Introduce OnReapAccount (#4585)

* Initial run and gun at `OnReapAccount`

* Fix some imports

* More fixes

* Whitespace

* More wack-a-mole

* Gotta catch em all

* Update lib.rs

* Small doc update

* Whitespace

bin/node-template/runtime/src/lib.rs

@@ -205,6 +205,8 @@ impl balances::Trait for Runtime {
 	type Balance = Balance;
 	/// What to do if an account's free balance gets zeroed.
 	type OnFreeBalanceZero = ();
+	/// What to do if an account is fully reaped from the system.
+	type OnReapAccount = System;
 	/// What to do if a new account is created.
 	type OnNewAccount = Indices;
 	/// The ubiquitous event type.

bin/node/runtime/src/lib.rs

@@ -174,6 +174,7 @@ parameter_types! {
 impl pallet_balances::Trait for Runtime {
 	type Balance = Balance;
 	type OnFreeBalanceZero = ((Staking, Contracts), Session);
+	type OnReapAccount = System;
 	type OnNewAccount = Indices;
 	type Event = Event;
 	type DustRemoval = ();

frame/balances/src/lib.rs

@@ -165,7 +165,7 @@ use codec::{Codec, Encode, Decode};
 use frame_support::{
 	StorageValue, Parameter, decl_event, decl_storage, decl_module, decl_error,
 	traits::{
-		UpdateBalanceOutcome, Currency, OnFreeBalanceZero, OnUnbalanced, TryDrop,
+		UpdateBalanceOutcome, Currency, OnFreeBalanceZero, OnReapAccount, OnUnbalanced, TryDrop,
 		WithdrawReason, WithdrawReasons, LockIdentifier, LockableCurrency, ExistenceRequirement,
 		Imbalance, SignedImbalance, ReservableCurrency, Get, VestingCurrency,
 	},
@@ -198,6 +198,12 @@ pub trait Subtrait<I: Instance = DefaultInstance>: frame_system::Trait {
 	/// Gives a chance to clean up resources associated with the given account.
 	type OnFreeBalanceZero: OnFreeBalanceZero<Self::AccountId>;
 
+	/// A function that is invoked when the free-balance and the reserved-balance has fallen below
+	/// the existential deposit and both have been reduced to zero.
+	///
+	/// All resources should be cleaned up all resources associated with the given account.
+	type OnReapAccount: OnReapAccount<Self::AccountId>;
+
 	/// Handler for when a new account is created.
 	type OnNewAccount: OnNewAccount<Self::AccountId>;
 
@@ -222,6 +228,12 @@ pub trait Trait<I: Instance = DefaultInstance>: frame_system::Trait {
 	/// Gives a chance to clean up resources associated with the given account.
 	type OnFreeBalanceZero: OnFreeBalanceZero<Self::AccountId>;
 
+	/// A function that is invoked when the free-balance and the reserved-balance has fallen below
+	/// the existential deposit and both have been reduced to zero.
+	///
+	/// All resources should be cleaned up all resources associated with the given account.
+	type OnReapAccount: OnReapAccount<Self::AccountId>;
+
 	/// Handler for when a new account is created.
 	type OnNewAccount: OnNewAccount<Self::AccountId>;
 
@@ -248,6 +260,7 @@ pub trait Trait<I: Instance = DefaultInstance>: frame_system::Trait {
 impl<T: Trait<I>, I: Instance> Subtrait<I> for T {
 	type Balance = T::Balance;
 	type OnFreeBalanceZero = T::OnFreeBalanceZero;
+	type OnReapAccount = T::OnReapAccount;
 	type OnNewAccount = T::OnNewAccount;
 	type ExistentialDeposit = T::ExistentialDeposit;
 	type TransferFee = T::TransferFee;
@@ -597,7 +610,7 @@ impl<T: Trait<I>, I: Instance> Module<T, I> {
 	///
 	/// This just removes the nonce and leaves an event.
 	fn reap_account(who: &T::AccountId, dust: T::Balance) {
-		<frame_system::AccountNonce<T>>::remove(who);
+		T::OnReapAccount::on_reap_account(who);
 		Self::deposit_event(RawEvent::ReapedAccount(who.clone(), dust));
 	}
 
