Account Model Refactor

[xlc]

Account Model Refactor

ExistentialDeposit causes a lot of issues and edgecases:

• System account have to be initialized first
  • https://github.com/paritytech/substrate/blob/812d94d42c6d6d7fd28cf18a08d84f14c49d8544/frame/treasury/src/lib.rs#L230-L233
• Almost impossible to guantaree a transfer is going to success
  • https://github.com/paritytech/substrate/blob/812d94d42c6d6d7fd28cf18a08d84f14c49d8544/frame/treasury/src/lib.rs#L676-L682

So we will be dropping it.

The main reason of ExistentialDeposit is to prevent dust account attack, avoid state bloating to ensure people will not be unnecessarily to store account balances with near zero value and the storages are paid with enough compensation.

To prevent such attack without ExistentialDeposit, a new account model is required.

An account needs to be opened before it can exists and holding balances and system account data (i.e. nonce and ref count). A deposit will be reserved in order to open an account. This deposit essentially pays for the state usage of account data and all the token balances.

Account can be closed to wipe account data and token balances and release the reserved token.

System account ref counting can be used to prevent an account to be closed, so pallets can know it only need to dealing with opened accounts.

Workflow

• any kind of transfer
  • check if receiver has account opened
  • if no, use the transfered fund to open account
    • if fund is not ACA
      • use DEX to convert required amount to ACA
    • reserve required ACA and mark account is open
• to close account
  • parameter dest: Option<AccountId>
  • if no dest, use treasury address
  • check allow_death from system ref counting
  • if yes
    • iterate all token balances, send remaining fund including reserved ones to dest
    • kill account

Spec

• orml-traits
  • Types
    • trait OnReceived
      • on_received(account: AccountId, currency: CurrencyId, amount: Balance)
• orml-tokens
  • Trait
    • OnReceived: OnReceived
  • Module
    • update deposit & transfer to call OnReceived
• module-accounts
  • Trait
    • OnCreatedAccount: Happened
    • KillAccount: Happened
    • NewAccountDeposit: Get<Balance>
    • Treasury: Get<ModuleId>
  • Calls
    • close_account(origin, to: Option<AccountId>)
      • ensure signed
      • ensure system::Module::allow_death
      • iterate all currencies and send all remaining funds to to
      • call KillAccount
  • Module
    • open_account(who: AccountId, currency: CurrencyId) -> DispatchResult
      • if currency is not ACA, use DEX to buy ACA
        • if failed, send all fund to treasury
      • Reserve NewAccountDeposit
        • if failed, send all fund to treasury
      • call OnCreatedAccount
    • impl OnReceived
      • on_received
        • if account did not exist
          • open_account
    • impl StoredMap
      • Duplicate https://github.com/paritytech/substrate/blob/812d94d42c6d6d7fd28cf18a08d84f14c49d8544/frame/system/src/lib.rs#L1271
      • Still use system::Account as storage
      • When account received balances (via mutate / insert)
        • if account did not exist
          • open_account
• runtime
  • pallet_balances
    • type ExistentialDeposit = 1
      • pallet balances with zero ExistentialDeposit paritytech/substrate#6083 (comment)
    • AccountStore = Accounts
  • module_accounts
    • OnCreatedAccount = system::CallOnCreatedAccount
    • KillAccount = system::CallKillAccount
    • NewAccountDeposit = 0.01 ACA
      • exact value to be decided later

[xlc]

Just found StorageMapShim can be used to detect on receive balance

https://github.com/paritytech/substrate/blob/cd305af2a9f8ec7d13081b6dc6807014b207dab8/frame/support/src/traits.rs#L91