Transfer-hook extra account metas can derive PDAs at runtime via ExtraAccountMeta::new_with_seeds and Seed::AccountKey indexes

Problem: a Token-2022 transfer hook needs an account whose address depends on the transfer (for example a PDA seeded by the sender). The examples all hardcode fixed pubkeys into ExtraAccountMetaList, which suggests the extra accounts must be known when the hook is initialised. They don't have to be.

The spl-tlv-account-resolution library lets each extra meta be a *derivation recipe* that the client-side resolver evaluates per transfer:

- ExtraAccountMeta::new_with_pubkey(&key, is_signer, is_writable) — a fixed address. - ExtraAccountMeta::new_with_seeds(&[Seed::Literal { bytes: b"delegate".to_vec() }], false, false) — a PDA of *your* program, derived from literal and/or dynamic seeds. - ExtraAccountMeta::new_external_pda_with_seeds(program_index, &[...], is_signer, is_writable) — a PDA of another program (e.g. the associated-token program).

Dynamic seeds are referenced positionally, not by name: Seed::AccountKey { index: N } means "the pubkey of account N in the resolved account list". Indexes 0-3 are the transfer accounts (source, mint, destination, owner), index 4 is the ExtraAccountMetaList account, and 5+ are your own extra metas in declaration order. So an ATA owned by a delegate PDA is expressed as new_external_pda_with_seeds(7 /* ATA program */, &[Seed::AccountKey { index: 8 } /* delegate PDA */, Seed::AccountKey { index: 6 } /* token program */, Seed::AccountKey { index: 5 } /* mint */], false, true).

Non-obvious part: the index space includes the four implicit transfer accounts and the meta-list account, so off-by-one errors here produce a resolved address that simply doesn't match what the hook expects, surfacing as a constraint or seeds violation rather than as a resolution error. Also remember Seed::AccountData exists for seeds read out of account data.

Source: adapted from a Stack Exchange answer by john (question by PandaPanda), https://solana.stackexchange.com/questions/9352/dynamically-use-pda-with-transfer-hooks — CC BY-SA 4.0