Close PDA account dynamically and conditionally in secure way

Question

I want to close the PDA account only on specif condition. Macro [account(mut, close = <target_account>)] does not work in this case because it will always be triggered after instruction completion. Also security should also be taken into account in order to avoid sealevel-attacks. I followed the instruction provided here but I am not sure what is the purpose of force_defund instruction and how to integrate it with my code:

#[derive(Accounts)]
pub struct Debt<'info> {
    pub user: SystemAccount<'info>,
    #[account(
        mut,
        seeds = [b"vault".as_ref(), player.key().as_ref()],
        bump = vault.bump
    )]
    pub user_vault: Account<'info, Vault>,
    #[account(mut)]
    pub vault: SystemAccount<'info>,
    pub system_program: Program<'info, System>,
}

impl Debt<'_> {
    pub fn debt(ctx: Context<Self>) -> Result<()> {
        ctx.accounts.user_vault.calculate_debt();

        if ctx.accounts.user_vault.result == Status::Inactive {
            // CLOSING SECURELY THE USER VAULT PDA ACCOUNT

            **ctx.accounts.user.to_account_info().try_borrow_mut_lamports()? +=
                **ctx.accounts.user_vault.to_account_info().try_borrow_lamports()?;
            **ctx.accounts.user_vault.to_account_info().try_borrow_mut_lamports()? = 0;

            let user_vault_account = ctx.accounts.user_vault.to_account_info();
            let mut data = user_vault_account.try_borrow_mut_data()?;
            for byte in data.deref_mut().iter_mut() {
                *byte = 0;
            }

            let dst: &mut [u8] = &mut data;
            let mut cursor = Cursor::new(dst);
            cursor.write_all(&CLOSED_ACCOUNT_DISCRIMINATOR).unwrap();
        }

        // DO SOME OTHER STUFF

        Ok(())
    }
}

Answer 1

I found the source code for close macro (docs) which has a lot of similarities with my code:

use crate::bpf_writer::BpfWriter;
use crate::error::ErrorCode;
use crate::prelude::error;
use crate::Result;
use solana_program::account_info::AccountInfo;
use std::io::Write;

pub fn close<'info>(info: AccountInfo<'info>, sol_destination: AccountInfo<'info>) -> Result<()> {
    // Transfer tokens from the account to the sol_destination.
    let dest_starting_lamports = sol_destination.lamports();
    **sol_destination.lamports.borrow_mut() =
        dest_starting_lamports.checked_add(info.lamports()).unwrap();
    **info.lamports.borrow_mut() = 0;

    // Mark the account discriminator as closed.
    let mut data = info.try_borrow_mut_data()?;
    let dst: &mut [u8] = &mut data;
    let mut writer = BpfWriter::new(dst);
    writer
        .write_all(&crate::__private::CLOSED_ACCOUNT_DISCRIMINATOR)
        .map_err(|_| error!(ErrorCode::AccountDidNotSerialize))
}

So my final version of code is:

#[derive(Accounts)]
pub struct Debt<'info> {
    pub user: SystemAccount<'info>,
    #[account(
        mut,
        seeds = [b"vault".as_ref(), player.key().as_ref()],
        bump = vault.bump
    )]
    pub user_vault: Account<'info, Vault>,
    #[account(mut)]
    pub vault: SystemAccount<'info>,
    pub system_program: Program<'info, System>,
}

impl Debt<'_> {
    pub fn debt(ctx: Context<Self>) -> Result<()> {
        ctx.accounts.user_vault.calculate_debt();

        if ctx.accounts.user_vault.result == Status::Inactive {
            // CLOSING SECURELY THE USER VAULT PDA ACCOUNT

            **ctx.accounts.user.to_account_info().try_borrow_mut_lamports()? +=
                **ctx.accounts.user_vault.to_account_info().try_borrow_lamports()?;
            **ctx.accounts.user_vault.to_account_info().try_borrow_mut_lamports()? = 0;

            let user_vault_account = ctx.accounts.user_vault.to_account_info();
            let mut data = user_vault_account.try_borrow_mut_data()?;
            for byte in data.deref_mut().iter_mut() {
                *byte = 0;
            }

            let dst: &mut [u8] = &mut data;
            let mut cursor = Cursor::new(dst);
            cursor.write_all(&CLOSED_ACCOUNT_DISCRIMINATOR)
                  .map_err(|_| error!(ErrorCode::AccountDidNotSerialize))
                  .unwrap();
        }

        // DO SOME OTHER STUFF

        Ok(())
    }
}

Answer 2

For now, there's no actual way to safely close an account outside of the close constraint, unless the account is passed in as an UncheckedAccount or within the remaining accounts. There's currently a PR open to fix this: github.com/coral-xyz/anchor/pull/2209.

Answer 3

I think this is new approach, which can be used in new anchor versions. https://github.com/coral-xyz/anchor/pull/2169/files