@@ -850,6 +863,7 @@ impl<T: Subtrait<I>, I: Instance> frame_system::Trait for ElevatedTrait<T, I> {
 impl<T: Subtrait<I>, I: Instance> Trait<I> for ElevatedTrait<T, I> {
 	type Balance = T::Balance;
 	type OnFreeBalanceZero = T::OnFreeBalanceZero;
+	type OnReapAccount = T::OnReapAccount;
 	type OnNewAccount = T::OnNewAccount;
 	type Event = ();
 	type TransferPayment = ();

frame/balances/src/mock.rs

@@ -93,6 +93,7 @@ impl pallet_transaction_payment::Trait for Test {
 impl Trait for Test {
 	type Balance = u64;
 	type OnFreeBalanceZero = ();
+	type OnReapAccount = System;
 	type OnNewAccount = ();
 	type Event = ();
 	type DustRemoval = ();

frame/contracts/src/tests.rs

@@ -122,6 +122,7 @@ impl frame_system::Trait for Test {
 impl pallet_balances::Trait for Test {
 	type Balance = u64;
 	type OnFreeBalanceZero = Contract;
+	type OnReapAccount = System;
 	type OnNewAccount = ();
 	type Event = MetaEvent;
 	type DustRemoval = ();

frame/democracy/src/lib.rs

@@ -1205,6 +1205,7 @@ mod tests {
 	impl pallet_balances::Trait for Test {
 		type Balance = u64;
 		type OnFreeBalanceZero = ();
+		type OnReapAccount = System;
 		type OnNewAccount = ();
 		type Event = ();
 		type TransferPayment = ();

frame/elections-phragmen/src/lib.rs

@@ -824,6 +824,7 @@ mod tests {
 		type Balance = u64;
 		type OnNewAccount = ();
 		type OnFreeBalanceZero = ();
+		type OnReapAccount = System;
 		type Event = Event;
 		type TransferPayment = ();
 		type DustRemoval = ();

frame/elections/src/mock.rs

@@ -65,6 +65,7 @@ impl pallet_balances::Trait for Test {
 	type Balance = u64;
 	type OnNewAccount = ();
 	type OnFreeBalanceZero = ();
+	type OnReapAccount = System;
 	type Event = Event;
 	type TransferPayment = ();
 	type DustRemoval = ();

frame/example/src/lib.rs

@@ -695,6 +695,7 @@ mod tests {
 	impl pallet_balances::Trait for Test {
 		type Balance = u64;
 		type OnFreeBalanceZero = ();
+		type OnReapAccount = System;
 		type OnNewAccount = ();
 		type Event = ();
 		type TransferPayment = ();
@@ -706,6 +707,7 @@ mod tests {
 	impl Trait for Test {
 		type Event = ();
 	}
+	type System = frame_system::Module<Test>;
 	type Example = Module<Test>;
 
 	// This function basically just builds a genesis storage key/value store according to

frame/executive/src/lib.rs

@@ -425,6 +425,7 @@ mod tests {
 	impl pallet_balances::Trait for Runtime {
 		type Balance = u64;
 		type OnFreeBalanceZero = ();
+		type OnReapAccount = System;
 		type OnNewAccount = ();
 		type Event = MetaEvent;
 		type DustRemoval = ();

frame/identity/src/lib.rs

@@ -921,6 +921,7 @@ mod tests {
 	impl pallet_balances::Trait for Test {
 		type Balance = u64;
 		type OnFreeBalanceZero = ();
+		type OnReapAccount = System;
 		type OnNewAccount = ();
 		type Event = ();
 		type TransferPayment = ();
@@ -950,6 +951,7 @@ mod tests {
 		type RegistrarOrigin = EnsureSignedBy<One, u64>;
 		type ForceOrigin = EnsureSignedBy<Two, u64>;
 	}
+	type System = frame_system::Module<Test>;
 	type Balances = pallet_balances::Module<Test>;
 	type Identity = Module<Test>;
 

frame/nicks/src/lib.rs

@@ -294,6 +294,7 @@ mod tests {
 	impl pallet_balances::Trait for Test {
 		type Balance = u64;
 		type OnFreeBalanceZero = ();
+		type OnReapAccount = System;
 		type OnNewAccount = ();
 		type Event = ();
 		type TransferPayment = ();
@@ -319,6 +320,7 @@ mod tests {
 		type MinLength = MinLength;
 		type MaxLength = MaxLength;
 	}
+	type System = frame_system::Module<Test>;
 	type Balances = pallet_balances::Module<Test>;
 	type Nicks = Module<Test>;
 

frame/scored-pool/src/mock.rs

@@ -77,6 +77,7 @@ impl frame_system::Trait for Test {
 impl pallet_balances::Trait for Test {
 	type Balance = u64;
 	type OnFreeBalanceZero = ();
+	type OnReapAccount = System;
 	type OnNewAccount = ();
 	type Event = ();
 	type TransferPayment = ();
@@ -118,13 +119,16 @@ impl Trait for Test {
 	type KickOrigin = EnsureSignedBy<KickOrigin, u64>;
 	type MembershipInitialized = TestChangeMembers;
 	type MembershipChanged = TestChangeMembers;
-	type Currency = pallet_balances::Module<Self>;
+	type Currency = Balances;
 	type CandidateDeposit = CandidateDeposit;
 	type Period = Period;
 	type Score = u64;
 	type ScoreOrigin = EnsureSignedBy<ScoreOrigin, u64>;
 }
 
