Make sure reward pool never gets dusted

frame/nomination-pools/src/lib.rs

@@ -1036,7 +1036,8 @@ pub mod pallet {
 		DoesNotHavePermission,
 		/// Metadata exceeds [`T::MaxMetadataLen`]
 		MetadataExceedsMaxLen,
-		/// Some error occurred that should never happen. This should be reported to the maintainers.
+		/// Some error occurred that should never happen. This should be reported to the
+		/// maintainers.
 		DefensiveError,
 	}
 
@@ -1327,9 +1328,9 @@ pub mod pallet {
 				// 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.
-				debug_assert_eq!(
-					T::Currency::free_balance(&bonded_pool.reward_account()),
-					Zero::zero()
+				debug_assert!(
+					T::Currency::free_balance(&bonded_pool.reward_account()) >=
+						T::Currency::minimum_balance()
 				);
 				debug_assert_eq!(
 					T::Currency::free_balance(&bonded_pool.bonded_account()),
@@ -1366,6 +1367,12 @@ pub mod pallet {
 		/// * `root` - The account to set as [`BondedPool::root`].
 		/// * `nominator` - The account to set as the [`BondedPool::nominator`].
 		/// * `state_toggler` - The account to set as the [`BondedPool::state_toggler`].
+		///
+		/// # Notes
+		///
+		/// The caller will transfer `amount` to the bonded account and existential deposit to the
+		/// reward account. While the former is returned when the caller withdraws unbonded funds,
+		/// the latter is not guaranteed to be returned.
 		// TODO: The creator needs to transfer ED to the pool account and then have their delegators
 		// `reward_pool_total_earnings` ever set to the balance of the reward pool. This will make
 		// an invariant that the reward pool account will always have ED until destroyed.
@@ -1395,8 +1402,12 @@ pub mod pallet {
 				Error::<T>::MaxPools
 			);
 			ensure!(!Delegators::<T>::contains_key(&who), Error::<T>::AccountBelongsToOtherPool);
+			ensure!(
+				T::Currency::free_balance(&who) >=
+					amount.saturating_add(T::Currency::minimum_balance()),
+				Error::<T>::MinimumBondNotMet
+			);
 
-			// TODO: transfer ED to reward pool and update reward pool total earnings and balance.
 			let pool_id = LastPoolId::<T>::mutate(|id| {
 				*id += 1;
 				*id
@@ -1408,6 +1419,25 @@ pub mod pallet {
 
 			let points = bonded_pool.try_bond_delegator(&who, amount, PoolBond::Create)?;
 
+			// The depositor transfers ED to the pool account and then have their delegator
+			// `reward_pool_total_earnings` ever set to the balance of the reward pool. This will
+			// ensure the invariant that the reward pool account will not be dusted until the pool
+			// is destroyed.
+			T::Currency::transfer(
+				&who,
+				&bonded_pool.reward_account(),
+				T::Currency::minimum_balance(),
+				ExistenceRequirement::AllowDeath,
+			)?;
+			let mut reward_pool = RewardPool::<T> {
+				balance: Zero::zero(),
+				points: U256::zero(),
+				total_earnings: Zero::zero(),
+			};
+			// Make sure the reward pool has correct balance and earnings so the first payout claim
+			// does not wipe the ED. This must be done after the transfer
+			reward_pool.update_total_earnings_and_balance(bonded_pool.id);
+
 			Delegators::<T>::insert(
 				who.clone(),
 				Delegator::<T> {
@@ -1417,14 +1447,7 @@ pub mod pallet {
 					unbonding_era: None,
 				},
 			);
-			RewardPools::<T>::insert(
-				pool_id,
-				RewardPool::<T> {
-					balance: Zero::zero(),
-					points: U256::zero(),
-					total_earnings: Zero::zero(),
-				},
-			);
+			RewardPools::<T>::insert(pool_id, reward_pool);
 			ReversePoolIdLookup::<T>::insert(bonded_pool.bonded_account(), pool_id);
 			Self::deposit_event(Event::<T>::Created { depositor: who, pool_id });
 			bonded_pool.put();