Consecutive seed pdas

Question

Is it possible to have an account with a counter, and an instruction that can be called by any user at any time and will init a new pda with the counter number as seed and add 1 to the counter?

I find difficult to do it because, although I can fetch the counter value from the counter account to pass it in the instruction, as any user can make this transaction at any time, if more than one user transacts at the same time, only one will succeed since the pda with that seed will already be created.

Is it a way to make pdas like this, with a counter on chain? Is it any other way to do it?

Thanks

Answer 1

Several ways to handle it:

1. Use init_if_needed , hence the first one creates and the rest update.

2. If your PDA is global, you can initialize it on your own and then all users will only update.

Answer 2

In a previous project I used the same architecture, but it is very dangerous. This is because anyone could deny access to a specific user by fetching their main account. But in case you want to execute that type of architecture, here is an example.

#[derive(Accounts)]
pub struct Update<'info> {
    #[account(mut, seeds = [b"SOLotery", main.authority.key().as_ref()], bump = main.bump_original)]
    pub main: Account<'info, Main>,
    #[account(init, seeds = [main.counter.to_le_bytes().as_ref()], bump = main.bump_winner, space = 8, payer=user)]
    pub unsafe_pda: Account<'info, UnsafePDA>,
    #[account(mut)]
    pub user: Signer<'info>,
}
#[account]
pub struct Main {
    pub authority: Pubkey, 
    pub counter: u8,
    pub bump_winner: u8, 
}
 #[account]
    pub struct UnsafePDA {
    pub authority: Pubkey, 
    pub counter: u8,
    pub bump_winner: u8, 
}