Configurable call fee refund for signed submissions

bin/node/runtime/src/lib.rs

@@ -672,6 +672,7 @@ impl pallet_election_provider_multi_phase::Config for Runtime {
 	type SignedRewardBase = SignedRewardBase;
 	type SignedDepositBase = SignedDepositBase;
 	type SignedDepositByte = SignedDepositByte;
+	type SignedMaxRefunds = SignedMaxRefunds;
 	type SignedDepositWeight = ();
 	type SignedMaxWeight = MinerMaxWeight;
 	type SlashHandler = (); // burn slashes

frame/election-provider-multi-phase/src/benchmarking.rs

@@ -225,14 +225,24 @@ frame_benchmarking::benchmarks! {
 			compute: Default::default()
 		};
 		let deposit: BalanceOf<T> = 10u32.into();
-		let reward: BalanceOf<T> = 20u32.into();
+
+		let reward: BalanceOf<T> = T::SignedRewardBase::get();
+		let call_fee: BalanceOf<T> = 30u32.into();
 
 		assert_ok!(T::Currency::reserve(&receiver, deposit));
 		assert_eq!(T::Currency::free_balance(&receiver), initial_balance - 10u32.into());
 	}: {
-		<MultiPhase<T>>::finalize_signed_phase_accept_solution(ready, &receiver, deposit, reward)
+		<MultiPhase<T>>::finalize_signed_phase_accept_solution(
+			ready,
+			&receiver,
+			deposit,
+			call_fee
+		)
 	} verify {
-		assert_eq!(T::Currency::free_balance(&receiver), initial_balance + 20u32.into());
+		assert_eq!(
+			T::Currency::free_balance(&receiver),
+			initial_balance + reward + call_fee
+		);
 		assert_eq!(T::Currency::reserved_balance(&receiver), 0u32.into());
 	}
 
@@ -333,7 +343,7 @@ frame_benchmarking::benchmarks! {
 				raw_solution,
 				who: account("submitters", i, SEED),
 				deposit: Default::default(),
-				reward: Default::default(),
+				call_fee: Default::default(),
 			};
 			signed_submissions.insert(signed_submission);
 		}

frame/election-provider-multi-phase/src/lib.rs

@@ -242,7 +242,7 @@ use frame_support::{
 use frame_system::{ensure_none, offchain::SendTransactionTypes};
 use scale_info::TypeInfo;
 use sp_arithmetic::{
-	traits::{Bounded, CheckedAdd, Saturating, Zero},
+	traits::{Bounded, CheckedAdd, Zero},
 	UpperOf,
 };
 use sp_npos_elections::{
@@ -623,6 +623,10 @@ pub mod pallet {
 		#[pallet::constant]
 		type SignedMaxWeight: Get<Weight>;
 
+		/// The maximum amount of unchecked solutions to refund the call fee for.
+		#[pallet::constant]
+		type SignedMaxRefunds: Get<u32>;
+
 		/// Base reward for a signed solution
 		#[pallet::constant]
 		type SignedRewardBase: Get<BalanceOf<Self>>;
@@ -843,6 +847,11 @@ pub mod pallet {
 				<T::DataProvider as ElectionDataProvider>::MaxVotesPerVoter::get(),
 				<SolutionOf<T> as NposSolution>::LIMIT as u32,
 			);
+
+			// While it won't cause any failures, setting `SignedMaxRefunds` gt
+			// `SignedMaxSubmissions` is a red flag that the developer does not understand how to
+			// configure this pallet.
+			assert!(T::SignedMaxSubmissions::get() >= T::SignedMaxRefunds::get());
 		}
 	}
 
@@ -983,14 +992,17 @@ pub mod pallet {
 
 			// create the submission
 			let deposit = Self::deposit_for(&raw_solution, size);
-			let reward = {
+			let call_fee = {
 				let call = Call::submit { raw_solution: raw_solution.clone() };
-				let call_fee = T::EstimateCallFee::estimate_call_fee(&call, None.into());
-				T::SignedRewardBase::get().saturating_add(call_fee)
+				T::EstimateCallFee::estimate_call_fee(&call, None.into())
 			};
 
