[NPoS] Fix for Reward Deficit in the pool (#1255)

closes https://github.com/paritytech/polkadot-sdk/issues/158.
partially addresses
https://github.com/paritytech/polkadot-sdk/issues/226.

Instead of fragile calculation of current balance by looking at `free
balance - ED`, Nomination Pool now freezes ED in the pool reward account
to restrict an account from going below minimum balance. This also has a
nice side effect that if ED changes, we know how much is the imbalance
in ED frozen in the pool and the current required ED. A pool operator
can diligently top up the pool with the deficit in ED or vice versa,
withdraw the excess they transferred to the pool.

## Notable changes
- New call `adjust_pool_deposit`: Allows to top up the deficit or
withdraw the excess deposited funds to the pool.
- Uses Fungible trait (instead of Currency trait). Since NP was not
doing any locking/reserving previously, no migration is needed for this.
- One time migration of freezing ED from each of the existing pools (not
very PoV friendly but fine for relay chain).

polkadot/runtime/westend/src/lib.rs

@@ -292,9 +292,9 @@ impl pallet_balances::Config for Runtime {
 	type ReserveIdentifier = [u8; 8];
 	type WeightInfo = weights::pallet_balances::WeightInfo<Runtime>;
 	type RuntimeHoldReason = RuntimeHoldReason;
-	type FreezeIdentifier = ();
+	type FreezeIdentifier = RuntimeFreezeReason;
+	type MaxFreezes = ConstU32<1>;
 	type MaxHolds = ConstU32<1>;
-	type MaxFreezes = ConstU32<0>;
 }
 
 parameter_types! {
@@ -1311,6 +1311,7 @@ impl pallet_nomination_pools::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
 	type WeightInfo = weights::pallet_nomination_pools::WeightInfo<Self>;
 	type Currency = Balances;
+	type RuntimeFreezeReason = RuntimeFreezeReason;
 	type RewardCounter = FixedU128;
 	type BalanceToU256 = BalanceToU256;
 	type U256ToBalance = U256ToBalance;
@@ -1398,7 +1399,7 @@ construct_runtime! {
 		VoterList: pallet_bags_list::<Instance1>::{Pallet, Call, Storage, Event<T>} = 25,
 
 		// Nomination pools for staking.
-		NominationPools: pallet_nomination_pools::{Pallet, Call, Storage, Event<T>, Config<T>} = 29,
+		NominationPools: pallet_nomination_pools::{Pallet, Call, Storage, Event<T>, Config<T>, FreezeReason} = 29,
 
 		// Fast unstake pallet: extension to staking.
 		FastUnstake: pallet_fast_unstake = 30,
@@ -1505,6 +1506,7 @@ pub mod migrations {
 		UpgradeSessionKeys,
 		parachains_configuration::migration::v9::MigrateToV9<Runtime>,
 		paras_registrar::migration::VersionCheckedMigrateToV1<Runtime, ()>,
+		pallet_nomination_pools::migration::versioned_migrations::V5toV6<Runtime>,
 		pallet_referenda::migration::v1::MigrateV0ToV1<Runtime, ()>,
 	);
 }

prdoc/pr_1255.prdoc

@@ -0,0 +1,22 @@
+# Schema: Parity PR Documentation Schema (prdoc)
+# See doc at https://github.com/paritytech/prdoc
+
+title: Fix for Reward Deficit in the pool
+
+doc:
+  - audience: Core Dev
+    description: Instead of fragile calculation of current balance by looking at free balance - ED, Nomination Pool now freezes ED in the pool reward account to restrict an account from going below minimum balance. This also has a nice side effect that if ED changes, we know how much is the imbalance in ED frozen in the pool and the current required ED. A pool operator can diligently top up the pool with the deficit in ED or vice versa, withdraw the excess they transferred to the pool.
+    notes:
+      - Introduces new call `adjust_pool_deposit` that allows to top up the deficit or withdraw the excess deposit for the pool.
+      - Switch to using Fungible trait from Currency trait.
+
+migrations:
+  db: []
+
+  runtime:
+    - { pallet: "pallet-nomination-pools", description: "One time migration of freezing ED from each of the existing pools."}
+
+crates:
+  - name: pallet-nomination-pools
+
+host_functions: []

substrate/bin/node/runtime/src/lib.rs

@@ -521,8 +521,8 @@ impl pallet_balances::Config for Runtime {
 	type ExistentialDeposit = ExistentialDeposit;
 	type AccountStore = frame_system::Pallet<Runtime>;
 	type WeightInfo = pallet_balances::weights::SubstrateWeight<Runtime>;
-	type FreezeIdentifier = ();
+	type FreezeIdentifier = RuntimeFreezeReason;
-	type MaxFreezes = ();
+	type MaxFreezes = ConstU32<1>;
 	type RuntimeHoldReason = RuntimeHoldReason;
 	type MaxHolds = ConstU32<2>;
 }
@@ -882,6 +882,7 @@ impl pallet_nomination_pools::Config for Runtime {
 	type WeightInfo = ();
 	type RuntimeEvent = RuntimeEvent;
 	type Currency = Balances;
+	type RuntimeFreezeReason = RuntimeFreezeReason;
 	type RewardCounter = FixedU128;
 	type BalanceToU256 = BalanceToU256;
 	type U256ToBalance = U256ToBalance;

substrate/frame/nomination-pools/benchmarking/src/lib.rs

@@ -27,7 +27,10 @@ use frame_benchmarking::v1::{account, whitelist_account};
 use frame_election_provider_support::SortedListProvider;
 use frame_support::{
 	assert_ok, ensure,
-	traits::{Currency, Get},
+	traits::{
+		fungible::{Inspect, Mutate, Unbalanced},
+		Get,
+	},
 };
 use frame_system::RawOrigin as RuntimeOrigin;
 use pallet_nomination_pools::{
@@ -67,7 +70,7 @@ fn create_funded_user_with_balance<T: pallet_nomination_pools::Config>(
 	balance: BalanceOf<T>,
 ) -> T::AccountId {
 	let user = account(string, n, USER_SEED);
-	T::Currency::make_free_balance_be(&user, balance);
+	T::Currency::set_balance(&user, balance);
 	user
 }
 
@@ -148,8 +151,7 @@ impl<T: Config> ListScenario<T> {
 		);
 
 		// Burn the entire issuance.
-		let i = CurrencyOf::<T>::burn(CurrencyOf::<T>::total_issuance());
+		CurrencyOf::<T>::set_total_issuance(Zero::zero());
-		sp_std::mem::forget(i);
 
 		// Create accounts with the origin weight
 		let (pool_creator1, pool_origin1) =
@@ -206,7 +208,7 @@ impl<T: Config> ListScenario<T> {
 
 		let joiner: T::AccountId = account("joiner", USER_SEED, 0);
 		self.origin1_member = Some(joiner.clone());
-		CurrencyOf::<T>::make_free_balance_be(&joiner, amount * 2u32.into());
+		CurrencyOf::<T>::set_balance(&joiner, amount * 2u32.into());
 
 		let original_bonded = T::Staking::active_stake(&self.origin1).unwrap();
 
@@ -254,7 +256,7 @@ frame_benchmarking::benchmarks! {
 		whitelist_account!(joiner);
 	}: _(RuntimeOrigin::Signed(joiner.clone()), max_additional, 1)
 	verify {
-		assert_eq!(CurrencyOf::<T>::free_balance(&joiner), joiner_free - max_additional);
+		assert_eq!(CurrencyOf::<T>::balance(&joiner), joiner_free - max_additional);
 		assert_eq!(
 			T::Staking::active_stake(&scenario.origin1).unwrap(),
 			scenario.dest_weight
@@ -289,7 +291,7 @@ frame_benchmarking::benchmarks! {
 		// transfer exactly `extra` to the depositor of the src pool (1),
 		let reward_account1 = Pools::<T>::create_reward_account(1);
 		assert!(extra >= CurrencyOf::<T>::minimum_balance());
-		CurrencyOf::<T>::deposit_creating(&reward_account1, extra);
+		let _ = CurrencyOf::<T>::mint_into(&reward_account1, extra);
 
 	}: _(RuntimeOrigin::Signed(claimer), T::Lookup::unlookup(scenario.creator1.clone()), BondExtra::Rewards)
 	verify {
@@ -309,7 +311,7 @@ frame_benchmarking::benchmarks! {
 		let reward_account = Pools::<T>::create_reward_account(1);
 
 		// Send funds to the reward account of the pool
-		CurrencyOf::<T>::make_free_balance_be(&reward_account, ed + origin_weight);
+		CurrencyOf::<T>::set_balance(&reward_account, ed + origin_weight);
 
 		// set claim preferences to `PermissionlessAll` so any account can claim rewards on member's
 		// behalf.
@@ -317,7 +319,7 @@ frame_benchmarking::benchmarks! {
 
 		// Sanity check
 		assert_eq!(
-			CurrencyOf::<T>::free_balance(&depositor),
+			CurrencyOf::<T>::balance(&depositor),
 			origin_weight
 		);
 
@@ -325,11 +327,11 @@ frame_benchmarking::benchmarks! {
 	}:claim_payout_other(RuntimeOrigin::Signed(claimer), depositor.clone())
 	verify {
 		assert_eq!(
-			CurrencyOf::<T>::free_balance(&depositor),
+			CurrencyOf::<T>::balance(&depositor),
 			origin_weight + commission * origin_weight
 		);
 		assert_eq!(
-			CurrencyOf::<T>::free_balance(&reward_account),
+			CurrencyOf::<T>::balance(&reward_account),
 			ed + commission * origin_weight
 		);
 	}
@@ -383,7 +385,7 @@ frame_benchmarking::benchmarks! {
 			T::Staking::active_stake(&pool_account).unwrap(),
 			min_create_bond + min_join_bond
 		);
-		assert_eq!(CurrencyOf::<T>::free_balance(&joiner), min_join_bond);
+		assert_eq!(CurrencyOf::<T>::balance(&joiner), min_join_bond);
 
