Governance Fee Collection (#535)

* Increase precision on approxEquals helper to just a single +1 or -1 of a 1e6 fixed point.

* Prettier on previous code.

* Collect a percentage of yield during rebasing.

* More tests.

* Spelling fix in old code.

* Rename to trustee. Thanks Micah!

* Fix safe math. Thanks Tom!

* Contract designed for the these values to initialize at zero.

* Prettier

* Rename vaultSupply to vaultValue.

* Remove old return comments.

* Rename basis to bps in new functions and variables.

* Add view functions to ivault for parameters

* Update variable name for slither

Co-authored-by: Franck <franck@originprotocol.com>

contracts/contracts/interfaces/IVault.sol

@@ -49,6 +49,14 @@ interface IVault {
 
     function maxSupplyDiff() external view returns (uint256);
 
+    function setTrusteeAddress(address _address) external;
+
+    function trusteeAddress() external view returns (address);
+
+    function setTrusteeFeeBps(uint256 _basis) external;
+
+    function trusteeFeeBps() external view returns (uint256);
+
     function supportAsset(address _asset) external;
 
     function approveStrategy(address _addr) external;
@@ -117,7 +125,7 @@ interface IVault {
         uint256[] calldata _amounts
     ) external;
 
-    function rebase() external returns (uint256);
+    function rebase() external;
 
     function totalValue() external view returns (uint256 value);
 

contracts/contracts/vault/VaultAdmin.sol

@@ -235,6 +235,25 @@ contract VaultAdmin is VaultStorage {
         emit MaxSupplyDiffChanged(_maxSupplyDiff);
     }
 
+    /**
+     * @dev Sets the trusteeAddress that can receive a portion of yield.
+     *      Setting to the zero address disables this feature.
+     */
+    function setTrusteeAddress(address _address) external onlyGovernor {
+        trusteeAddress = _address;
+        emit TrusteeAddressChanged(_address);
+    }
+
+    /**
+     * @dev Sets the TrusteeFeeBps to the percentage of yield that should be
+     *      received in basis points.
+     */
+    function setTrusteeFeeBps(uint256 _basis) external onlyGovernor {
+        require(_basis <= 5000, "basis cannot exceed 50%");
+        trusteeFeeBps = _basis;
+        emit TrusteeFeeBpsChanged(_basis);
+    }
+
     /***************************************
                     Pause
     ****************************************/

contracts/contracts/vault/VaultCore.sol

@@ -357,33 +357,39 @@ contract VaultCore is VaultStorage {
     /**
      * @dev Calculate the total value of assets held by the Vault and all
      *      strategies and update the supply of OUSD.
-     * @return uint256 New total supply of OUSD
      */
-    function rebase()
-        public
-        whenNotRebasePaused
-        nonReentrant
-        returns (uint256 newTotalSupply)
-    {
-        return _rebase();
+    function rebase() public whenNotRebasePaused nonReentrant {
+        _rebase();
     }
 
     /**
      * @dev Calculate the total value of assets held by the Vault and all
-     *      strategies and update the supply of OUSD.
-     * @return uint256 New total supply of OUSD
+     *      strategies and update the supply of OUSD, optionaly sending a
+     *      portion of the yield to the trustee.
      */
-    function _rebase()
-        internal
-        whenNotRebasePaused
-        returns (uint256 newTotalSupply)
-    {
-        if (oUSD.totalSupply() == 0) return 0;
-        uint256 oldTotalSupply = oUSD.totalSupply();
-        newTotalSupply = _totalValue();
-        // Only rachet upwards
-        if (newTotalSupply > oldTotalSupply) {
-            oUSD.changeSupply(newTotalSupply);
+    function _rebase() internal whenNotRebasePaused {
+        uint256 ousdSupply = oUSD.totalSupply();
+        if (ousdSupply == 0) {
+            return;
+        }
+        uint256 vaultValue = _totalValue();
+
+        // Yield fee collection
+        address _trusteeAddress = trusteeAddress; // gas savings
+        if (_trusteeAddress != address(0) && (vaultValue > ousdSupply)) {
+            uint256 yield = vaultValue.sub(ousdSupply);
+            uint256 fee = yield.mul(trusteeFeeBps).div(10000);
+            require(yield > fee, "Fee must not be greater than yield");
+            if (fee > 0) {
+                oUSD.mint(_trusteeAddress, fee);
+            }
+            emit YieldDistribution(_trusteeAddress, yield, fee);
+        }
+
+        // Only rachet OUSD supply upwards
+        ousdSupply = oUSD.totalSupply(); // Final check should use latest value
+        if (vaultValue > ousdSupply) {
+            oUSD.changeSupply(vaultValue);
         }
     }
 

contracts/contracts/vault/VaultStorage.sol

@@ -44,6 +44,9 @@ contract VaultStorage is Initializable, Governable {
     event UniswapUpdated(address _address);
     event StrategistUpdated(address _address);
     event MaxSupplyDiffChanged(uint256 maxSupplyDiff);
+    event YieldDistribution(address _to, uint256 _yield, uint256 _fee);
+    event TrusteeFeeBpsChanged(uint256 _basis);
+    event TrusteeAddressChanged(address _address);
 
