USTB GSM Implementation

docs/buidl-gsm-converter.md

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+# BUIDL GSM Converter
+
+## Overview
+
+The BUIDL GSM Converter is designed to allow for integration of the GSM ([GHO Stability Module](https://docs.gho.xyz/developer-docs/gho-stability-module)) with [Blackrock BUIDL fund](https://securitize.io/learn/press/blackrock-launches-first-tokenized-fund-buidl-on-the-ethereum-network) subscriptions and redemptions, adhering to the [BUIDL GSM Temp Check](https://governance.aave.com/t/temp-check-buidl-gsm/18775).
+
+The combined BUIDL GSM instance with GsmConverter will enable 1:1 fixed-ratio swaps between USDC and GHO, utilizing the USDC surplus to mint BUIDL tokens. The converter architecture as an intermediary allows for minimal required code changes to the GSM, while also providing a standardized smart contract interface for users. Furthermore, BUIDL holders will be able to interact directly with the BUIDL GSM as they do with other existing GSMs.
+
+### BUIDL GSM ([Gsm.sol](../src/contracts/facilitators/gsm/Gsm.sol))
+
+The BUIDL GSM operates like other existing GSM instances, except it specifically facilitates conversions between GHO and BUIDL. BUIDL holders can interact with this contract directly.
+
+### GSM Converter ([GsmConverter.sol](../src/contracts/facilitators/gsm/converter/GsmConverter.sol))
+
+The GSM Converter will act as a middleware between the user and BUIDL GSM, facilitating conversions between USDC and BUIDL under the hood.
+
+During a `buyAsset` transaction, where users sell GHO to receive USDC, the GsmConverter first interacts with the BUIDL GSM to convert GHO to BUIDL. Then it integrates with the [Circle BUIDL Off-Ramp contract](https://etherscan.io/address/0x31d3f59ad4aac0eee2247c65ebe8bf6e9e470a53#code) in order to redeem USDC in exchange for BUIDL. This USDC is then returned to the user.
+
+![buyAsset](./img/buyAsset.png)
+
+During a `sellAsset` transaction, where users sell USDC to receive GHO, ...

src/contracts/facilitators/gsm/converter/GsmConverter.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.10;
+
+import {GPv2SafeERC20} from '@aave/core-v3/contracts/dependencies/gnosis/contracts/GPv2SafeERC20.sol';
+import {IERC20} from '@aave/core-v3/contracts/dependencies/openzeppelin/contracts/IERC20.sol';
+import {EIP712} from '@openzeppelin/contracts/utils/cryptography/EIP712.sol';
+import {SignatureChecker} from '@openzeppelin/contracts/utils/cryptography/SignatureChecker.sol';
+import {Ownable} from '@openzeppelin/contracts/access/Ownable.sol';
+import {IGhoToken} from '../../../gho/interfaces/IGhoToken.sol';
+import {IGsm} from '../interfaces/IGsm.sol';
+import {IGsmConverter} from './interfaces/IGsmConverter.sol';
+import {IRedemption} from '../dependencies/circle/IRedemption.sol';
+// TODO: replace with proper issuance implementation later
+import {MockBUIDLSubscription} from '../../../../test/mocks/MockBUIDLSubscription.sol';
+
+import 'forge-std/console2.sol';
+
+/**
+ * @title GsmConverter
+ * @author Aave
+ * @notice Converter that facilitates conversions/redemptions of underlying assets. Integrates with GSM to buy/sell to go to/from an underlying asset to/from GHO.
