✨ Macro to define and access extra accounts

[GabrielePicco]

✨ Macro to define and access extra accounts

Status	Type	⚠️ Core Change	Issue
Ready	Feature	Yes	#7

Problem

See #7 for details

Solution

• Propagate remaining accounts to the systems
• Added a macro to define the extra accounts the system needs, similarly to transfer hooks: https://github.com/solana-labs/solana-program-library/tree/master/token/transfer-hook/interface
• The macro also inject named helper functions to easily access extra accounts

Example

The example below show a system which defines 3 extra accounts:

#[system]
pub mod system_apply_velocity {
    pub fn execute(ctx: Context<Components>, _args: Vec<u8>) -> Result<Components>  {
        // ...
        Ok(ctx.accounts)
    }


    #[system_input]
    pub struct Components {
        pub velocity: Velocity,
        pub position: Position,
    }

    #[extra_accounts]
    pub struct ExtraAccounts {
        #[account(address = bolt_lang::solana_program::sysvar::clock::id())]
        pub sysvar_clock: AccountInfo,
        #[account(address = pubkey!("6LHhFVwif6N9Po3jHtSmMVtPjF6zRfL3xMosSzcrQAS8"))]
        pub some_extra_account: AccountInfo,
        #[account(address = Metadata::id())]
        pub program_metadata: Program<Metadata>,
    }
}

The system can define the logic, using the injected helper functions to access the extra accounts:

#[system]
pub mod system_apply_velocity {
    pub fn execute(ctx: Context<Components>, _args: Vec<u8>) -> Result<Components>  {
         // Access the clock account
         let sysvar_clock_account_info = ctx.sysvar_clock()?;
         clock = Clock::from_account_info(sysvar_clock_account_info)?;
        ...
        // Access an extra account
        let extra_account = ctx.some_extra_account()?;
        ...
        Ok(ctx.accounts)
    }
}

The IDL of the system will contain the extra accounts and their address

Comments

• Clients can use the IDL to automatically retrieve the extra accounts, similarly to tlv-account-resolution for token extensions

...
"accounts": [
        {
          "name": "sysvarClock",
          "address": "SysvarC1ock11111111111111111111111111111111"
        },
        {
          "name": "someExtraAccount",
          "address": "6LHhFVwif6N9Po3jHtSmMVtPjF6zRfL3xMosSzcrQAS8"
        },
        {
          "name": "programMetadata",
          "address": "metaqbxxUerdq28cj1RbAWkYQm3ybzjb6a8bt518x1s"
        }
 ],
 ...

• Additionally, needed AccountMetas could be saved onchain (in a PDA), following the token extensions standard

Issues

Closes #7