 		// Unbond the new member
 		Pools::<T>::fully_unbond(RuntimeOrigin::Signed(joiner.clone()).into(), joiner.clone()).unwrap();
@@ -403,7 +405,7 @@ frame_benchmarking::benchmarks! {
 	}: _(RuntimeOrigin::Signed(pool_account.clone()), 1, s)
 	verify {
 		// The joiners funds didn't change
-		assert_eq!(CurrencyOf::<T>::free_balance(&joiner), min_join_bond);
+		assert_eq!(CurrencyOf::<T>::balance(&joiner), min_join_bond);
 		// The unlocking chunk was removed
 		assert_eq!(pallet_staking::Ledger::<T>::get(pool_account).unwrap().unlocking.len(), 0);
 	}
@@ -426,7 +428,7 @@ frame_benchmarking::benchmarks! {
 			T::Staking::active_stake(&pool_account).unwrap(),
 			min_create_bond + min_join_bond
 		);
-		assert_eq!(CurrencyOf::<T>::free_balance(&joiner), min_join_bond);
+		assert_eq!(CurrencyOf::<T>::balance(&joiner), min_join_bond);
 
 		// Unbond the new member
 		pallet_staking::CurrentEra::<T>::put(0);
@@ -447,8 +449,7 @@ frame_benchmarking::benchmarks! {
 	}: withdraw_unbonded(RuntimeOrigin::Signed(joiner.clone()), joiner_lookup, s)
 	verify {
 		assert_eq!(
-			CurrencyOf::<T>::free_balance(&joiner),
+			CurrencyOf::<T>::balance(&joiner), min_join_bond * 2u32.into()
-			min_join_bond * 2u32.into()
 		);
 		// The unlocking chunk was removed
 		assert_eq!(pallet_staking::Ledger::<T>::get(&pool_account).unwrap().unlocking.len(), 0);
@@ -485,7 +486,7 @@ frame_benchmarking::benchmarks! {
 			Zero::zero()
 		);
 		assert_eq!(
-			CurrencyOf::<T>::free_balance(&pool_account),
+			CurrencyOf::<T>::balance(&pool_account),
 			min_create_bond
 		);
 		assert_eq!(pallet_staking::Ledger::<T>::get(&pool_account).unwrap().unlocking.len(), 1);
@@ -515,7 +516,7 @@ frame_benchmarking::benchmarks! {
 
 		// Funds where transferred back correctly
 		assert_eq!(
-			CurrencyOf::<T>::free_balance(&depositor),
+			CurrencyOf::<T>::balance(&depositor),
 			// gets bond back + rewards collecting when unbonding
 			min_create_bond * 2u32.into() + CurrencyOf::<T>::minimum_balance()
 		);
@@ -527,7 +528,7 @@ frame_benchmarking::benchmarks! {
 		let depositor_lookup = T::Lookup::unlookup(depositor.clone());
 
 		// Give the depositor some balance to bond
-		CurrencyOf::<T>::make_free_balance_be(&depositor, min_create_bond * 2u32.into());
+		CurrencyOf::<T>::set_balance(&depositor, min_create_bond * 2u32.into());
 
 		// Make sure no Pools exist at a pre-condition for our verify checks
 		assert_eq!(RewardPools::<T>::count(), 0);
@@ -782,7 +783,7 @@ frame_benchmarking::benchmarks! {
 		let ed = CurrencyOf::<T>::minimum_balance();
 		let (depositor, pool_account) = create_pool_account::<T>(0, origin_weight, Some(commission));
 		let reward_account = Pools::<T>::create_reward_account(1);
-		CurrencyOf::<T>::make_free_balance_be(&reward_account, ed + origin_weight);
+		CurrencyOf::<T>::set_balance(&reward_account, ed + origin_weight);
 
 		// member claims a payout to make some commission available.
 		let _ = Pools::<T>::claim_payout(RuntimeOrigin::Signed(claimer).into());
@@ -791,15 +792,29 @@ frame_benchmarking::benchmarks! {
 	}:_(RuntimeOrigin::Signed(depositor.clone()), 1u32.into())
 	verify {
 		assert_eq!(
-			CurrencyOf::<T>::free_balance(&depositor),
+			CurrencyOf::<T>::balance(&depositor),
 			origin_weight + commission * origin_weight
 		);
 		assert_eq!(
-			CurrencyOf::<T>::free_balance(&reward_account),
+			CurrencyOf::<T>::balance(&reward_account),
 			ed + commission * origin_weight
 		);
 	}
 
+	adjust_pool_deposit {
+		// Create a pool
+		let (depositor, _) = create_pool_account::<T>(0, Pools::<T>::depositor_min_bond() * 2u32.into(), None);
+
+		// Remove ed freeze to create a scenario where the ed deposit needs to be adjusted.
+		let _ = Pools::<T>::unfreeze_pool_deposit(&Pools::<T>::create_reward_account(1));
+		assert!(&Pools::<T>::check_ed_imbalance().is_err());
+
+		whitelist_account!(depositor);
+	}:_(RuntimeOrigin::Signed(depositor), 1)
+	verify {
+		assert!(&Pools::<T>::check_ed_imbalance().is_ok());
+	}
+
 	impl_benchmark_test_suite!(
 		Pallet,
 		crate::mock::new_test_ext(),

substrate/frame/nomination-pools/benchmarking/src/mock.rs

@@ -74,8 +74,8 @@ impl pallet_balances::Config for Runtime {
 	type ExistentialDeposit = ExistentialDeposit;
 	type AccountStore = System;
 	type WeightInfo = ();
-	type FreezeIdentifier = ();
+	type FreezeIdentifier = RuntimeFreezeReason;
-	type MaxFreezes = ();
+	type MaxFreezes = ConstU32<1>;
 	type RuntimeHoldReason = ();
 	type MaxHolds = ();
 }
@@ -160,6 +160,7 @@ impl pallet_nomination_pools::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
 	type WeightInfo = ();
 	type Currency = Balances;
+	type RuntimeFreezeReason = RuntimeFreezeReason;
 	type RewardCounter = FixedU128;
 	type BalanceToU256 = BalanceToU256;
 	type U256ToBalance = U256ToBalance;
@@ -183,7 +184,7 @@ frame_support::construct_runtime!(
 		Balances: pallet_balances::{Pallet, Call, Storage, Config<T>, Event<T>},
 		Staking: pallet_staking::{Pallet, Call, Config<T>, Storage, Event<T>},
 		VoterList: pallet_bags_list::<Instance1>::{Pallet, Call, Storage, Event<T>},
-		Pools: pallet_nomination_pools::{Pallet, Call, Storage, Event<T>},
+		Pools: pallet_nomination_pools::{Pallet, Call, Storage, Event<T>, FreezeReason},
 	}
 );
 

substrate/frame/nomination-pools/src/lib.rs

@@ -353,12 +353,16 @@
 
 use codec::Codec;
 use frame_support::{
-	defensive, ensure,
+	defensive, defensive_assert, ensure,
 	pallet_prelude::{MaxEncodedLen, *},
 	storage::bounded_btree_map::BoundedBTreeMap,
 	traits::{
-		Currency, Defensive, DefensiveOption, DefensiveResult, DefensiveSaturating,
+		fungible::{
-		ExistenceRequirement, Get,
+			Inspect as FunInspect, InspectFreeze, Mutate as FunMutate,
+			MutateFreeze as FunMutateFreeze,
+		},
+		tokens::{Fortitude, Preservation},
+		Defensive, DefensiveOption, DefensiveResult, DefensiveSaturating, Get,
 	},
 	DefaultNoBound, PalletError,
 };
@@ -380,7 +384,6 @@ use sp_runtime::TryRuntimeError;
 
 /// The log target of this pallet.
 pub const LOG_TARGET: &str = "runtime::nomination-pools";
-
 // syntactic sugar for logging.
 #[macro_export]
 macro_rules! log {
@@ -405,7 +408,7 @@ pub use weights::WeightInfo;
 
 /// The balance type used by the currency system.
 pub type BalanceOf<T> =
-	<<T as Config>::Currency as Currency<<T as frame_system::Config>::AccountId>>::Balance;
+	<<T as Config>::Currency as FunInspect<<T as frame_system::Config>::AccountId>>::Balance;
 /// Type used for unique identifier of each pool.
 pub type PoolId = u32;
 
@@ -1005,10 +1008,15 @@ impl<T: Config> BondedPool<T> {
 		self
 	}
 
-	/// The pools balance that is transferrable.
+	/// The pools balance that is transferable provided it is expendable by staking pallet.
-	fn transferrable_balance(&self) -> BalanceOf<T> {
+	fn transferable_balance(&self) -> BalanceOf<T> {
 		let account = self.bonded_account();
-		T::Currency::free_balance(&account)
+		// Note on why we can't use `Currency::reducible_balance`: Since pooled account has a
+		// provider (staking pallet), the account can not be set expendable by
+		// `pallet-nomination-pool`. This means reducible balance always returns balance preserving
+		// ED in the account. What we want though is transferable balance given the account can be
+		// dusted.
+		T::Currency::balance(&account)
 			.saturating_sub(T::Staking::active_stake(&account).unwrap_or_default())
 	}
 
@@ -1201,8 +1209,8 @@ impl<T: Config> BondedPool<T> {
 			&bonded_account,
 			amount,
 			match ty {
-				BondType::Create => ExistenceRequirement::AllowDeath,
+				BondType::Create => Preservation::Expendable,
-				BondType::Later => ExistenceRequirement::KeepAlive,
+				BondType::Later => Preservation::Preserve,
 			},
 		)?;
 		// We must calculate the points issued *before* we bond who's funds, else points:balance
@@ -1300,13 +1308,22 @@ impl<T: Config> RewardPool<T> {
 		self.total_commission_pending =
 			self.total_commission_pending.saturating_add(new_pending_commission);
 