     // Assets supported by the Vault, i.e. Stablecoins
     struct Asset {
@@ -94,6 +97,12 @@ contract VaultStorage is Initializable, Governable {
 
     uint256 public maxSupplyDiff;
 
+    // Trustee address that can collect a percentage of yield
+    address public trusteeAddress;
+
+    // Amount of yield collected in basis points
+    uint256 public trusteeFeeBps;
+
     /**
      * @dev set the implementation for the admin, this needs to be in a base class else we cannot set it
      * @param newImpl address of the implementation

contracts/test/helpers.js

@@ -8,8 +8,8 @@ const addresses = require("../utils/addresses");
 
 chai.Assertion.addMethod("approxEqual", function (expected, message) {
   const actual = this._obj;
-  chai.expect(actual, message).gte(expected.mul("9999").div("10000"));
-  chai.expect(actual, message).lte(expected.mul("10001").div("10000"));
+  chai.expect(actual, message).gte(expected.mul("99999").div("100000"));
+  chai.expect(actual, message).lte(expected.mul("100001").div("100000"));
 });
 
 chai.Assertion.addMethod("approxBalanceOf", async function (

contracts/test/vault/rebase.js

@@ -9,6 +9,7 @@ const {
   tusdUnits,
   getOracleAddress,
   setOracleTokenPriceUsd,
+  expectApproxSupply,
   loadFixture,
 } = require("../helpers");
 
@@ -174,7 +175,7 @@ describe("Vault rebasing", async () => {
     await expect(await vault.getStrategyCount()).to.equal(0);
     await vault.connect(governor).allocate();
 
-    // All assets sould still remain in Vault
+    // All assets should still remain in Vault
 
     // Note defaultFixture sets up with 200 DAI already in the Strategy
     // 200 + 100 = 300
@@ -217,3 +218,37 @@ describe("Vault rebasing", async () => {
     await expect(await vault.priceProvider()).to.be.equal(oracle);
   });
 });
+
+describe("Vault yield accrual to OGN", async () => {
+  [
+    { yield: "1000", basis: 100, expectedFee: "10" },
+    { yield: "1000", basis: 5000, expectedFee: "500" },
+    { yield: "1523", basis: 900, expectedFee: "137.07" },
+    { yield: "0.000001", basis: 10, expectedFee: "0.00000001" },
+    { yield: "0", basis: 1000, expectedFee: "0" },
+  ].forEach((options) => {
+    const { yield, basis, expectedFee } = options;
+    it(`should collect on rebase a ${expectedFee} fee from ${yield} yield at ${basis}bp `, async function () {
+      const fixture = await loadFixture(defaultFixture);
+      const { matt, governor, ousd, usdt, vault, mockNonRebasing } = fixture;
+      const trustee = mockNonRebasing;
+
+      // Setup trustee trustee on vault
+      await vault.connect(governor).setTrusteeAddress(trustee.address);
+      await vault.connect(governor).setTrusteeFeeBps(900);
+      await expect(trustee).has.a.balanceOf("0", ousd);
+
+      // Create yield for the vault
+      await usdt.connect(matt).mint(usdcUnits("1523"));
+      await usdt.connect(matt).transfer(vault.address, usdcUnits("1523"));
+      // Do rebase
+      const supplyBefore = await ousd.totalSupply();
+      await vault.rebase();
+      // OUSD supply increases correctly
+      await expectApproxSupply(ousd, supplyBefore.add(ousdUnits("1523")));
+      // ogntrustee address increases correctly
+      // 1523 * 0.09 = 137.07
+      await expect(trustee).has.a.balanceOf("137.07", ousd);
+    });
+  });
+});

contracts/test/vault/redeem.js

@@ -345,7 +345,7 @@ describe("Vault Redeem", function () {
           // Manually call rebase because not triggered by mint
           await vault.rebase();
           // Rebase could have changed user balance
-          // as there could have been yeild from different
+          // as there could have been yield from different
           // oracle prices on redeems during a previous loop.
           let userBalance = await getUserOusdBalance(user);
           for (const amount of amounts) {