-			let submission =
-				SignedSubmission { who: who.clone(), deposit, raw_solution: *raw_solution, reward };
+			let submission = SignedSubmission {
+				who: who.clone(),
+				deposit,
+				raw_solution: *raw_solution,
+				call_fee,
+			};
 
 			// insert the submission if the queue has space or it's better than the weakest
 			// eject the weakest if the queue was full

frame/election-provider-multi-phase/src/mock.rs

@@ -257,6 +257,7 @@ parameter_types! {
 	pub static SignedPhase: BlockNumber = 10;
 	pub static UnsignedPhase: BlockNumber = 5;
 	pub static SignedMaxSubmissions: u32 = 5;
+	pub static SignedMaxRefunds: u32 = 1;
 	pub static SignedDepositBase: Balance = 5;
 	pub static SignedDepositByte: Balance = 0;
 	pub static SignedDepositWeight: Balance = 0;
@@ -424,6 +425,7 @@ impl crate::Config for Runtime {
 	type SignedDepositWeight = ();
 	type SignedMaxWeight = SignedMaxWeight;
 	type SignedMaxSubmissions = SignedMaxSubmissions;
+	type SignedMaxRefunds = SignedMaxRefunds;
 	type SlashHandler = ();
 	type RewardHandler = ();
 	type DataProvider = StakingMock;

frame/election-provider-multi-phase/src/signed.rs

@@ -51,8 +51,8 @@ pub struct SignedSubmission<AccountId, Balance: HasCompact, Solution> {
 	pub deposit: Balance,
 	/// The raw solution itself.
 	pub raw_solution: RawSolution<Solution>,
-	/// The reward that should potentially be paid for this solution, if accepted.
-	pub reward: Balance,
+	// The estimated fee `who` paid to submit the solution.
+	pub call_fee: Balance,
 }
 
 impl<AccountId, Balance, Solution> Ord for SignedSubmission<AccountId, Balance, Solution>
@@ -235,20 +235,34 @@ impl<T: Config> SignedSubmissions<T> {
 	}
 
 	/// Empty the set of signed submissions, returning an iterator of signed submissions in
-	/// arbitrary order.
+	/// order of submission.
 	///
 	/// Note that if the iterator is dropped without consuming all elements, not all may be removed
 	/// from the underlying `SignedSubmissionsMap`, putting the storages into an invalid state.
 	///
 	/// Note that, like `put`, this function consumes `Self` and modifies storage.
-	fn drain(mut self) -> impl Iterator<Item = SignedSubmissionOf<T>> {
+	fn drain_submitted_order(mut self) -> impl Iterator<Item = SignedSubmissionOf<T>> {
+		let mut keys = SignedSubmissionsMap::<T>::iter_keys()
+			.filter(|k| {
+				if self.deletion_overlay.contains(k) {
+					// Remove submissions that should be deleted.
+					SignedSubmissionsMap::<T>::remove(k);
+					false
+				} else {
+					true
+				}
+			})
+			.chain(self.insertion_overlay.keys().map(|n| *n))
+			.collect::<Vec<_>>();
+		keys.sort();
+
 		SignedSubmissionIndices::<T>::kill();
 		SignedSubmissionNextIndex::<T>::kill();
-		let insertion_overlay = sp_std::mem::take(&mut self.insertion_overlay);
-		SignedSubmissionsMap::<T>::drain()
-			.filter(move |(k, _v)| !self.deletion_overlay.contains(k))
-			.map(|(_k, v)| v)
-			.chain(insertion_overlay.into_iter().map(|(_k, v)| v))
+		let _ = sp_std::mem::take(&mut self.deletion_overlay);
+
+		keys.into_iter().filter_map(move |index| {
+			SignedSubmissionsMap::<T>::take(index).or_else(|| self.insertion_overlay.remove(&index))
+		})
 	}
 