-		// Store the total payouts at the time of this update. Total payouts are essentially the
+		// Total payouts are essentially the entire historical balance of the reward pool, equating
-		// entire historical balance of the reward pool, equating to the current balance + the total
+		// to the current balance + the total rewards that have left the pool + the total commission
-		// rewards that have left the pool + the total commission that has left the pool.
+		// that has left the pool.
-		self.last_recorded_total_payouts = balance
+		let last_recorded_total_payouts = balance
 			.checked_add(&self.total_rewards_claimed.saturating_add(self.total_commission_claimed))
 			.ok_or(Error::<T>::OverflowRisk)?;
 
+		// Store the total payouts at the time of this update.
+		//
+		// An increase in ED could cause `last_recorded_total_payouts` to decrease but we should not
+		// allow that to happen since an already paid out reward cannot decrease. The reward account
+		// might go in deficit temporarily in this exceptional case but it will be corrected once
+		// new rewards are added to the pool.
+		self.last_recorded_total_payouts =
+			self.last_recorded_total_payouts.max(last_recorded_total_payouts);
+
 		Ok(())
 	}
 
@@ -1380,8 +1397,11 @@ impl<T: Config> RewardPool<T> {
 	///
 	/// This is sum of all the rewards that are claimable by pool members.
 	fn current_balance(id: PoolId) -> BalanceOf<T> {
-		T::Currency::free_balance(&Pallet::<T>::create_reward_account(id))
+		T::Currency::reducible_balance(
-			.saturating_sub(T::Currency::minimum_balance())
+			&Pallet::<T>::create_reward_account(id),
+			Preservation::Expendable,
+			Fortitude::Polite,
+		)
 	}
 }
 
@@ -1487,6 +1507,7 @@ impl<T: Config> SubPools<T> {
 /// `no_era` pool. This is guaranteed to at least be equal to the staking `UnbondingDuration`. For
 /// improved UX [`Config::PostUnbondingPoolsWindow`] should be configured to a non-zero value.
 pub struct TotalUnbondingPools<T: Config>(PhantomData<T>);
+
 impl<T: Config> Get<u32> for TotalUnbondingPools<T> {
 	fn get() -> u32 {
 		// NOTE: this may be dangerous in the scenario bonding_duration gets decreased because
@@ -1504,7 +1525,7 @@ pub mod pallet {
 	use sp_runtime::Perbill;
 
 	/// The current storage version.
-	const STORAGE_VERSION: StorageVersion = StorageVersion::new(5);
+	const STORAGE_VERSION: StorageVersion = StorageVersion::new(6);
 
 	#[pallet::pallet]
 	#[pallet::storage_version(STORAGE_VERSION)]
@@ -1518,8 +1539,12 @@ pub mod pallet {
 		/// Weight information for extrinsics in this pallet.
 		type WeightInfo: weights::WeightInfo;
 
-		/// The nominating balance.
+		/// The currency type used for nomination pool.
-		type Currency: Currency<Self::AccountId>;
+		type Currency: FunMutate<Self::AccountId>
+			+ FunMutateFreeze<Self::AccountId, Id = Self::RuntimeFreezeReason>;
+
+		/// The overarching freeze reason.
+		type RuntimeFreezeReason: From<FreezeReason>;
 
 		/// The type that is used for reward counter.
 		///
@@ -1685,6 +1710,7 @@ pub mod pallet {
 		fn build(&self) {
 			MinJoinBond::<T>::put(self.min_join_bond);
 			MinCreateBond::<T>::put(self.min_create_bond);
+
 			if let Some(max_pools) = self.max_pools {
 				MaxPools::<T>::put(max_pools);
 			}
@@ -1770,6 +1796,10 @@ pub mod pallet {
 		},
 		/// Pool commission has been claimed.
 		PoolCommissionClaimed { pool_id: PoolId, commission: BalanceOf<T> },
+		/// Topped up deficit in frozen ED of the reward pool.
+		MinBalanceDeficitAdjusted { pool_id: PoolId, amount: BalanceOf<T> },
+		/// Claimed excess frozen ED of af the reward pool.
+		MinBalanceExcessAdjusted { pool_id: PoolId, amount: BalanceOf<T> },
 	}
 
 	#[pallet::error]
@@ -1845,6 +1875,8 @@ pub mod pallet {
 		InvalidPoolId,
 		/// Bonding extra is restricted to the exact pending reward amount.
 		BondExtraRestricted,
+		/// No imbalance in the ED deposit for the pool.
+		NothingToAdjust,
 	}
 
 	#[derive(Encode, Decode, PartialEq, TypeInfo, PalletError, RuntimeDebug)]
@@ -1868,6 +1900,14 @@ pub mod pallet {
 		}
 	}
 
+	/// A reason for freezing funds.
+	#[pallet::composite_enum]
+	pub enum FreezeReason {
+		/// Pool reward account is restricted from going below Existential Deposit.
+		#[codec(index = 0)]
+		PoolMinBalance,
+	}
+
 	#[pallet::call]
 	impl<T: Config> Pallet<T> {
 		/// Stake funds with a pool. The amount to bond is transferred from the member to the
@@ -2140,7 +2180,7 @@ pub mod pallet {
 			ensure!(!withdrawn_points.is_empty(), Error::<T>::CannotWithdrawAny);
 
 			// Before calculating the `balance_to_unbond`, we call withdraw unbonded to ensure the
-			// `transferrable_balance` is correct.
+			// `transferable_balance` is correct.
 			let stash_killed =
 				T::Staking::withdraw_unbonded(bonded_pool.bonded_account(), num_slashing_spans)?;
 
@@ -2175,13 +2215,13 @@ pub mod pallet {
 				// don't exist. This check is also defensive in cases where the unbond pool does not
 				// update its balance (e.g. a bug in the slashing hook.) We gracefully proceed in
 				// order to ensure members can leave the pool and it can be destroyed.
-				.min(bonded_pool.transferrable_balance());
+				.min(bonded_pool.transferable_balance());
 
 			T::Currency::transfer(
 				&bonded_pool.bonded_account(),
 				&member_account,
 				balance_to_unbond,
-				ExistenceRequirement::AllowDeath,
+				Preservation::Expendable,
 			)
 			.defensive()?;
 
@@ -2237,7 +2277,7 @@ pub mod pallet {
 		/// # Note
 		///
 		/// In addition to `amount`, the caller will transfer the existential deposit; so the caller
-		/// needs at have at least `amount + existential_deposit` transferrable.
+		/// needs at have at least `amount + existential_deposit` transferable.
 		#[pallet::call_index(6)]
 		#[pallet::weight(T::WeightInfo::create())]
 		pub fn create(
@@ -2631,6 +2671,20 @@ pub mod pallet {
 			let who = ensure_signed(origin)?;
 			Self::do_claim_commission(who, pool_id)
 		}
+
+		/// Top up the deficit or withdraw the excess ED from the pool.
+		///
+		/// When a pool is created, the pool depositor transfers ED to the reward account of the
+		/// pool. ED is subject to change and over time, the deposit in the reward account may be
+		/// insufficient to cover the ED deficit of the pool or vice-versa where there is excess
+		/// deposit to the pool. This call allows anyone to adjust the ED deposit of the
+		/// pool by either topping up the deficit or claiming the excess.
+		#[pallet::call_index(21)]
+		#[pallet::weight(T::WeightInfo::adjust_pool_deposit())]
+		pub fn adjust_pool_deposit(origin: OriginFor<T>, pool_id: PoolId) -> DispatchResult {
+			let who = ensure_signed(origin)?;
+			Self::do_adjust_pool_deposit(who, pool_id)
+		}
 	}
 
 	#[pallet::hooks]
@@ -2681,6 +2735,9 @@ impl<T: Config> Pallet<T> {
 		RewardPools::<T>::remove(bonded_pool.id);
 		SubPoolsStorage::<T>::remove(bonded_pool.id);
 
+		// remove the ED restriction from the pool reward account.
+		let _ = Self::unfreeze_pool_deposit(&bonded_pool.reward_account()).defensive();
+
 		// Kill accounts from storage by making their balance go below ED. We assume that the
 		// accounts have no references that would prevent destruction once we get to this point. We
 		// don't work with the system pallet directly, but
@@ -2688,26 +2745,44 @@ impl<T: Config> Pallet<T> {
 		// consumers anyway.
 		// 2. the bonded account should become a 'killed stash' in the staking system, and all of
 		//    its consumers removed.
-		debug_assert_eq!(frame_system::Pallet::<T>::consumers(&reward_account), 0);
+		defensive_assert!(
-		debug_assert_eq!(frame_system::Pallet::<T>::consumers(&bonded_account), 0);
+			frame_system::Pallet::<T>::consumers(&reward_account) == 0,
-		debug_assert_eq!(
+			"reward account of dissolving pool should have no consumers"
-			T::Staking::total_stake(&bonded_account).unwrap_or_default(),
+		);
-			Zero::zero()
+		defensive_assert!(
+			frame_system::Pallet::<T>::consumers(&bonded_account) == 0,
+			"bonded account of dissolving pool should have no consumers"
+		);
+		defensive_assert!(
+			T::Staking::total_stake(&bonded_account).unwrap_or_default() == Zero::zero(),
+			"dissolving pool should not have any stake in the staking pallet"
 		);
 
 		// This shouldn't fail, but if it does we don't really care. Remaining balance can consist
-		// of unclaimed pending commission, errorneous transfers to the reward account, etc.
+		// of unclaimed pending commission, erroneous transfers to the reward account, etc.
-		let reward_pool_remaining = T::Currency::free_balance(&reward_account);
+		let reward_pool_remaining = T::Currency::reducible_balance(
+			&reward_account,
+			Preservation::Expendable,
+			Fortitude::Polite,
+		);
 		let _ = T::Currency::transfer(
 			&reward_account,
 			&bonded_pool.roles.depositor,
 			reward_pool_remaining,
-			ExistenceRequirement::AllowDeath,
+			Preservation::Expendable,
 		);
 