+type System = frame_system::Module<Test>;
+type Balances = pallet_balances::Module<Test>;
+
 // This function basically just builds a genesis storage key/value store according to
 // our desired mockup.
 pub fn new_test_ext() -> sp_io::TestExternalities {

frame/staking/src/mock.rs

@@ -146,6 +146,7 @@ parameter_types! {
 impl pallet_balances::Trait for Test {
 	type Balance = Balance;
 	type OnFreeBalanceZero = Staking;
+	type OnReapAccount = System;
 	type OnNewAccount = ();
 	type Event = ();
 	type TransferPayment = ();

frame/support/src/traits.rs

@@ -67,10 +67,17 @@ pub trait Contains<T: Ord> {
 /// The account with the given id was killed.
 #[impl_trait_for_tuples::impl_for_tuples(30)]
 pub trait OnFreeBalanceZero<AccountId> {
-	/// The account was the given id was killed.
+	/// The account with the given id was killed.
 	fn on_free_balance_zero(who: &AccountId);
 }
 
+/// The account with the given id was reaped.
+#[impl_trait_for_tuples::impl_for_tuples(30)]
+pub trait OnReapAccount<AccountId> {
+	/// The account with the given id was reaped.
+	fn on_reap_account(who: &AccountId);
+}
+
 /// Outcome of a balance update.
 pub enum UpdateBalanceOutcome {
 	/// Account balance was simply updated.

frame/system/src/lib.rs

@@ -113,7 +113,7 @@ use sp_runtime::{
 use sp_core::storage::well_known_keys;
 use frame_support::{
 	decl_module, decl_event, decl_storage, decl_error, storage, Parameter,
-	traits::{Contains, Get, ModuleToIndex},
+	traits::{Contains, Get, ModuleToIndex, OnReapAccount},
 	weights::{Weight, DispatchInfo, DispatchClass, SimpleDispatchInfo},
 };
 use codec::{Encode, Decode};
@@ -789,6 +789,13 @@ impl<T: Trait> Module<T> {
 	}
 }
 
+impl<T: Trait> OnReapAccount<T::AccountId> for Module<T> {
+	/// Remove the nonce for the account. Account is considered fully removed from the system.
+	fn on_reap_account(who: &T::AccountId) {
+		<AccountNonce<T>>::remove(who);
+	}
+}
+
 /// resource limit check.
 #[derive(Encode, Decode, Clone, Eq, PartialEq)]
 pub struct CheckWeight<T: Trait + Send + Sync>(PhantomData<T>);

frame/transaction-payment/src/lib.rs

@@ -311,6 +311,7 @@ mod tests {
 	impl pallet_balances::Trait for Runtime {
 		type Balance = u64;
 		type OnFreeBalanceZero = ();
+		type OnReapAccount = System;
 		type OnNewAccount = ();
 		type Event = ();
 		type TransferPayment = ();

frame/treasury/src/lib.rs

@@ -766,6 +766,7 @@ mod tests {
 		type Balance = u64;
 		type OnNewAccount = ();
 		type OnFreeBalanceZero = ();
+		type OnReapAccount = System;
 		type Event = ();
 		type TransferPayment = ();
 		type DustRemoval = ();

frame/utility/src/lib.rs

@@ -698,6 +698,7 @@ mod tests {
 	impl pallet_balances::Trait for Test {
 		type Balance = u64;
 		type OnFreeBalanceZero = ();
+		type OnReapAccount = System;
 		type OnNewAccount = ();
 		type Event = TestEvent;
 		type TransferPayment = ();
@@ -719,6 +720,7 @@ mod tests {
 		type MultisigDepositFactor = MultisigDepositFactor;
 		type MaxSignatories = MaxSignatories;
 	}
+	type System = frame_system::Module<Test>;
 	type Balances = pallet_balances::Module<Test>;
 	type Utility = Module<Test>;