Fix listing validation: add failsafe for nonexistent tokens

[kumaryash90]

No description provided.

[joaquim-verges]

wouldn't this just return the zero address? it doesn't throw in solidity right?

[kumaryash90]

Yeah it won't throw. The operator and owner variables are zero address by default. The check below will evaluate to false in case these remain zero due to external calls failing.

[joaquim-verges]

then why do we need the try catch?

[nkrishang]

The ownerOf function does throw an error in some ERC-721 contracts out there. For example, this is the ownerOf code in the OpenZeppelin ERC-721 contract:

function ownerOf(uint256 tokenId) public view virtual override returns (address) {
        address owner = _owners[tokenId];
        require(owner != address(0), "ERC721: invalid token ID");
        return owner;
}

[joaquim-verges]

oh is this the call that throws? i would assume that also just returns false if the tokenId doesnt exist?

[kumaryash90]

Yeah both ownerOf and getApproved check if the token-id exists, and revert if not.

[kumaryash90]

For most cases, including create and update listing, this is fine. The call would just revert.
But it affects the view function for getting the valid listings.

[Vunguyen0909]

Yes