-		// NOTE: this is purely defensive.
+		defensive_assert!(
-		T::Currency::make_free_balance_be(&reward_account, Zero::zero());
+			T::Currency::total_balance(&reward_account) == Zero::zero(),
-		T::Currency::make_free_balance_be(&bonded_pool.bonded_account(), Zero::zero());
+			"could not transfer all amount to depositor while dissolving pool"
+		);
+		defensive_assert!(
+			T::Currency::total_balance(&bonded_pool.bonded_account()) == Zero::zero(),
+			"dissolving pool should not have any balance"
+		);
+		// NOTE: Defensively force set balance to zero.
+		T::Currency::set_balance(&reward_account, Zero::zero());
+		T::Currency::set_balance(&bonded_pool.bonded_account(), Zero::zero());
 
 		Self::deposit_event(Event::<T>::Destroyed { pool_id: bonded_pool.id });
 		// Remove bonded pool metadata.
@@ -2838,7 +2913,7 @@ impl<T: Config> Pallet<T> {
 			pending_rewards,
 			// defensive: the depositor has put existential deposit into the pool and it stays
 			// untouched, reward account shall not die.
-			ExistenceRequirement::KeepAlive,
+			Preservation::Preserve,
 		)?;
 
 		Self::deposit_event(Event::<T>::PaidOut {
@@ -2846,7 +2921,6 @@ impl<T: Config> Pallet<T> {
 			pool_id: member.pool_id,
 			payout: pending_rewards,
 		});
-
 		Ok(pending_rewards)
 	}
 
@@ -2881,13 +2955,17 @@ impl<T: Config> Pallet<T> {
 		bonded_pool.try_inc_members()?;
 		let points = bonded_pool.try_bond_funds(&who, amount, BondType::Create)?;
 
+		// Transfer the minimum balance for the reward account.
 		T::Currency::transfer(
 			&who,
 			&bonded_pool.reward_account(),
 			T::Currency::minimum_balance(),
-			ExistenceRequirement::AllowDeath,
+			Preservation::Expendable,
 		)?;
 
+		// Restrict reward account balance from going below ED.
+		Self::freeze_pool_deposit(&bonded_pool.reward_account())?;
+
 		PoolMembers::<T>::insert(
 			who.clone(),
 			PoolMember::<T> {
@@ -2999,7 +3077,7 @@ impl<T: Config> Pallet<T> {
 			&bonded_pool.reward_account(),
 			&payee,
 			commission,
-			ExistenceRequirement::KeepAlive,
+			Preservation::Preserve,
 		)?;
 
 		// Add pending commission to total claimed counter.
@@ -3007,7 +3085,6 @@ impl<T: Config> Pallet<T> {
 			reward_pool.total_commission_claimed.saturating_add(commission);
 		// Reset total pending commission counter to zero.
 		reward_pool.total_commission_pending = Zero::zero();
-		// Commit reward pool updates
 		RewardPools::<T>::insert(pool_id, reward_pool);
 
 		Self::deposit_event(Event::<T>::PoolCommissionClaimed { pool_id, commission });
@@ -3029,6 +3106,55 @@ impl<T: Config> Pallet<T> {
 		Ok(())
 	}
 
+	fn do_adjust_pool_deposit(who: T::AccountId, pool: PoolId) -> DispatchResult {
+		let bonded_pool = BondedPool::<T>::get(pool).ok_or(Error::<T>::PoolNotFound)?;
+		let reward_acc = &bonded_pool.reward_account();
+		let pre_frozen_balance =
+			T::Currency::balance_frozen(&FreezeReason::PoolMinBalance.into(), reward_acc);
+		let min_balance = T::Currency::minimum_balance();
+
+		if pre_frozen_balance == min_balance {
+			return Err(Error::<T>::NothingToAdjust.into())
+		}
+
+		// Update frozen amount with current ED.
+		Self::freeze_pool_deposit(reward_acc)?;
+
+		if pre_frozen_balance > min_balance {
+			// Transfer excess back to depositor.
+			let excess = pre_frozen_balance.saturating_sub(min_balance);
+			T::Currency::transfer(reward_acc, &who, excess, Preservation::Preserve)?;
+			Self::deposit_event(Event::<T>::MinBalanceExcessAdjusted {
+				pool_id: pool,
+				amount: excess,
+			});
+		} else {
+			// Transfer ED deficit from depositor to the pool
+			let deficit = min_balance.saturating_sub(pre_frozen_balance);
+			T::Currency::transfer(&who, reward_acc, deficit, Preservation::Expendable)?;
+			Self::deposit_event(Event::<T>::MinBalanceDeficitAdjusted {
+				pool_id: pool,
+				amount: deficit,
+			});
+		}
+
+		Ok(())
+	}
+
+	/// Apply freeze on reward account to restrict it from going below ED.
+	pub(crate) fn freeze_pool_deposit(reward_acc: &T::AccountId) -> DispatchResult {
+		T::Currency::set_freeze(
+			&FreezeReason::PoolMinBalance.into(),
+			reward_acc,
+			T::Currency::minimum_balance(),
+		)
+	}
+
+	/// Removes the ED freeze on the reward account of `pool_id`.
+	pub fn unfreeze_pool_deposit(reward_acc: &T::AccountId) -> DispatchResult {
+		T::Currency::thaw(&FreezeReason::PoolMinBalance.into(), reward_acc)
+	}
+
 	/// Ensure the correctness of the state of this pallet.
 	///
 	/// This should be valid before or after each state transition of this pallet.
@@ -3094,14 +3220,20 @@ impl<T: Config> Pallet<T> {
 
 		for id in reward_pools {
 			let account = Self::create_reward_account(id);
-			if T::Currency::free_balance(&account) < T::Currency::minimum_balance() {
+			if T::Currency::reducible_balance(&account, Preservation::Expendable, Fortitude::Polite) <
+				T::Currency::minimum_balance()
+			{
 				log!(
 					warn,
 					"reward pool of {:?}: {:?} (ed = {:?}), should only happen because ED has \
 					changed recently. Pool operators should be notified to top up the reward \
 					account",
 					id,
-					T::Currency::free_balance(&account),
+					T::Currency::reducible_balance(
+						&account,
+						Preservation::Expendable,
+						Fortitude::Polite
+					),
 					T::Currency::minimum_balance(),
 				)
 			}
@@ -3135,9 +3267,8 @@ impl<T: Config> Pallet<T> {
 			let pending_rewards_lt_leftover_bal = RewardPool::<T>::current_balance(id) >=
 				pools_members_pending_rewards.get(&id).copied().unwrap_or_default();
 
-			// this is currently broken in Kusama, a fix is being worked on in
+			// If this happens, this is most likely due to an old bug and not a recent code change.
-			// <https://github.com/paritytech/polkadot-sdk/pull/1255>. until it is fixed, log a
+			// We warn about this in try-runtime checks but do not panic.
-			// warning instead of panicing with an `ensure` statement.
 			if !pending_rewards_lt_leftover_bal {
 				log::warn!(
 					"pool {:?}, sum pending rewards = {:?}, remaining balance = {:?}",
@@ -3199,9 +3330,39 @@ impl<T: Config> Pallet<T> {
 			);
 		}
 
+		// Warn if any pool has incorrect ED frozen. We don't want to fail hard as this could be a
+		// result of an intentional ED change.
+		let _ = Self::check_ed_imbalance()?;
+
 		Ok(())
 	}
 
+	/// Check if any pool have an incorrect amount of ED frozen.
+	///
+	/// This can happen if the ED has changed since the pool was created.
+	#[cfg(any(feature = "try-runtime", feature = "runtime-benchmarks", test, debug_assertions))]
+	pub fn check_ed_imbalance() -> Result<(), DispatchError> {
+		let mut failed: u32 = 0;
+		BondedPools::<T>::iter_keys().for_each(|id| {
+			let reward_acc = Self::create_reward_account(id);
+			let frozen_balance =
+				T::Currency::balance_frozen(&FreezeReason::PoolMinBalance.into(), &reward_acc);
+
+			let expected_frozen_balance = T::Currency::minimum_balance();
+			if frozen_balance != expected_frozen_balance {
+				failed += 1;
+				log::warn!(
+					"pool {:?} has incorrect ED frozen that can result from change in ED. Expected  = {:?},  Actual = {:?}",
+					id,
+					expected_frozen_balance,
+					frozen_balance,
+				);
+			}
+		});
+
+		ensure!(failed == 0, "Some pools do not have correct ED frozen");
+		Ok(())
+	}
 	/// Fully unbond the shares of `member`, when executed from `origin`.
 	///
 	/// This is useful for backwards compatibility with the majority of tests that only deal with

substrate/frame/nomination-pools/src/migration.rs

@@ -23,55 +23,95 @@ use sp_std::{collections::btree_map::BTreeMap, vec::Vec};
 #[cfg(feature = "try-runtime")]
 use sp_runtime::TryRuntimeError;
 
