withdraw_secure.rs

use anchor_lang::prelude::*;
use crate::state::Vault;

/// Withdraw lamports from vault - SECURE version
///
/// ✅ SECURE: This instruction checks that the remaining balance
/// stays above the rent-exempt minimum before allowing withdrawal.
#[derive(Accounts)]
pub struct WithdrawSecure<'info> {
    #[account(
        mut,
        seeds = [b"vault", owner.key().as_ref()],
        bump = vault.bump,
        has_one = owner
    )]
    pub vault: Account<'info, Vault>,

    #[account(mut)]
    pub owner: Signer<'info>,
}

pub fn handler(ctx: Context<WithdrawSecure>, amount: u64) -> Result<()> {
    let vault = &ctx.accounts.vault;
    let vault_info = vault.to_account_info();

    // Get current balance
    let vault_balance = vault_info.lamports();

    // ✅ SECURE: Calculate rent-exempt minimum for this account
    let rent = Rent::get()?;
    let rent_exempt_minimum = rent.minimum_balance(vault_info.data_len());

    // ✅ SECURE: Calculate maximum withdrawable amount
    let max_withdrawable = vault_balance
        .checked_sub(rent_exempt_minimum)
        .unwrap_or(0);

    // ✅ SECURE: Check that withdrawal won't violate rent exemption
    require!(
        amount <= max_withdrawable,
        crate::error::ErrorCode::WouldViolateRentExemption
    );

    // Safe to transfer
    **vault_info.try_borrow_mut_lamports()? -= amount;
    **ctx.accounts.owner.try_borrow_mut_lamports()? += amount;

    let remaining = vault_info.lamports();
    msg!(
        "SECURE: Withdrew {} lamports. Remaining: {} (rent-exempt min: {})",
        amount,
        remaining,
        rent_exempt_minimum
    );

    Ok(())
}