transfer hook in token 2022 - how to actually transfer?

Question

I'm trying to understand how the transfer hook works in the token 2022 program. I have deployed a program that implements the Execute and InitializeExtraAccountMetas functions (they both currently do nothing), and have been able to create tokens that reference that program in the transfer hook extension, but how do I actually make use of it during a transfer?

Calling the TransferChecked function fails with the following error:

https://explorer.solana.com/tx/5gvoeqwsWa6D6v4pKB91SbDBrmpoqmjggJyriiKV4vg63mTtrtDNXPVAjjvEBU23ANd7SZUsjxS9vS7y1RNBMwP?cluster=devnet

Program logged: "Instruction: TransferChecked" Program logged: "Error: Lamport balance below rent-exempt threshold" Program consumed: 12796 of 575094 compute units Program returned error: "custom program error: 0x0"

I don't really understand what account it refers to, certainly none of the token accounts, or the transfer hook program.. but i'm also just totally confused about how this works at all. TransferChecked doesn't have an option to include the transfer hook program i deployed, so how can it get called in the first place?

Thanks for any help you can offer!

Answer 1

In order to create a correct transfer instruction, you must fetch all of the additional required accounts and add them to the instruction. The Rust crates currently expose some helpers to make this easier, found in offchain.rs

Here's an example using it:

            let mut instruction = spl_token_2022::instruction::transfer_checked(
                &spl_token_2022::id(),
                source,
                mint,
                destination,
                authority,
                &multisig_signers,
                amount,
                decimals,
            )?;
            spl_token_2022::offchain::resolve_extra_transfer_account_metas(
                &mut instruction,
                |address| {
                    rpc_client
                        .get_account(address)
                        .map_ok(|opt| opt.map(|acc| acc.data))
                },
                mint,
            )
            .await.unwrap();
            // add the instruction to a transaction, sign and send!

Lifted from the token client: https://github.com/solana-labs/solana-program-library/blob/5a07a106b38b3107c4c89da1015d458ac9dcc5fe/token/client/src/token.rs#L907

Side note: I'll get around to adding these helpers and more docs to make it clearer. This is still very bleeding edge, sorry!