-pub mod v1 {
+/// Exports for versioned migration `type`s for this pallet.
+pub mod versioned_migrations {
+	use super::*;
+
+	/// Wrapper over `MigrateToV6` with convenience version checks.
+	pub type V5toV6<T> = frame_support::migrations::VersionedMigration<
+		5,
+		6,
+		v6::MigrateToV6<T>,
+		crate::pallet::Pallet<T>,
+		<T as frame_system::Config>::DbWeight,
+	>;
+}
+
+mod v6 {
+	use super::*;
+
+	/// This migration would restrict reward account of pools to go below ED by doing a named
+	/// freeze on all the existing pools.
+	pub struct MigrateToV6<T>(sp_std::marker::PhantomData<T>);
+
+	impl<T: Config> MigrateToV6<T> {
+		fn freeze_ed(pool_id: PoolId) -> Result<(), ()> {
+			let reward_acc = Pallet::<T>::create_reward_account(pool_id);
+			Pallet::<T>::freeze_pool_deposit(&reward_acc).map_err(|e| {
+				log!(error, "Failed to freeze ED for pool {} with error: {:?}", pool_id, e);
+				()
+			})
+		}
+	}
+	impl<T: Config> OnRuntimeUpgrade for MigrateToV6<T> {
+		fn on_runtime_upgrade() -> Weight {
+			let mut success = 0u64;
+			let mut fail = 0u64;
+
+			BondedPools::<T>::iter_keys().for_each(|p| {
+				if Self::freeze_ed(p).is_ok() {
+					success.saturating_inc();
+				} else {
+					fail.saturating_inc();
+				}
+			});
+
+			if fail > 0 {
+				log!(error, "Failed to freeze ED for {} pools", fail);
+			} else {
+				log!(info, "Freezing ED succeeded for {} pools", success);
+			}
+
+			let total = success.saturating_add(fail);
+			// freeze_ed = r:2 w:2
+			// reads: (freeze_ed + bonded pool key) * total
+			// writes: freeze_ed * total
+			T::DbWeight::get().reads_writes(3u64.saturating_mul(total), 2u64.saturating_mul(total))
+		}
+
+		#[cfg(feature = "try-runtime")]
+		fn post_upgrade(_data: Vec<u8>) -> Result<(), TryRuntimeError> {
+			// there should be no ED imbalances anymore..
+			Pallet::<T>::check_ed_imbalance()
+		}
+	}
+}
+pub mod v5 {
 	use super::*;
 
 	#[derive(Decode)]
-	pub struct OldPoolRoles<AccountId> {
+	pub struct OldRewardPool<T: Config> {
-		pub depositor: AccountId,
+		last_recorded_reward_counter: T::RewardCounter,
-		pub root: AccountId,
+		last_recorded_total_payouts: BalanceOf<T>,
-		pub nominator: AccountId,
+		total_rewards_claimed: BalanceOf<T>,
-		pub bouncer: AccountId,
 	}
 
-	impl<AccountId> OldPoolRoles<AccountId> {
+	impl<T: Config> OldRewardPool<T> {
-		fn migrate_to_v1(self) -> PoolRoles<AccountId> {
+		fn migrate_to_v5(self) -> RewardPool<T> {
-			PoolRoles {
+			RewardPool {
-				depositor: self.depositor,
+				last_recorded_reward_counter: self.last_recorded_reward_counter,
-				root: Some(self.root),
+				last_recorded_total_payouts: self.last_recorded_total_payouts,
-				nominator: Some(self.nominator),
+				total_rewards_claimed: self.total_rewards_claimed,
-				bouncer: Some(self.bouncer),
+				total_commission_pending: Zero::zero(),
+				total_commission_claimed: Zero::zero(),
 			}
 		}
 	}
 
-	#[derive(Decode)]
+	/// This migration adds `total_commission_pending` and `total_commission_claimed` field to every
-	pub struct OldBondedPoolInner<T: Config> {
+	/// `RewardPool`, if any.
-		pub points: BalanceOf<T>,
+	pub struct MigrateToV5<T>(sp_std::marker::PhantomData<T>);
-		pub state: PoolState,
+	impl<T: Config> OnRuntimeUpgrade for MigrateToV5<T> {
-		pub member_counter: u32,
-		pub roles: OldPoolRoles<T::AccountId>,
-	}
-
-	impl<T: Config> OldBondedPoolInner<T> {
-		fn migrate_to_v1(self) -> BondedPoolInner<T> {
-			// Note: `commission` field not introduced to `BondedPoolInner` until
-			// migration 4.
-			BondedPoolInner {
-				points: self.points,
-				commission: Commission::default(),
-				member_counter: self.member_counter,
-				state: self.state,
-				roles: self.roles.migrate_to_v1(),
-			}
-		}
-	}
-
-	/// Trivial migration which makes the roles of each pool optional.
-	///
-	/// Note: The depositor is not optional since they can never change.
-	pub struct MigrateToV1<T>(sp_std::marker::PhantomData<T>);
-	impl<T: Config> OnRuntimeUpgrade for MigrateToV1<T> {
 		fn on_runtime_upgrade() -> Weight {
 			let current = Pallet::<T>::current_storage_version();
 			let onchain = Pallet::<T>::on_chain_storage_version();
@@ -83,33 +123,284 @@ pub mod v1 {
 				onchain
 			);
 
-			if current == 1 && onchain == 0 {
+			if current == 5 && onchain == 4 {
-				// this is safe to execute on any runtime that has a bounded number of pools.
 				let mut translated = 0u64;
-				BondedPools::<T>::translate::<OldBondedPoolInner<T>, _>(|_key, old_value| {
+				RewardPools::<T>::translate::<OldRewardPool<T>, _>(|_id, old_value| {
 					translated.saturating_inc();
-					Some(old_value.migrate_to_v1())
+					Some(old_value.migrate_to_v5())
 				});
 
 				current.put::<Pallet<T>>();
-
 				log!(info, "Upgraded {} pools, storage to version {:?}", translated, current);
 
+				// reads: translated + onchain version.
+				// writes: translated + current.put.
 				T::DbWeight::get().reads_writes(translated + 1, translated + 1)
 			} else {
-				log!(info, "Migration did not executed. This probably should be removed");
+				log!(info, "Migration did not execute. This probably should be removed");
 				T::DbWeight::get().reads(1)
 			}
 		}
 
