Add a dust removal whitelist to pallet-balances

[maltekliemann]

A whitelisted account is an account whose dust will not be collected even if it's balances is below the existential deposit or zero. orml_tokens implements whitelisted accounts using a DustRemovalWhitelist config parameter which implements frame_support::traits::Contains<Self::AccountId>; an account is whitelisted if DustRemovalWhitelist::contains(&account) is true.

This type of feature seems like a good fit for pallet-balances, as well. For example, accounts holding some type of collateral should be whitelisted. Unless, of course, I'm completely overlooking some other feature which allows us to do this already.

I'd be willing to write the PR myself, if that helps.

[shawntabrizi]

Substrate already provides this using reference counters.

Balances pallet is not what ultimately ends up "killing" an account, the lack of reference counters does. If you create a pallet or system logic which simply adds a reference counter to a user, its account will not be dusted.

[maltekliemann]

Thanks. I had convinced myself earlier that pallet-balances dusting is separate from reference counter. Must have overlooked something.

[maltekliemann]

@shawntabrizi I took another look and wrote a test (for pallet-balances). So it seems that if you've got providers, consumers and sufficients, the accounts does not get reaped, but it appears to get dusted. Is this correct? If so, I wasn't aware of this distinction.

#[test]
fn reap_fails_but_account_gets_dusted() {
    <$ext_builder>::default().existential_deposit(2).monied(true).build().execute_with(|| {
        assert_eq!(System::inc_providers(&1), frame_system::IncRefStatus::Existed);
        assert_eq!(System::providers(&1), 2);
        assert_ok!(System::inc_consumers(&1));
        System::inc_sufficients(&1);
        assert_eq!(Balances::free_balance(1), 20);
        assert_ok!(<Balances as Currency<_>>::transfer(&1, &2, 19, AllowDeath));
        assert!(System::account_exists(&1));
        assert_eq!(Balances::free_balance(1), 0);
        assert_eq!(System::providers(&1), 1);
    });
}

Anyways, what I'm looking for is a way to prevent dust collection in addition to preventing the account from getting reaped.

[maltekliemann]

bump After a discussion on stackexchange I think the claim made by @shawntabrizi that the account won't be dusted is incorrect and the above is correct behavior.

Is a whitelist for dusting (not reaping) is something that Parity would consider a good feature? If so, I'm offering to implement the feature.