+ */
+contract GsmConverter is Ownable, EIP712, IGsmConverter {
+  using GPv2SafeERC20 for IERC20;
+
+  /// @inheritdoc IGsmConverter
+  bytes32 public constant BUY_ASSET_WITH_SIG_TYPEHASH =
+    keccak256(
+      'BuyAssetWithSig(address originator,uint256 minAmount,address receiver,uint256 nonce,uint256 deadline)'
+    );
+
+  /// @inheritdoc IGsmConverter
+  bytes32 public constant SELL_ASSET_WITH_SIG_TYPEHASH =
+    keccak256(
+      'SellAssetWithSig(address originator,uint256 maxAmount,address receiver,uint256 nonce,uint256 deadline)'
+    );
+
+  /// @inheritdoc IGsmConverter
+  address public immutable GHO_TOKEN;
+
+  /// @inheritdoc IGsmConverter
+  address public immutable GSM;
+
+  /// @inheritdoc IGsmConverter
+  address public immutable ISSUED_ASSET;
+
+  /// @inheritdoc IGsmConverter
+  address public immutable REDEEMED_ASSET;
+
+  /// @inheritdoc IGsmConverter
+  address public immutable REDEMPTION_CONTRACT;
+
+  /// @inheritdoc IGsmConverter
+  address public immutable SUBSCRIPTION_CONTRACT;
+
+  /// @inheritdoc IGsmConverter
+  mapping(address => uint256) public nonces;
+
+  /**
+   * @dev Constructor
+   * @param gsm The address of the associated GSM contract
+   * @param redemptionContract The address of the redemption contract associated with the asset conversion
+   * @param issuanceReceiverContract The address of the contract receiving the payment associated with the asset conversion
+   * @param issuedAsset The address of the asset being redeemed
+   * @param redeemedAsset The address of the asset being received from redemption
+   */
+  constructor(
+    address admin,
+    address gsm,
+    address redemptionContract,
+    address issuanceReceiverContract,
+    address issuedAsset,
+    address redeemedAsset
+  ) EIP712('GSMConverter', '1') {
+    require(admin != address(0), 'ZERO_ADDRESS_NOT_VALID');
+    require(gsm != address(0), 'ZERO_ADDRESS_NOT_VALID');
+    require(redemptionContract != address(0), 'ZERO_ADDRESS_NOT_VALID');
+    require(issuanceReceiverContract != address(0), 'ZERO_ADDRESS_NOT_VALID');
+    require(issuedAsset != address(0), 'ZERO_ADDRESS_NOT_VALID');
+    require(redeemedAsset != address(0), 'ZERO_ADDRESS_NOT_VALID');
+
+    GSM = gsm;
+    REDEMPTION_CONTRACT = redemptionContract;
+    SUBSCRIPTION_CONTRACT = issuanceReceiverContract;
+    ISSUED_ASSET = issuedAsset; // BUIDL
+    REDEEMED_ASSET = redeemedAsset; // USDC
+    GHO_TOKEN = IGsm(GSM).GHO_TOKEN();
+
+    transferOwnership(admin);
+  }
+
+  /// @inheritdoc IGsmConverter
+  function sellAsset(uint256 maxAmount, address receiver) external returns (uint256, uint256) {
+    require(maxAmount > 0, 'INVALID_MAX_AMOUNT');
+
+    return _sellAsset(msg.sender, maxAmount, receiver);
+  }
+
+  /// @inheritdoc IGsmConverter
+  function buyAsset(uint256 minAmount, address receiver) external returns (uint256, uint256) {
+    require(minAmount > 0, 'INVALID_MIN_AMOUNT');
+
+    return _buyAsset(msg.sender, minAmount, receiver);
+  }
+
+  /// @inheritdoc IGsmConverter
+  function buyAssetWithSig(
+    address originator,
+    uint256 minAmount,
+    address receiver,
+    uint256 deadline,
+    bytes calldata signature
+  ) external returns (uint256, uint256) {
+    require(deadline >= block.timestamp, 'SIGNATURE_DEADLINE_EXPIRED');
+    bytes32 digest = keccak256(