 	/// Decode the length of the signed submissions without actually reading the entire struct into
@@ -362,7 +376,7 @@ impl<T: Config> Pallet<T> {
 			Self::snapshot_metadata().unwrap_or_default();
 
 		while let Some(best) = all_submissions.pop_last() {
-			let SignedSubmission { raw_solution, who, deposit, reward } = best;
+			let SignedSubmission { raw_solution, who, deposit, call_fee } = best;
 			let active_voters = raw_solution.solution.voter_count() as u32;
 			let feasibility_weight = {
 				// defensive only: at the end of signed phase, snapshot will exits.
@@ -377,7 +391,7 @@ impl<T: Config> Pallet<T> {
 						ready_solution,
 						&who,
 						deposit,
-						reward,
+						call_fee,
 					);
 					found_solution = true;
 
@@ -396,10 +410,23 @@ impl<T: Config> Pallet<T> {
 		// Any unprocessed solution is pointless to even consider. Feasible or malicious,
 		// they didn't end up being used. Unreserve the bonds.
 		let discarded = all_submissions.len();
-		for SignedSubmission { who, deposit, .. } in all_submissions.drain() {
+		let mut refund_count = 0;
+		let max_refunds = T::SignedMaxRefunds::get();
+
+		for SignedSubmission { who, deposit, call_fee, .. } in
+			all_submissions.drain_submitted_order()
+		{
+			if refund_count < max_refunds {
+				// Refund fee
+				let positive_imbalance = T::Currency::deposit_creating(&who, call_fee);
+				T::RewardHandler::on_unbalanced(positive_imbalance);
+				refund_count += 1;
+			}
+
+			// Unreserve deposit
 			let _remaining = T::Currency::unreserve(&who, deposit);
-			weight = weight.saturating_add(T::DbWeight::get().writes(1));
 			debug_assert!(_remaining.is_zero());
+			weight = weight.saturating_add(T::DbWeight::get().reads_writes(1, 2));
 		}
 
 		debug_assert!(!SignedSubmissionIndices::<T>::exists());
@@ -424,20 +451,22 @@ impl<T: Config> Pallet<T> {
 		ready_solution: ReadySolution<T::AccountId>,
 		who: &T::AccountId,
 		deposit: BalanceOf<T>,
-		reward: BalanceOf<T>,
+		call_fee: BalanceOf<T>,
 	) {
 		// write this ready solution.
 		<QueuedSolution<T>>::put(ready_solution);
 
+		let reward = T::SignedRewardBase::get();
 		// emit reward event
 		Self::deposit_event(crate::Event::Rewarded { account: who.clone(), value: reward });
 
-		// unreserve deposit.
+		// Unreserve deposit.
 		let _remaining = T::Currency::unreserve(who, deposit);
 		debug_assert!(_remaining.is_zero());
 
-		// Reward.
-		let positive_imbalance = T::Currency::deposit_creating(who, reward);
+		// Reward and refund the call fee.
+		let positive_imbalance =
+			T::Currency::deposit_creating(who, reward.saturating_add(call_fee));
 		T::RewardHandler::on_unbalanced(positive_imbalance);
 	}
 
@@ -496,8 +525,8 @@ mod tests {
 	use super::*;
 	use crate::{
 		mock::{
-			balances, raw_solution, roll_to, ExtBuilder, MultiPhase, Origin, Runtime,
-			SignedMaxSubmissions, SignedMaxWeight,
+			balances, raw_solution, roll_to, Balances, ExtBuilder, MultiPhase, Origin, Runtime,
+			SignedMaxRefunds, SignedMaxSubmissions, SignedMaxWeight,
 		},
 		Error, Phase,
 	};
@@ -599,8 +628,8 @@ mod tests {
 
