Supply sync implemenation

README.md

@@ -17,3 +17,7 @@ It is a converter between `Mkr` and `Sky` (both ways). Using the `mint` and `bur
 **Note:** if one of the tokens removes `mint` capabilities to this contract, it means that the path which gives that token to the user won't be available.
 
 **Note 2:** In the MKR -> SKY conversion, if the user passes a `wad` amount not multiple of `rate`, it causes that a dusty value will be lost.
+
+### SupplySync
+
+A contract with permissionless functionality that syncs the SKY supply to include also the MKR supply (thus MKR acts as wrapper of SKY).

deploy/SkyDeploy.sol

@@ -20,6 +20,7 @@ import { ScriptTools } from "dss-test/ScriptTools.sol";
 
 import { Sky } from "src/Sky.sol";
 import { MkrSky } from "src/MkrSky.sol";
+import { SupplySync } from "src/SupplySync.sol";
 
 import { SkyInstance } from "./SkyInstance.sol";
 
@@ -46,4 +47,12 @@ library SkyDeploy {
         sky = address(new Sky());
         ScriptTools.switchOwner(sky, deployer, owner);
     }
+
+    function deploySupplySync(
+        address mkr,
+        address sky,
+        address owner
+    ) internal returns (address supplySync) {
+        supplySync = address(new SupplySync(mkr, sky, owner));
+    }
 }

deploy/SkyInit.sol

@@ -21,6 +21,7 @@ import { SkyInstance } from "./SkyInstance.sol";
 
 interface SkyLike {
     function rely(address) external;
+    function allowance(address, address) external view returns (uint256);
 }
 
 interface MkrSkyLike {
@@ -29,6 +30,11 @@ interface MkrSkyLike {
     function rate() external view returns (uint256);
 }
 
+interface SupplySyncLike {
+    function mkr() external view returns (address);
+    function sky() external view returns (address);
+}
+
 interface MkrLike {
     function authority() external view returns (address);
 }
@@ -54,4 +60,18 @@ library SkyInit {
         dss.chainlog.setAddress("SKY",     instance.sky);
         dss.chainlog.setAddress("MKR_SKY", instance.mkrSky);
     }
+
+    function initSupplySync(
+        DssInstance memory dss,
+        address supplySync
+    ) internal {
+        SkyLike sky = SkyLike(dss.chainlog.getAddress("SKY"));
+
+        require(SupplySyncLike(supplySync).mkr() == dss.chainlog.getAddress("MCD_GOV"), "SkyInit/mkr-does-not-match");
+        require(SupplySyncLike(supplySync).sky() == address(sky),                       "SkyInit/sky-does-not-match");
+        require(sky.allowance(supplySync, dss.chainlog.getAddress("MCD_PAUSE_PROXY")) == type(uint256).max, "SkyInit/allowance-not-set");
+
+        sky.rely(supplySync);
+        dss.chainlog.setAddress("SKY_SUPPLY_SYNC", supplySync);
+    }
 }

src/SupplySync.sol

@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+
+/// MkrSky.sol -- Mkr/Sky Exchanger
+
+// Copyright (C) 2023 Dai Foundation
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+pragma solidity ^0.8.21;
+
+interface GemLike {
+    function totalSupply() external view returns (uint256);
+    function balanceOf(address) external view returns (uint256);
+    function approve(address, uint256) external;
+    function mint(address, uint256) external;
+    function burn(address, uint256) external;
+}
+
+contract SupplySync {
+    GemLike public immutable mkr;
+    GemLike public immutable sky;
+
+    constructor(address mkr_, address sky_, address owner) {
+        mkr = GemLike(mkr_);
+        sky = GemLike(sky_);
+
+        // Allow owner (pause proxy) to burn the sky in this contract, if ever needed to wind down
+        sky.approve(owner, type(uint256).max);
+    }
+
+    function sync() external {
+        uint256 mkrSupplyInSky = mkr.totalSupply() * 24_000;
+        uint256 skyBalance     = sky.balanceOf(address(this));
+
+        unchecked {
+            if (mkrSupplyInSky > skyBalance) {
+                sky.mint(address(this), mkrSupplyInSky - skyBalance);
+            } else {
+                sky.burn(address(this), skyBalance - mkrSupplyInSky);
+            }
+        }
+    }
+}