+      abi.encode(
+        '\x19\x01',
+        _domainSeparatorV4(),
+        BUY_ASSET_WITH_SIG_TYPEHASH,
+        abi.encode(originator, minAmount, receiver, nonces[originator]++, deadline)
+      )
+    );
+    require(
+      SignatureChecker.isValidSignatureNow(originator, digest, signature),
+      'SIGNATURE_INVALID'
+    );
+
+    return _buyAsset(originator, minAmount, receiver);
+  }
+
+  /// @inheritdoc IGsmConverter
+  function sellAssetWithSig(
+    address originator,
+    uint256 maxAmount,
+    address receiver,
+    uint256 deadline,
+    bytes calldata signature
+  ) external returns (uint256, uint256) {
+    require(deadline >= block.timestamp, 'SIGNATURE_DEADLINE_EXPIRED');
+    bytes32 digest = keccak256(
+      abi.encode(
+        '\x19\x01',
+        _domainSeparatorV4(),
+        SELL_ASSET_WITH_SIG_TYPEHASH,
+        abi.encode(originator, maxAmount, receiver, nonces[originator]++, deadline)
+      )
+    );
+    require(
+      SignatureChecker.isValidSignatureNow(originator, digest, signature),
+      'SIGNATURE_INVALID'
+    );
+
+    return _sellAsset(originator, maxAmount, receiver);
+  }
+
+  /// @inheritdoc IGsmConverter
+  function rescueTokens(address token, address to, uint256 amount) external onlyOwner {
+    require(amount > 0, 'INVALID_AMOUNT');
+    IERC20(token).safeTransfer(to, amount);
+    emit TokensRescued(token, to, amount);
+  }
+
+  /// @inheritdoc IGsmConverter
+  function DOMAIN_SEPARATOR() external view returns (bytes32) {
+    return _domainSeparatorV4();
+  }
+
+  /**
+   * @notice Buys the GSM underlying asset in exchange for selling GHO, after asset redemption
+   * @param minAmount The minimum amount of the underlying asset to buy (ie BUIDL)
+   * @param receiver Recipient address of the underlying asset being purchased
+   * @return The amount of underlying asset bought, after asset redemption
+   * @return The amount of GHO sold by the user
+   */
+  function _buyAsset(
+    address originator,
+    uint256 minAmount,
+    address receiver
+  ) internal returns (uint256, uint256) {
+    uint256 initialGhoBalance = IGhoToken(GHO_TOKEN).balanceOf(address(this));
+    uint256 initialIssuedAssetBalance = IERC20(ISSUED_ASSET).balanceOf(address(this));
+    uint256 initialRedeemedAssetBalance = IERC20(REDEEMED_ASSET).balanceOf(address(this));
+
+    (, uint256 ghoAmount, , ) = IGsm(GSM).getGhoAmountForBuyAsset(minAmount);
+
+    IGhoToken(GHO_TOKEN).transferFrom(originator, address(this), ghoAmount);
+    IGhoToken(GHO_TOKEN).approve(address(GSM), ghoAmount);
+    (uint256 boughtAssetAmount, uint256 ghoSold) = IGsm(GSM).buyAsset(minAmount, address(this));
+    require(ghoAmount == ghoSold, 'INVALID_GHO_SOLD');
+    IGhoToken(GHO_TOKEN).approve(address(GSM), 0);
+
+    IERC20(ISSUED_ASSET).approve(address(REDEMPTION_CONTRACT), boughtAssetAmount);
+    IRedemption(REDEMPTION_CONTRACT).redeem(boughtAssetAmount);
+    // Redemption exchanges in 1:1 ratio between BUIDL and USDC
+    require(
+      IERC20(REDEEMED_ASSET).balanceOf(address(this)) ==
+        initialRedeemedAssetBalance + boughtAssetAmount,
+      'INVALID_REDEMPTION'
+    );
+    IERC20(ISSUED_ASSET).approve(address(REDEMPTION_CONTRACT), 0);
+    IERC20(REDEEMED_ASSET).safeTransfer(receiver, boughtAssetAmount);
+
+    require(
