Allow the re-initialization of contracts

contracts/proxy/utils/Initializable.sol

@@ -35,8 +35,9 @@ import "../../utils/Address.sol";
 abstract contract Initializable {
     /**
      * @dev Indicates that the contract has been initialized.
+     * @custom:oz-upgrades-retyped-from bool
      */
-    bool private _initialized;
+    uint8 private _initialized;
 
     /**
      * @dev Indicates that the contract is in the process of being initialized.
@@ -47,19 +48,21 @@ abstract contract Initializable {
      * @dev Modifier to protect an initializer function from being invoked twice.
      */
     modifier initializer() {
-        // If the contract is initializing we ignore whether _initialized is set in order to support multiple
-        // inheritance patterns, but we only do this in the context of a constructor, because in other contexts the
-        // contract may have been reentered.
-        require(_initializing ? _isConstructor() : !_initialized, "Initializable: contract is already initialized");
-
-        bool isTopLevelCall = !_initializing;
+        bool isTopLevelCall = _setInitializeStep(1);
+        _;
         if (isTopLevelCall) {
-            _initializing = true;
-            _initialized = true;
+            _initializing = false;
         }
+    }
 
+    /**
+     * @dev Modifier to allow the execution of `onlyInitializing`-protected methods, after the original initialization
+     * step, through "reinitializer". These "reinitializer" are essential to configure modules that are added through
+     * upgrades and that require an initialization step.
+     */
+    modifier reinitializer(uint8 version) {
+        bool isTopLevelCall = _setInitializeStep(version);
         _;
-
         if (isTopLevelCall) {
             _initializing = false;
         }
@@ -74,7 +77,15 @@ abstract contract Initializable {
         _;
     }
 
-    function _isConstructor() private view returns (bool) {
-        return !Address.isContract(address(this));
+    function _setInitializeStep(uint8 version) private returns (bool) {
+        if (_initializing) {
+            require(version == 1 && !Address.isContract(address(this)), "Initializable: contract is already initialized");
+            return false;
+        } else {
+            require(_initialized < version, "Initializable: contract is already initialized");
+            _initializing = true;
+            _initialized = version;
+            return true;
+        }
     }
 }