Change execution order to avoid reentry through the _beforeTokenTransfer hook

CHANGELOG.md

@@ -5,10 +5,11 @@
  * `ERC165Checker`: add `supportsERC165InterfaceUnchecked` for consulting individual interfaces without the full ERC165 protocol. ([#3339](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3339))
  * `Address`: optimize `functionCall` by calling `functionCallWithValue` directly. ([#3468](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3468))
  * `Address`: optimize `functionCall` functions by checking contract size only if there is no returned data. ([#3469](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3469))
- * `GovernorCompatibilityBravo`: remove unused `using` statements ([#3506](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3506))
+ * `GovernorCompatibilityBravo`: remove unused `using` statements. ([#3506](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3506))
  * `ERC20`: optimize `_transfer`, `_mint` and `_burn` by using `unchecked` arithmetic when possible. ([#3513](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3513))
  * `ERC721`: optimize transfers by making approval clearing implicit instead of emitting an event. ([#3481](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3481))
  * `ERC721`: optimize burn by making approval clearing implicit instead of emitting an event. ([#3538](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3538))
+ * `ERC721`: Fix balance accounting when a custom `_beforeTokenTransfer` hook results in a transfer of the token under consideration. ([#3611](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3611))
  * `ReentrancyGuard`: Reduce code size impact of the modifier by using internal functions. ([#3515](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3515))
  * `SafeCast`: optimize downcasting of signed integers. ([#3565](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3565))
  * `ERC20FlashMint`: add an internal `_flashFee` function for overriding. ([#3551](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/3551))

contracts/token/ERC721/ERC721.sol

@@ -282,6 +282,9 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
 
         _beforeTokenTransfer(address(0), to, tokenId);
 
+        // Check that tokenId was not minted by `_beforeTokenTransfer` hook
+        require(!_exists(tokenId), "ERC721: token already minted");
+
         _balances[to] += 1;
         _owners[tokenId] = to;
 
@@ -306,6 +309,9 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
 
         _beforeTokenTransfer(owner, address(0), tokenId);
 
+        // Update ownership in case tokenId was transfered by `_beforeTokenTransfer` hook
+        owner = ERC721.ownerOf(tokenId);
+
         // Clear approvals
         delete _tokenApprovals[tokenId];
 
@@ -338,6 +344,9 @@ contract ERC721 is Context, ERC165, IERC721, IERC721Metadata {
 
         _beforeTokenTransfer(from, to, tokenId);
 
+        // Check that tokenId was not transfered by `_beforeTokenTransfer` hook
+        require(ERC721.ownerOf(tokenId) == from, "ERC721: transfer from incorrect owner");
+
         // Clear approvals from the previous owner
         delete _tokenApprovals[tokenId];