+      IGhoToken(GHO_TOKEN).balanceOf(address(this)) == initialGhoBalance,
+      'INVALID_REMAINING_GHO_BALANCE'
+    );
+    require(
+      IERC20(ISSUED_ASSET).balanceOf(address(this)) == initialIssuedAssetBalance,
+      'INVALID_REMAINING_ISSUED_ASSET_BALANCE'
+    );
+
+    emit BuyAssetThroughRedemption(originator, receiver, boughtAssetAmount, ghoSold);
+    return (boughtAssetAmount, ghoSold);
+  }
+
+  /**
+   * @notice Sells the GSM underlying asset in exchange for buying GHO, after asset conversion
+   * @param originator The originator of the request
+   * @param maxAmount The maximum amount of the underlying asset to sell
+   * @param receiver Recipient address of the GHO being purchased
+   * @return The amount of underlying asset sold, after asset conversion
+   * @return The amount of GHO bought by the user
+   */
+  function _sellAsset(
+    address originator,
+    uint256 maxAmount,
+    address receiver
+  ) internal returns (uint256, uint256) {
+    uint256 initialGhoBalance = IGhoToken(GHO_TOKEN).balanceOf(address(this));
+    uint256 initialIssuedAssetBalance = IERC20(ISSUED_ASSET).balanceOf(address(this));
+    uint256 initialRedeemedAssetBalance = IERC20(REDEEMED_ASSET).balanceOf(address(this));
+
+    (uint256 assetAmount, , , ) = IGsm(GSM).getGhoAmountForSellAsset(maxAmount); // asset is BUIDL
+    IERC20(REDEEMED_ASSET).transferFrom(originator, address(this), assetAmount);
+    IERC20(REDEEMED_ASSET).approve(SUBSCRIPTION_CONTRACT, assetAmount);
+    //TODO: replace with proper issuance implementation later
+    MockBUIDLSubscription(SUBSCRIPTION_CONTRACT).issuance(assetAmount);
+    uint256 subscribedAssetAmount = IERC20(ISSUED_ASSET).balanceOf(address(this)) -
+      initialIssuedAssetBalance;
+    // TODO: probably will be fees from issuance, so need to adjust the logic
+    // only use this require only if preview of issuance is possible, otherwise it is redundant
+    require(
+      IERC20(ISSUED_ASSET).balanceOf(address(this)) ==
+        initialIssuedAssetBalance + subscribedAssetAmount,
+      'INVALID_ISSUANCE'
+    );
+    // reset approval after issuance
+    IERC20(REDEEMED_ASSET).approve(SUBSCRIPTION_CONTRACT, 0);
+
+    // TODO: account for fees for sellAsset amount param
+    (assetAmount, , , ) = IGsm(GSM).getGhoAmountForSellAsset(subscribedAssetAmount); // recalculate based on actual issuance amount, < maxAmount
+    IERC20(ISSUED_ASSET).approve(GSM, assetAmount);
+    (uint256 soldAssetAmount, uint256 ghoBought) = IGsm(GSM).sellAsset(
+      subscribedAssetAmount,
+      receiver
+    );
+    // reset approval after sellAsset
+    IERC20(ISSUED_ASSET).approve(GSM, 0);
+
+    // by the end of the transaction, this contract should not retain any of the assets
+    require(
+      IGhoToken(GHO_TOKEN).balanceOf(address(this)) == initialGhoBalance,
+      'INVALID_REMAINING_GHO_BALANCE'
+    );
+    require(
+      IERC20(REDEEMED_ASSET).balanceOf(address(this)) == initialRedeemedAssetBalance,
+      'INVALID_REMAINING_REDEEMED_ASSET_BALANCE'
+    );
+    require(
+      IERC20(ISSUED_ASSET).balanceOf(address(this)) == initialIssuedAssetBalance,
+      'INVALID_REMAINING_ISSUED_ASSET_BALANCE'
+    );
+
+    emit SellAssetThroughSubscription(originator, receiver, soldAssetAmount, ghoBought);
+    return (soldAssetAmount, ghoBought);
+  }
+}