 		#[cfg(feature = "try-runtime")]
-		fn post_upgrade(_: Vec<u8>) -> Result<(), TryRuntimeError> {
+		fn pre_upgrade() -> Result<Vec<u8>, TryRuntimeError> {
-			// new version must be set.
+			let rpool_keys = RewardPools::<T>::iter_keys().count();
+			let rpool_values = RewardPools::<T>::iter_values().count();
+			if rpool_keys != rpool_values {
+				log!(info, "🔥 There are {} undecodable RewardPools in storage. This migration will try to correct them. keys: {}, values: {}", rpool_keys.saturating_sub(rpool_values), rpool_keys, rpool_values);
+			}
+
 			ensure!(
-				Pallet::<T>::on_chain_storage_version() == 1,
+				PoolMembers::<T>::iter_keys().count() == PoolMembers::<T>::iter_values().count(),
-				"The onchain version must be updated after the migration."
+				"There are undecodable PoolMembers in storage. This migration will not fix that."
+			);
+			ensure!(
+				BondedPools::<T>::iter_keys().count() == BondedPools::<T>::iter_values().count(),
+				"There are undecodable BondedPools in storage. This migration will not fix that."
+			);
+			ensure!(
+				SubPoolsStorage::<T>::iter_keys().count() ==
+					SubPoolsStorage::<T>::iter_values().count(),
+				"There are undecodable SubPools in storage. This migration will not fix that."
+			);
+			ensure!(
+				Metadata::<T>::iter_keys().count() == Metadata::<T>::iter_values().count(),
+				"There are undecodable Metadata in storage. This migration will not fix that."
+			);
+
+			Ok((rpool_values as u64).encode())
+		}
+
+		#[cfg(feature = "try-runtime")]
+		fn post_upgrade(data: Vec<u8>) -> Result<(), TryRuntimeError> {
+			let old_rpool_values: u64 = Decode::decode(&mut &data[..]).unwrap();
+			let rpool_keys = RewardPools::<T>::iter_keys().count() as u64;
+			let rpool_values = RewardPools::<T>::iter_values().count() as u64;
+			ensure!(
+				rpool_keys == rpool_values,
+				"There are STILL undecodable RewardPools - migration failed"
+			);
+
+			if old_rpool_values != rpool_values {
+				log!(
+					info,
+					"🎉 Fixed {} undecodable RewardPools.",
+					rpool_values.saturating_sub(old_rpool_values)
+				);
+			}
+
+			// ensure all RewardPools items now contain `total_commission_pending` and
+			// `total_commission_claimed` field.
+			ensure!(
+				RewardPools::<T>::iter().all(|(_, reward_pool)| reward_pool
+					.total_commission_pending >=
+					Zero::zero() && reward_pool
+					.total_commission_claimed >=
+					Zero::zero()),
+				"a commission value has been incorrectly set"
+			);
+			ensure!(
+				Pallet::<T>::on_chain_storage_version() >= 5,
+				"nomination-pools::migration::v5: wrong storage version"
+			);
+
+			// These should not have been touched - just in case.
+			ensure!(
+				PoolMembers::<T>::iter_keys().count() == PoolMembers::<T>::iter_values().count(),
+				"There are undecodable PoolMembers in storage."
+			);
+			ensure!(
+				BondedPools::<T>::iter_keys().count() == BondedPools::<T>::iter_values().count(),
+				"There are undecodable BondedPools in storage."
+			);
+			ensure!(
+				SubPoolsStorage::<T>::iter_keys().count() ==
+					SubPoolsStorage::<T>::iter_values().count(),
+				"There are undecodable SubPools in storage."
+			);
+			ensure!(
+				Metadata::<T>::iter_keys().count() == Metadata::<T>::iter_values().count(),
+				"There are undecodable Metadata in storage."
+			);
+
+			Ok(())
+		}
+	}
+}
+
+pub mod v4 {
+	use super::*;
+
+	#[derive(Decode)]
+	pub struct OldBondedPoolInner<T: Config> {
+		pub points: BalanceOf<T>,
+		pub state: PoolState,
+		pub member_counter: u32,
+		pub roles: PoolRoles<T::AccountId>,
+	}
+
+	impl<T: Config> OldBondedPoolInner<T> {
+		fn migrate_to_v4(self) -> BondedPoolInner<T> {
+			BondedPoolInner {
+				commission: Commission::default(),
+				member_counter: self.member_counter,
+				points: self.points,
+				state: self.state,
+				roles: self.roles,
+			}
+		}
+	}
+
+	/// Migrates from `v3` directly to `v5` to avoid the broken `v4` migration.
+	#[allow(deprecated)]
+	pub type MigrateV3ToV5<T, U> = (v4::MigrateToV4<T, U>, v5::MigrateToV5<T>);
+
+	/// # Warning
+	///
+	/// To avoid mangled storage please use `MigrateV3ToV5` instead.
+	/// See: github.com/paritytech/substrate/pull/13715
+	///
+	/// This migration adds a `commission` field to every `BondedPoolInner`, if
+	/// any.
+	#[deprecated(
+		note = "To avoid mangled storage please use `MigrateV3ToV5` instead. See: github.com/paritytech/substrate/pull/13715"
+	)]
+	pub struct MigrateToV4<T, U>(sp_std::marker::PhantomData<(T, U)>);
+	#[allow(deprecated)]
+	impl<T: Config, U: Get<Perbill>> OnRuntimeUpgrade for MigrateToV4<T, U> {
+		fn on_runtime_upgrade() -> Weight {
+			let current = Pallet::<T>::current_storage_version();
+			let onchain = Pallet::<T>::on_chain_storage_version();
+
+			log!(
+				info,
+				"Running migration with current storage version {:?} / onchain {:?}",
+				current,
+				onchain
+			);
+
+			if onchain == 3 {
+				log!(warn, "Please run MigrateToV5 immediately after this migration. See github.com/paritytech/substrate/pull/13715");
+				let initial_global_max_commission = U::get();
+				GlobalMaxCommission::<T>::set(Some(initial_global_max_commission));
+				log!(
+					info,
+					"Set initial global max commission to {:?}.",
+					initial_global_max_commission
+				);
+
+				let mut translated = 0u64;
+				BondedPools::<T>::translate::<OldBondedPoolInner<T>, _>(|_key, old_value| {
+					translated.saturating_inc();
+					Some(old_value.migrate_to_v4())
+				});
+
+				StorageVersion::new(4).put::<Pallet<T>>();
+				log!(info, "Upgraded {} pools, storage to version {:?}", translated, current);
+
+				// reads: translated + onchain version.
+				// writes: translated + current.put + initial global commission.
+				T::DbWeight::get().reads_writes(translated + 1, translated + 2)
+			} else {
+				log!(info, "Migration did not execute. This probably should be removed");
+				T::DbWeight::get().reads(1)
+			}
+		}
+
+		#[cfg(feature = "try-runtime")]
+		fn pre_upgrade() -> Result<Vec<u8>, TryRuntimeError> {
+			Ok(Vec::new())
+		}
+
+		#[cfg(feature = "try-runtime")]
+		fn post_upgrade(_: Vec<u8>) -> Result<(), TryRuntimeError> {
+			// ensure all BondedPools items now contain an `inner.commission: Commission` field.
+			ensure!(
+				BondedPools::<T>::iter().all(|(_, inner)|
+					// Check current
+					(inner.commission.current.is_none() ||
+					inner.commission.current.is_some()) &&
+					// Check max
+					(inner.commission.max.is_none() || inner.commission.max.is_some()) &&
+					// Check change_rate
+					(inner.commission.change_rate.is_none() ||
+					inner.commission.change_rate.is_some()) &&
+					// Check throttle_from
+					(inner.commission.throttle_from.is_none() ||
+					inner.commission.throttle_from.is_some())),
+				"a commission value has not been set correctly"
+			);
+			ensure!(
+				GlobalMaxCommission::<T>::get() == Some(U::get()),
+				"global maximum commission error"
+			);
+			ensure!(
+				Pallet::<T>::on_chain_storage_version() >= 4,
+				"nomination-pools::migration::v4: wrong storage version"
+			);
+			Ok(())
+		}
+	}
+}
+
+pub mod v3 {
+	use super::*;
+
+	/// This migration removes stale bonded-pool metadata, if any.
+	pub struct MigrateToV3<T>(sp_std::marker::PhantomData<T>);
+	impl<T: Config> OnRuntimeUpgrade for MigrateToV3<T> {
+		fn on_runtime_upgrade() -> Weight {
+			let current = Pallet::<T>::current_storage_version();
+			let onchain = Pallet::<T>::on_chain_storage_version();
+
+			if onchain == 2 {
+				log!(
+					info,
+					"Running migration with current storage version {:?} / onchain {:?}",
+					current,
+					onchain
+				);
+
+				let mut metadata_iterated = 0u64;
+				let mut metadata_removed = 0u64;
+				Metadata::<T>::iter_keys()
+					.filter(|id| {
+						metadata_iterated += 1;
+						!BondedPools::<T>::contains_key(&id)
+					})
+					.collect::<Vec<_>>()
+					.into_iter()
+					.for_each(|id| {
+						metadata_removed += 1;
+						Metadata::<T>::remove(&id);
+					});
+				StorageVersion::new(3).put::<Pallet<T>>();
+				// metadata iterated + bonded pools read + a storage version read
+				let total_reads = metadata_iterated * 2 + 1;
+				// metadata removed + a storage version write
+				let total_writes = metadata_removed + 1;
+				T::DbWeight::get().reads_writes(total_reads, total_writes)
+			} else {
+				log!(info, "MigrateToV3 should be removed");
+				T::DbWeight::get().reads(1)
+			}
+		}
+
+		#[cfg(feature = "try-runtime")]
+		fn pre_upgrade() -> Result<Vec<u8>, TryRuntimeError> {
+			Ok(Vec::new())
+		}
+
+		#[cfg(feature = "try-runtime")]
+		fn post_upgrade(_: Vec<u8>) -> Result<(), TryRuntimeError> {
+			ensure!(
+				Metadata::<T>::iter_keys().all(|id| BondedPools::<T>::contains_key(&id)),
+				"not all of the stale metadata has been removed"
+			);
+			ensure!(
+				Pallet::<T>::on_chain_storage_version() >= 3,
+				"nomination-pools::migration::v3: wrong storage version"
 			);
-			Pallet::<T>::try_state(frame_system::Pallet::<T>::block_number())?;
 			Ok(())
 		}
 	}
@@ -127,7 +418,7 @@ pub mod v2 {
 		use crate::mock::*;
 		ExtBuilder::default().build_and_execute(|| {
 			let join = |x| {
-				Balances::make_free_balance_be(&x, Balances::minimum_balance() + 10);
+				Currency::set_balance(&x, Balances::minimum_balance() + 10);
 				frame_support::assert_ok!(Pools::join(RuntimeOrigin::signed(x), 10, 1));
 			};
 
@@ -279,7 +570,7 @@ pub mod v2 {
 								&reward_account,
 								&who,
 								last_claim,
-								ExistenceRequirement::KeepAlive,
+								Preservation::Preserve,
 							);
 
 							if let Err(reason) = outcome {
@@ -304,7 +595,7 @@ pub mod v2 {
 							&reward_account,
 							&bonded_pool.roles.depositor,
 							leftover,
-							ExistenceRequirement::KeepAlive,
+							Preservation::Preserve,
 						);
 						log!(warn, "paying {:?} leftover to the depositor: {:?}", leftover, o);
 					}
@@ -362,7 +653,7 @@ pub mod v2 {
 			// all reward accounts must have more than ED.
 			RewardPools::<T>::iter().try_for_each(|(id, _)| -> Result<(), TryRuntimeError> {
 				ensure!(
-					T::Currency::free_balance(&Pallet::<T>::create_reward_account(id)) >=
+					<T::Currency as frame_support::traits::fungible::Inspect<T::AccountId>>::balance(&Pallet::<T>::create_reward_account(id)) >=
 						T::Currency::minimum_balance(),
 					"Reward accounts must have greater balance than ED."
 				);
@@ -406,109 +697,55 @@ pub mod v2 {
 	}
 }
 
-pub mod v3 {
+pub mod v1 {
 	use super::*;
 
-	/// This migration removes stale bonded-pool metadata, if any.
+	#[derive(Decode)]
-	pub struct MigrateToV3<T>(sp_std::marker::PhantomData<T>);
+	pub struct OldPoolRoles<AccountId> {
-	impl<T: Config> OnRuntimeUpgrade for MigrateToV3<T> {
+		pub depositor: AccountId,
-		fn on_runtime_upgrade() -> Weight {
+		pub root: AccountId,
-			let current = Pallet::<T>::current_storage_version();
+		pub nominator: AccountId,
-			let onchain = Pallet::<T>::on_chain_storage_version();
+		pub bouncer: AccountId,
+	}
 
-			if onchain == 2 {
+	impl<AccountId> OldPoolRoles<AccountId> {
-				log!(
+		fn migrate_to_v1(self) -> PoolRoles<AccountId> {
-					info,
+			PoolRoles {
-					"Running migration with current storage version {:?} / onchain {:?}",
+				depositor: self.depositor,
-					current,
+				root: Some(self.root),
-					onchain
+				nominator: Some(self.nominator),
-				);
+				bouncer: Some(self.bouncer),
-
-				let mut metadata_iterated = 0u64;
-				let mut metadata_removed = 0u64;
-				Metadata::<T>::iter_keys()
-					.filter(|id| {
-						metadata_iterated += 1;
-						!BondedPools::<T>::contains_key(&id)
-					})
-					.collect::<Vec<_>>()
-					.into_iter()
-					.for_each(|id| {
-						metadata_removed += 1;
-						Metadata::<T>::remove(&id);
-					});
-				StorageVersion::new(3).put::<Pallet<T>>();
-				// metadata iterated + bonded pools read + a storage version read
-				let total_reads = metadata_iterated * 2 + 1;
-				// metadata removed + a storage version write
-				let total_writes = metadata_removed + 1;
-				T::DbWeight::get().reads_writes(total_reads, total_writes)
-			} else {
-				log!(info, "MigrateToV3 should be removed");
-				T::DbWeight::get().reads(1)
 			}
 		}
-
-		#[cfg(feature = "try-runtime")]
-		fn pre_upgrade() -> Result<Vec<u8>, TryRuntimeError> {
-			Ok(Vec::new())
-		}
-
-		#[cfg(feature = "try-runtime")]
-		fn post_upgrade(_: Vec<u8>) -> Result<(), TryRuntimeError> {
-			ensure!(
-				Metadata::<T>::iter_keys().all(|id| BondedPools::<T>::contains_key(&id)),
-				"not all of the stale metadata has been removed"
-			);
-			ensure!(
-				Pallet::<T>::on_chain_storage_version() >= 3,
-				"nomination-pools::migration::v3: wrong storage version"
-			);
-			Ok(())
-		}
 	}
-}
-
-pub mod v4 {
-	use super::*;
 
