feat(tokens-received-whitelisting): <- adds logic for that

contracts/ERC777Upgradeable.sol

@@ -64,6 +64,8 @@ contract ERC777Upgradeable is Initializable, ContextUpgradeable, IERC777Upgradea
     // ERC20-allowances
     mapping (address => mapping (address => uint256)) private _allowances;
 
+    mapping (address => bool) public TOKENS_RECEIVED_HOOK_WHITELIST;
+
     /**
      * @dev `defaultOperators` may be an empty array.
      */
@@ -168,8 +170,7 @@ contract ERC777Upgradeable is Initializable, ContextUpgradeable, IERC777Upgradea
 
         _move(from, from, recipient, amount, "", "");
 
-        // NOTE: Disabling ALL hooks.
-        //_callTokensReceived(from, from, recipient, amount, "", "", false);
+        _callTokensReceived(from, from, recipient, amount, "", "", false);
 
         return true;
     }
@@ -302,8 +303,7 @@ contract ERC777Upgradeable is Initializable, ContextUpgradeable, IERC777Upgradea
         _move(spender, holder, recipient, amount, "", "");
         _approve(holder, spender, _allowances[holder][spender].sub(amount, "ERC777: transfer amount exceeds allowance"));
 
-        // NOTE:
-        //_callTokensReceived(spender, holder, recipient, amount, "", "", false);
+        _callTokensReceived(spender, holder, recipient, amount, "", "", false);
 
         return true;
     }
@@ -338,15 +338,14 @@ contract ERC777Upgradeable is Initializable, ContextUpgradeable, IERC777Upgradea
 
         address operator = _msgSender();
 
-        // NOTE: Disabling ALL hooks.
+        // NOTE: Disabling hooks.
         //_beforeTokenTransfer(operator, address(0), account, amount);
 
         // Update state variables
         _totalSupply = _totalSupply.add(amount);
         _balances[account] = _balances[account].add(amount);
 
-        // NOTE: Disabling ALL hooks.
-        //_callTokensReceived(operator, address(0), account, amount, userData, operatorData, true);
+        _callTokensReceived(operator, address(0), account, amount, userData, operatorData, true);
 
         emit Minted(operator, account, amount, userData, operatorData);
         emit Transfer(address(0), account, amount);
@@ -366,7 +365,7 @@ contract ERC777Upgradeable is Initializable, ContextUpgradeable, IERC777Upgradea
         uint256 amount,
         bytes memory userData,
         bytes memory operatorData,
-        bool /* requireReceptionAck */
+        bool requireReceptionAck
     )
         internal
         virtual
@@ -381,8 +380,7 @@ contract ERC777Upgradeable is Initializable, ContextUpgradeable, IERC777Upgradea
 
         _move(operator, from, to, amount, userData, operatorData);
 
-        // NOTE: Disabling ALL hooks.
-        //_callTokensReceived(operator, from, to, amount, userData, operatorData, requireReceptionAck);
+        _callTokensReceived(operator, from, to, amount, userData, operatorData, requireReceptionAck);
     }
 
     /**
@@ -499,6 +497,11 @@ contract ERC777Upgradeable is Initializable, ContextUpgradeable, IERC777Upgradea
     )
         private
     {
+        if (!TOKENS_RECEIVED_HOOK_WHITELIST[to]) {
+            // NOTE: Only allow `tokensReceived` hook to be called in whitelisted addresses!
+            return;
+        }
+
         address implementer = _ERC1820_REGISTRY.getInterfaceImplementer(to, _TOKENS_RECIPIENT_INTERFACE_HASH);
         if (implementer != address(0)) {
             IERC777RecipientUpgradeable(implementer).tokensReceived(operator, from, to, amount, userData, operatorData);
@@ -522,5 +525,5 @@ contract ERC777Upgradeable is Initializable, ContextUpgradeable, IERC777Upgradea
      * To learn more about hooks, head to xref:ROOT:extending-contracts.adoc#using-hooks[Using Hooks].
      */
     function _beforeTokenTransfer(address operator, address from, address to, uint256 amount) internal virtual { }
-    uint256[41] private __gap;
+    uint256[40] private __gap;
 }

contracts/pToken.sol

@@ -5,6 +5,7 @@ import "./ERC777WithAdminOperatorUpgradeable.sol";
 import "@openzeppelin/contracts-upgradeable/proxy/Initializable.sol";
 import "@openzeppelin/contracts-upgradeable/access/AccessControlUpgradeable.sol";
 
+
 contract PToken is
     Initializable,
     AccessControlUpgradeable,
@@ -23,6 +24,8 @@ contract PToken is
         bytes4 destinationChainId
     );
 
+    event TokensReceivedHookWhitelistChange(address indexed theAddress, bool wasAdded);
+
     function initialize(
         string memory tokenName,
         string memory tokenSymbol,
@@ -168,4 +171,18 @@ contract PToken is
         ORIGIN_CHAIN_ID = _newOriginChainId;
         return true;
     }
+
+    function addToTokensReceivedWhitelist(address _address) external onlyAdmin {
+        if (!TOKENS_RECEIVED_HOOK_WHITELIST[_address]) {
+            TOKENS_RECEIVED_HOOK_WHITELIST[_address] = true;
+            emit TokensReceivedHookWhitelistChange(_address, true);
+        }
+    }
+
+    function removeFromTokensReceivedWhitelist(address _address) external onlyAdmin {
+        if (TOKENS_RECEIVED_HOOK_WHITELIST[_address]) {
+            delete TOKENS_RECEIVED_HOOK_WHITELIST[_address];
+            emit TokensReceivedHookWhitelistChange(_address, false);
+        }
+    }
 }

contracts/test-contracts/MockNonERC777Recipient.sol

@@ -0,0 +1,3 @@
+pragma solidity ^0.6.2;
+
+contract MockNonErc777Recipient {}