In an on-chain program I would like to find the wallet holding a particular NFT, is it possible? how can I do that?
Please note - I do NOT want to do this in the client, but totally inside an on-chain program.
I have a Mint address only, how to I find the wallet address? I can do it off-chain with getProgramAccount() but how to do it on-chain?
It's not possible.
Mint -> Token Accounts is a 1:Many relationship, the getProgramAccount() off-chain RPC call wades through a ton of data to find the Token Accounts for a Mint, you could not do this on-chain.
In the special case of an NFT (supply=1) there is just one Token Account you want, but that relationship is not maintained on-chain.
If this was Ethereum, you could just change your ERC20 token contract to store the Owner Address on-chain, but it's Solana... you can't modify the SPL program (without your token no longer being a token)... so you are stuffed.
Solana is very constrained when it comes to development possibilities.
nb: the limitation isn't down to not being able to dynamically load accounts, as you would know the account that holds the relationship (PDA) upfront... the problem is you can't make a modded SPL program like you can mod an ERC20 token.