 	#[derive(Decode)]
 	pub struct OldBondedPoolInner<T: Config> {
 		pub points: BalanceOf<T>,
 		pub state: PoolState,
 		pub member_counter: u32,
-		pub roles: PoolRoles<T::AccountId>,
+		pub roles: OldPoolRoles<T::AccountId>,
 	}
 
 	impl<T: Config> OldBondedPoolInner<T> {
-		fn migrate_to_v4(self) -> BondedPoolInner<T> {
+		fn migrate_to_v1(self) -> BondedPoolInner<T> {
+			// Note: `commission` field not introduced to `BondedPoolInner` until
+			// migration 4.
 			BondedPoolInner {
+				points: self.points,
 				commission: Commission::default(),
 				member_counter: self.member_counter,
-				points: self.points,
 				state: self.state,
-				roles: self.roles,
+				roles: self.roles.migrate_to_v1(),
 			}
 		}
 	}
 
-	/// Migrates from `v3` directly to `v5` to avoid the broken `v4` migration.
+	/// Trivial migration which makes the roles of each pool optional.
-	#[allow(deprecated)]
-	pub type MigrateV3ToV5<T, U> = (v4::MigrateToV4<T, U>, v5::MigrateToV5<T>);
-
-	/// # Warning
 	///
-	/// To avoid mangled storage please use `MigrateV3ToV5` instead.
+	/// Note: The depositor is not optional since they can never change.
-	/// See: github.com/paritytech/substrate/pull/13715
+	pub struct MigrateToV1<T>(sp_std::marker::PhantomData<T>);
-	///
+	impl<T: Config> OnRuntimeUpgrade for MigrateToV1<T> {
-	/// This migration adds a `commission` field to every `BondedPoolInner`, if
-	/// any.
-	#[deprecated(
-		note = "To avoid mangled storage please use `MigrateV3ToV5` instead. See: github.com/paritytech/substrate/pull/13715"
-	)]
-	pub struct MigrateToV4<T, U>(sp_std::marker::PhantomData<(T, U)>);
-	#[allow(deprecated)]
-	impl<T: Config, U: Get<Perbill>> OnRuntimeUpgrade for MigrateToV4<T, U> {
 		fn on_runtime_upgrade() -> Weight {
 			let current = Pallet::<T>::current_storage_version();
 			let onchain = Pallet::<T>::on_chain_storage_version();
@@ -520,207 +757,33 @@ pub mod v4 {
 				onchain
 			);
 
-			if onchain == 3 {
+			if current == 1 && onchain == 0 {
-				log!(warn, "Please run MigrateToV5 immediately after this migration. See github.com/paritytech/substrate/pull/13715");
+				// this is safe to execute on any runtime that has a bounded number of pools.
-				let initial_global_max_commission = U::get();
-				GlobalMaxCommission::<T>::set(Some(initial_global_max_commission));
-				log!(
-					info,
-					"Set initial global max commission to {:?}.",
-					initial_global_max_commission
-				);
-
 				let mut translated = 0u64;
 				BondedPools::<T>::translate::<OldBondedPoolInner<T>, _>(|_key, old_value| {
 					translated.saturating_inc();
-					Some(old_value.migrate_to_v4())
+					Some(old_value.migrate_to_v1())
 				});
 
-				StorageVersion::new(4).put::<Pallet<T>>();
+				current.put::<Pallet<T>>();
+
 				log!(info, "Upgraded {} pools, storage to version {:?}", translated, current);
 
-				// reads: translated + onchain version.
+				T::DbWeight::get().reads_writes(translated + 1, translated + 1)
-				// writes: translated + current.put + initial global commission.
-				T::DbWeight::get().reads_writes(translated + 1, translated + 2)
 			} else {
-				log!(info, "Migration did not execute. This probably should be removed");
+				log!(info, "Migration did not executed. This probably should be removed");
 				T::DbWeight::get().reads(1)
 			}
 		}
 
-		#[cfg(feature = "try-runtime")]
-		fn pre_upgrade() -> Result<Vec<u8>, TryRuntimeError> {
-			Ok(Vec::new())
-		}
-
 		#[cfg(feature = "try-runtime")]
 		fn post_upgrade(_: Vec<u8>) -> Result<(), TryRuntimeError> {
-			// ensure all BondedPools items now contain an `inner.commission: Commission` field.
+			// new version must be set.
 			ensure!(
-				BondedPools::<T>::iter().all(|(_, inner)|
+				Pallet::<T>::on_chain_storage_version() == 1,
-					// Check current
+				"The onchain version must be updated after the migration."
-					(inner.commission.current.is_none() ||
-					inner.commission.current.is_some()) &&
-					// Check max
-					(inner.commission.max.is_none() || inner.commission.max.is_some()) &&
-					// Check change_rate
-					(inner.commission.change_rate.is_none() ||
-					inner.commission.change_rate.is_some()) &&
-					// Check throttle_from
-					(inner.commission.throttle_from.is_none() ||
-					inner.commission.throttle_from.is_some())),
-				"a commission value has not been set correctly"
-			);
-			ensure!(
-				GlobalMaxCommission::<T>::get() == Some(U::get()),
-				"global maximum commission error"
-			);
-			ensure!(
-				Pallet::<T>::on_chain_storage_version() >= 4,
-				"nomination-pools::migration::v4: wrong storage version"
 			);
-			Ok(())
+			Pallet::<T>::try_state(frame_system::Pallet::<T>::block_number())?;
-		}
-	}
-}
-
-pub mod v5 {
-	use super::*;
-
-	#[derive(Decode)]
-	pub struct OldRewardPool<T: Config> {
-		last_recorded_reward_counter: T::RewardCounter,
-		last_recorded_total_payouts: BalanceOf<T>,
-		total_rewards_claimed: BalanceOf<T>,
-	}
-
-	impl<T: Config> OldRewardPool<T> {
-		fn migrate_to_v5(self) -> RewardPool<T> {
-			RewardPool {
-				last_recorded_reward_counter: self.last_recorded_reward_counter,
-				last_recorded_total_payouts: self.last_recorded_total_payouts,
-				total_rewards_claimed: self.total_rewards_claimed,
-				total_commission_pending: Zero::zero(),
-				total_commission_claimed: Zero::zero(),
-			}
-		}
-	}
-
-	/// This migration adds `total_commission_pending` and `total_commission_claimed` field to every
-	/// `RewardPool`, if any.
-	pub struct MigrateToV5<T>(sp_std::marker::PhantomData<T>);
-	impl<T: Config> OnRuntimeUpgrade for MigrateToV5<T> {
-		fn on_runtime_upgrade() -> Weight {
-			let current = Pallet::<T>::current_storage_version();
-			let onchain = Pallet::<T>::on_chain_storage_version();
-
-			log!(
-				info,
-				"Running migration with current storage version {:?} / onchain {:?}",
-				current,
-				onchain
-			);
-
-			if current == 5 && onchain == 4 {
-				let mut translated = 0u64;
-				RewardPools::<T>::translate::<OldRewardPool<T>, _>(|_id, old_value| {
-					translated.saturating_inc();
-					Some(old_value.migrate_to_v5())
-				});
-
-				current.put::<Pallet<T>>();
-				log!(info, "Upgraded {} pools, storage to version {:?}", translated, current);
-
-				// reads: translated + onchain version.
-				// writes: translated + current.put.
-				T::DbWeight::get().reads_writes(translated + 1, translated + 1)
-			} else {
-				log!(info, "Migration did not execute. This probably should be removed");
-				T::DbWeight::get().reads(1)
-			}
-		}
-
-		#[cfg(feature = "try-runtime")]
-		fn pre_upgrade() -> Result<Vec<u8>, TryRuntimeError> {
-			let rpool_keys = RewardPools::<T>::iter_keys().count();
-			let rpool_values = RewardPools::<T>::iter_values().count();
-			if rpool_keys != rpool_values {
-				log!(info, "🔥 There are {} undecodable RewardPools in storage. This migration will try to correct them. keys: {}, values: {}", rpool_keys.saturating_sub(rpool_values), rpool_keys, rpool_values);
-			}
-
-			ensure!(
-				PoolMembers::<T>::iter_keys().count() == PoolMembers::<T>::iter_values().count(),
-				"There are undecodable PoolMembers in storage. This migration will not fix that."
-			);
-			ensure!(
-				BondedPools::<T>::iter_keys().count() == BondedPools::<T>::iter_values().count(),
-				"There are undecodable BondedPools in storage. This migration will not fix that."
-			);
-			ensure!(
-				SubPoolsStorage::<T>::iter_keys().count() ==
-					SubPoolsStorage::<T>::iter_values().count(),
-				"There are undecodable SubPools in storage. This migration will not fix that."
-			);
-			ensure!(
-				Metadata::<T>::iter_keys().count() == Metadata::<T>::iter_values().count(),
-				"There are undecodable Metadata in storage. This migration will not fix that."
-			);
-
-			Ok((rpool_values as u64).encode())
-		}
-
-		#[cfg(feature = "try-runtime")]
-		fn post_upgrade(data: Vec<u8>) -> Result<(), TryRuntimeError> {
-			let old_rpool_values: u64 = Decode::decode(&mut &data[..]).unwrap();
-			let rpool_keys = RewardPools::<T>::iter_keys().count() as u64;
-			let rpool_values = RewardPools::<T>::iter_values().count() as u64;
-			ensure!(
-				rpool_keys == rpool_values,
-				"There are STILL undecodable RewardPools - migration failed"
-			);
-
-			if old_rpool_values != rpool_values {
-				log!(
-					info,
-					"🎉 Fixed {} undecodable RewardPools.",
-					rpool_values.saturating_sub(old_rpool_values)
-				);
-			}
-
-			// ensure all RewardPools items now contain `total_commission_pending` and
-			// `total_commission_claimed` field.
-			ensure!(
-				RewardPools::<T>::iter().all(|(_, reward_pool)| reward_pool
-					.total_commission_pending >=
-					Zero::zero() && reward_pool
-					.total_commission_claimed >=
-					Zero::zero()),
-				"a commission value has been incorrectly set"
-			);
-			ensure!(
-				Pallet::<T>::on_chain_storage_version() >= 5,
-				"nomination-pools::migration::v5: wrong storage version"
-			);
-
-			// These should not have been touched - just in case.
-			ensure!(
-				PoolMembers::<T>::iter_keys().count() == PoolMembers::<T>::iter_values().count(),
-				"There are undecodable PoolMembers in storage."
-			);
-			ensure!(
-				BondedPools::<T>::iter_keys().count() == BondedPools::<T>::iter_values().count(),
-				"There are undecodable BondedPools in storage."
-			);
-			ensure!(
-				SubPoolsStorage::<T>::iter_keys().count() ==
-					SubPoolsStorage::<T>::iter_values().count(),
-				"There are undecodable SubPools in storage."
-			);
-			ensure!(
-				Metadata::<T>::iter_keys().count() == Metadata::<T>::iter_values().count(),
-				"There are undecodable Metadata in storage."
-			);
-
 			Ok(())
 		}
 	}