 			// 99 is rewarded.
 			assert_eq!(balances(&99), (100 + 7 + 8, 0));
-			// 999 gets everything back.
-			assert_eq!(balances(&999), (100, 0));
+			// 999 gets everything back, including the call fee.
+			assert_eq!(balances(&999), (100 + 8, 0));
 		})
 	}
 
@@ -632,19 +661,60 @@ mod tests {
 		})
 	}
 
+	#[test]
+	fn call_fee_refund_is_limited_by_signed_max_refunds() {
+		ExtBuilder::default().build_and_execute(|| {
+			roll_to(15);
+			assert!(MultiPhase::current_phase().is_signed());
+			assert_eq!(SignedMaxRefunds::get(), 1);
+			assert!(SignedMaxSubmissions::get() > 2);
+
+			for s in 0..SignedMaxSubmissions::get() {
+				let account = 99 + s as u64;
+				Balances::make_free_balance_be(&account, 100);
+				// score is always decreasing
+				let mut solution = raw_solution();
+				solution.score.minimal_stake -= s as u128;
+
+				assert_ok!(MultiPhase::submit(Origin::signed(account), Box::new(solution)));
+				assert_eq!(balances(&account), (95, 5));
+			}
+
+			assert!(MultiPhase::finalize_signed_phase());
+
+			for s in 0..SignedMaxSubmissions::get() {
+				let account = 99 + s as u64;
+				// lower accounts have higher scores
+				if s == 0 {
+					// winning solution always gets call fee + reward
+					assert_eq!(balances(&account), (100 + 8 + 7, 0))
+				} else if s == 1 {
+					// 1 runner up gets their call fee refunded
+					assert_eq!(balances(&account), (100 + 8, 0))
+				} else {
+					// all other solutions don't get a call fee refund
+					assert_eq!(balances(&account), (100, 0));
+				}
+			}
+		});
+	}
+
 	#[test]
 	fn weakest_is_removed_if_better_provided() {
 		ExtBuilder::default().build_and_execute(|| {
 			roll_to(15);
 			assert!(MultiPhase::current_phase().is_signed());
 
 			for s in 0..SignedMaxSubmissions::get() {
+				let account = 99 + s as u64;
+				Balances::make_free_balance_be(&account, 100);
 				// score is always getting better
 				let solution = RawSolution {
 					score: ElectionScore { minimal_stake: (5 + s).into(), ..Default::default() },
 					..Default::default()
 				};
-				assert_ok!(MultiPhase::submit(Origin::signed(99), Box::new(solution)));
+				assert_ok!(MultiPhase::submit(Origin::signed(account), Box::new(solution)));
+				assert_eq!(balances(&account), (95, 5));
 			}
 
 			assert_eq!(
@@ -660,7 +730,7 @@ mod tests {
 				score: ElectionScore { minimal_stake: 20, ..Default::default() },
 				..Default::default()
 			};
-			assert_ok!(MultiPhase::submit(Origin::signed(99), Box::new(solution)));
+			assert_ok!(MultiPhase::submit(Origin::signed(999), Box::new(solution)));
 
 			// the one with score 5 was rejected, the new one inserted.
 			assert_eq!(
@@ -670,6 +740,9 @@ mod tests {
 					.collect::<Vec<_>>(),
 				vec![6, 7, 8, 9, 20]
 			);
+
+			// the submitter of the ejected solution does *not* get a call fee refund
+			assert_eq!(balances(&(99 + 0)), (100, 0));
 		})
 	}
 
@@ -825,8 +898,8 @@ mod tests {
 			assert_eq!(balances(&99), (100 + 7 + 8, 0));
 			// 999 is slashed.
 			assert_eq!(balances(&999), (95, 0));
-			// 9999 gets everything back.
-			assert_eq!(balances(&9999), (100, 0));
+			// 9999 gets everything back, including the call fee.
+			assert_eq!(balances(&9999), (100 + 8, 0));
 		})
 	}