Treasury rewards should pay the remainder of the 10% (#4026)

* Treasury rewards should pay the remainder of the 10%..

* Event for deposits coming into treasury

* Fix tests

* Remove OnDilution

substrate/srml/staking/src/lib.rs

@@ -256,7 +256,7 @@ use codec::{HasCompact, Encode, Decode};
 use support::{
 	decl_module, decl_event, decl_storage, ensure,
 	traits::{
-		Currency, OnFreeBalanceZero, OnDilution, LockIdentifier, LockableCurrency,
+		Currency, OnFreeBalanceZero, LockIdentifier, LockableCurrency,
 		WithdrawReasons, OnUnbalanced, Imbalance, Get, Time
 	}
 };
@@ -501,8 +501,8 @@ pub trait Trait: system::Trait {
 	/// The post-processing needs it but will be moved to off-chain. TODO: #2908
 	type CurrencyToVote: Convert<BalanceOf<Self>, u64> + Convert<u128, BalanceOf<Self>>;
 
-	/// Some tokens minted.
-	type OnRewardMinted: OnDilution<BalanceOf<Self>>;
+	/// Tokens have been minted and are unused for validator-reward.
+	type RewardRemainder: OnUnbalanced<NegativeImbalanceOf<Self>>;
 
 	/// The overarching event type.
 	type Event: From<Event<Self>> + Into<<Self as system::Trait>::Event>;
@@ -524,6 +524,10 @@ pub trait Trait: system::Trait {
 
 	/// The NPoS reward curve to use.
 	type RewardCurve: Get<&'static PiecewiseLinear<'static>>;
+
+	/// The maximum possible reward (in proportion of total issued tokens) that can be paid in one
+	/// reward cycle.
+	type MaxPossibleReward: Get<Perbill>;
 }
 
 /// Mode of era-forcing.
@@ -652,8 +656,9 @@ decl_storage! {
 
 decl_event!(
 	pub enum Event<T> where Balance = BalanceOf<T>, <T as system::Trait>::AccountId {
-		/// All validators have been rewarded by the given balance.
-		Reward(Balance),
+		/// All validators have been rewarded by the first balance; the second is the remainder
+		/// from the maximum amount of reward.
+		Reward(Balance, Balance),
 		/// One validator (and its nominators) has been slashed by the given amount.
 		Slash(AccountId, Balance),
 		/// An old slashing report from a prior era was discarded because it could
@@ -1218,9 +1223,13 @@ impl<T: Trait> Module<T> {
 			let total_reward = total_imbalance.peek();
 			// assert!(total_reward <= total_payout)
 
-			Self::deposit_event(RawEvent::Reward(total_reward));
+			let max_reward = T::MaxPossibleReward::get() * T::Currency::total_issuance();
+			let rest_reward = max_reward.saturating_sub(total_reward);
+
+			Self::deposit_event(RawEvent::Reward(total_reward, rest_reward));
+
 			T::Reward::on_unbalanced(total_imbalance);
-			T::OnRewardMinted::on_dilution(total_reward, total_rewarded_stake);
+			T::RewardRemainder::on_unbalanced(T::Currency::issue(rest_reward));
 		}
 
 		// Increment current era.

substrate/srml/staking/src/mock.rs

@@ -192,12 +192,13 @@ parameter_types! {
 	pub const SessionsPerEra: SessionIndex = 3;
 	pub const BondingDuration: EraIndex = 3;
 	pub const RewardCurve: &'static PiecewiseLinear<'static> = &I_NPOS;
+	pub const MaxReward: Perbill = Perbill::from_percent(10);
 }
 impl Trait for Test {
 	type Currency = balances::Module<Self>;
 	type Time = timestamp::Module<Self>;
 	type CurrencyToVote = CurrencyToVoteHandler;
-	type OnRewardMinted = ();
+	type RewardRemainder = ();
 	type Event = ();
 	type Slash = ();
 	type Reward = ();
@@ -205,6 +206,7 @@ impl Trait for Test {
 	type BondingDuration = BondingDuration;
 	type SessionInterface = Self;
 	type RewardCurve = RewardCurve;
+	type MaxPossibleReward = MaxReward;
 }
 
 pub struct ExtBuilder {