substrate/frame/nomination-pools/src/mock.rs

@@ -17,7 +17,7 @@
 
 use super::*;
 use crate::{self as pools};
-use frame_support::{assert_ok, parameter_types, PalletId};
+use frame_support::{assert_ok, parameter_types, traits::fungible::Mutate, PalletId};
 use frame_system::RawOrigin;
 use sp_runtime::{BuildStorage, FixedU128};
 use sp_staking::Stake;
@@ -29,6 +29,7 @@ pub type RewardCounter = FixedU128;
 // This sneaky little hack allows us to write code exactly as we would do in the pallet in the tests
 // as well, e.g. `StorageItem::<T>::get()`.
 pub type T = Runtime;
+pub type Currency = <T as Config>::Currency;
 
 // Ext builder creates a pool with id 1.
 pub fn default_bonded_account() -> AccountId {
@@ -51,8 +52,8 @@ parameter_types! {
 	pub static StakingMinBond: Balance = 10;
 	pub storage Nominations: Option<Vec<AccountId>> = None;
 }
-
 pub struct StakingMock;
+
 impl StakingMock {
 	pub(crate) fn set_bonded_balance(who: AccountId, bonded: Balance) {
 		let mut x = BondedBalanceMap::get();
@@ -221,8 +222,8 @@ impl pallet_balances::Config for Runtime {
 	type ExistentialDeposit = ExistentialDeposit;
 	type AccountStore = System;
 	type WeightInfo = ();
-	type FreezeIdentifier = ();
+	type FreezeIdentifier = RuntimeFreezeReason;
-	type MaxFreezes = ();
+	type MaxFreezes = ConstU32<1>;
 	type RuntimeHoldReason = ();
 	type MaxHolds = ();
 }
@@ -251,6 +252,7 @@ impl pools::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
 	type WeightInfo = ();
 	type Currency = Balances;
+	type RuntimeFreezeReason = RuntimeFreezeReason;
 	type RewardCounter = RewardCounter;
 	type BalanceToU256 = BalanceToU256;
 	type U256ToBalance = U256ToBalance;
@@ -268,7 +270,7 @@ frame_support::construct_runtime!(
 	{
 		System: frame_system::{Pallet, Call, Storage, Event<T>, Config<T>},
 		Balances: pallet_balances::{Pallet, Call, Storage, Config<T>, Event<T>},
-		Pools: pools::{Pallet, Call, Storage, Event<T>},
+		Pools: pools::{Pallet, Call, Storage, Event<T>, FreezeReason},
 	}
 );
 
@@ -356,12 +358,12 @@ impl ExtBuilder {
 
 			// make a pool
 			let amount_to_bond = Pools::depositor_min_bond();
-			Balances::make_free_balance_be(&10, amount_to_bond * 5);
+			Currency::set_balance(&10, amount_to_bond * 5);
 			assert_ok!(Pools::create(RawOrigin::Signed(10).into(), amount_to_bond, 900, 901, 902));
 			assert_ok!(Pools::set_metadata(RuntimeOrigin::signed(900), 1, vec![1, 1]));
 			let last_pool = LastPoolId::<Runtime>::get();
 			for (account_id, bonded) in self.members {
-				Balances::make_free_balance_be(&account_id, bonded * 2);
+				<Runtime as Config>::Currency::set_balance(&account_id, bonded * 2);
 				assert_ok!(Pools::join(RawOrigin::Signed(account_id).into(), bonded, last_pool));
 			}
 		});
@@ -440,6 +442,58 @@ pub fn fully_unbond_permissioned(member: AccountId) -> DispatchResult {
 	Pools::unbond(RuntimeOrigin::signed(member), member, points)
 }
 
+pub fn pending_rewards_for_delegator(delegator: AccountId) -> Balance {
+	let member = PoolMembers::<T>::get(delegator).unwrap();
+	let bonded_pool = BondedPools::<T>::get(member.pool_id).unwrap();
+	let reward_pool = RewardPools::<T>::get(member.pool_id).unwrap();
+
+	assert!(!bonded_pool.points.is_zero());
+
+	let commission = bonded_pool.commission.current();
+	let current_rc = reward_pool
+		.current_reward_counter(member.pool_id, bonded_pool.points, commission)
+		.unwrap()
+		.0;
+
+	member.pending_rewards(current_rc).unwrap_or_default()
+}
+
+#[derive(PartialEq, Debug)]
+pub enum RewardImbalance {
+	// There is no reward deficit.
+	Surplus(Balance),
+	// There is a reward deficit.
+	Deficit(Balance),
+}
+
+pub fn pool_pending_rewards(pool: PoolId) -> Result<BalanceOf<T>, sp_runtime::DispatchError> {
+	let bonded_pool = BondedPools::<T>::get(pool).ok_or(Error::<T>::PoolNotFound)?;
+	let reward_pool = RewardPools::<T>::get(pool).ok_or(Error::<T>::PoolNotFound)?;
+
+	let current_rc = if !bonded_pool.points.is_zero() {
+		let commission = bonded_pool.commission.current();
+		reward_pool.current_reward_counter(pool, bonded_pool.points, commission)?.0
+	} else {
+		Default::default()
+	};
+
+	Ok(PoolMembers::<T>::iter()
+		.filter(|(_, d)| d.pool_id == pool)
+		.map(|(_, d)| d.pending_rewards(current_rc).unwrap_or_default())
+		.fold(0u32.into(), |acc: BalanceOf<T>, x| acc.saturating_add(x)))
+}
+
+pub fn reward_imbalance(pool: PoolId) -> RewardImbalance {
+	let pending_rewards = pool_pending_rewards(pool).expect("pool should exist");
+	let current_balance = RewardPool::<Runtime>::current_balance(pool);
+
+	if pending_rewards > current_balance {
+		RewardImbalance::Deficit(pending_rewards - current_balance)
+	} else {
+		RewardImbalance::Surplus(current_balance - pending_rewards)
+	}
+}
+
 #[cfg(test)]
 mod test {
 	use super::*;

substrate/frame/nomination-pools/test-staking/src/mock.rs

@@ -85,8 +85,8 @@ impl pallet_balances::Config for Runtime {
 	type ExistentialDeposit = ExistentialDeposit;
 	type AccountStore = System;
 	type WeightInfo = ();
-	type FreezeIdentifier = ();
+	type FreezeIdentifier = RuntimeFreezeReason;
-	type MaxFreezes = ();
+	type MaxFreezes = ConstU32<1>;
 	type RuntimeHoldReason = ();
 	type MaxHolds = ();
 }
@@ -174,6 +174,7 @@ impl pallet_nomination_pools::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
 	type WeightInfo = ();
 	type Currency = Balances;
+	type RuntimeFreezeReason = RuntimeFreezeReason;
 	type RewardCounter = FixedU128;
 	type BalanceToU256 = BalanceToU256;
 	type U256ToBalance = U256ToBalance;
@@ -195,7 +196,7 @@ frame_support::construct_runtime!(
 		Balances: pallet_balances::{Pallet, Call, Storage, Config<T>, Event<T>},
 		Staking: pallet_staking::{Pallet, Call, Config<T>, Storage, Event<T>},
 		VoterList: pallet_bags_list::<Instance1>::{Pallet, Call, Storage, Event<T>},
-		Pools: pallet_nomination_pools::{Pallet, Call, Storage, Event<T>},
+		Pools: pallet_nomination_pools::{Pallet, Call, Storage, Event<T>, FreezeReason},
 	}
 );
 

substrate/frame/support/src/traits/tokens/misc.rs

@@ -20,7 +20,10 @@
 use codec::{Decode, Encode, FullCodec, MaxEncodedLen};
 use sp_arithmetic::traits::{AtLeast32BitUnsigned, Zero};
 use sp_core::RuntimeDebug;
-use sp_runtime::{traits::Convert, ArithmeticError, DispatchError, TokenError};
+use sp_runtime::{
+	traits::{Convert, MaybeSerializeDeserialize},
+	ArithmeticError, DispatchError, TokenError,
+};
 use sp_std::fmt::Debug;
 
 /// The origin of funds to be used for a deposit operation.
@@ -240,6 +243,7 @@ pub trait Balance:
 	+ MaxEncodedLen
 	+ Send
 	+ Sync
+	+ MaybeSerializeDeserialize
 	+ 'static
 {
 }
@@ -253,6 +257,7 @@ impl<
 			+ MaxEncodedLen
 			+ Send
 			+ Sync
+			+ MaybeSerializeDeserialize
 			+ 'static,
 	> Balance for T
 {