe("Blue", () => {
       await borrowableOracle.setPrice(BigNumber.WAD);
     }
   });
+
+  it("should simuate gas cost [flashloan]", async () => {
+    const user = signers[0];
+    const amount = BigNumber.WAD;
+
+    await blue.connect(user).supply(market, amount, user.address);
+
+    await blue.flashLoan(flashBorrower.address, borrowable.address, amount.div(2), []);
+  });
 });