bounding staking: BoundedElectionProvider trait (paritytech#12362)

* add a bounded election provider trait

* extract common trait election provider base

* fmt

* only bound the outer support vector

* fix tests

* docs

* fix rust docs

* fmt

* fix rustdocs

* docs

* improve docs

* small doc change

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

@@ -231,7 +231,8 @@
 
 use codec::{Decode, Encode};
 use frame_election_provider_support::{
-	ElectionDataProvider, ElectionProvider, InstantElectionProvider, NposSolution,
+	ElectionDataProvider, ElectionProvider, ElectionProviderBase, InstantElectionProvider,
+	NposSolution,
 };
 use frame_support::{
 	dispatch::DispatchClass,
@@ -289,7 +290,7 @@ pub type SolutionTargetIndexOf<T> = <SolutionOf<T> as NposSolution>::TargetIndex
 pub type SolutionAccuracyOf<T> =
 	<SolutionOf<<T as crate::Config>::MinerConfig> as NposSolution>::Accuracy;
 /// The fallback election type.
-pub type FallbackErrorOf<T> = <<T as crate::Config>::Fallback as ElectionProvider>::Error;
+pub type FallbackErrorOf<T> = <<T as crate::Config>::Fallback as ElectionProviderBase>::Error;
 
 /// Configuration for the benchmarks of the pallet.
 pub trait BenchmarkingConfig {
@@ -312,7 +313,7 @@ pub trait BenchmarkingConfig {
 /// A fallback implementation that transitions the pallet to the emergency phase.
 pub struct NoFallback<T>(sp_std::marker::PhantomData<T>);
 
-impl<T: Config> ElectionProvider for NoFallback<T> {
+impl<T: Config> ElectionProviderBase for NoFallback<T> {
 	type AccountId = T::AccountId;
 	type BlockNumber = T::BlockNumber;
 	type DataProvider = T::DataProvider;
@@ -321,7 +322,9 @@ impl<T: Config> ElectionProvider for NoFallback<T> {
 	fn ongoing() -> bool {
 		false
 	}
+}
 
+impl<T: Config> ElectionProvider for NoFallback<T> {
 	fn elect() -> Result<Supports<T::AccountId>, Self::Error> {
 		// Do nothing, this will enable the emergency phase.
 		Err("NoFallback.")
@@ -1563,7 +1566,7 @@ impl<T: Config> Pallet<T> {
 		<QueuedSolution<T>>::take()
 			.ok_or(ElectionError::<T>::NothingQueued)
 			.or_else(|_| {
-				T::Fallback::elect()
+				<T::Fallback as ElectionProvider>::elect()
 					.map(|supports| ReadySolution {
 						supports,
 						score: Default::default(),
@@ -1598,7 +1601,7 @@ impl<T: Config> Pallet<T> {
 	}
 }
 
-impl<T: Config> ElectionProvider for Pallet<T> {
+impl<T: Config> ElectionProviderBase for Pallet<T> {
 	type AccountId = T::AccountId;
 	type BlockNumber = T::BlockNumber;
 	type Error = ElectionError<T>;
@@ -1610,7 +1613,9 @@ impl<T: Config> ElectionProvider for Pallet<T> {
 			_ => true,
 		}
 	}
+}
 
+impl<T: Config> ElectionProvider for Pallet<T> {
 	fn elect() -> Result<Supports<T::AccountId>, Self::Error> {
 		match Self::do_elect() {
 			Ok(supports) => {
@@ -1627,7 +1632,6 @@ impl<T: Config> ElectionProvider for Pallet<T> {
 		}
 	}
 }
-
 /// convert a DispatchError to a custom InvalidTransaction with the inner code being the error
 /// number.
 pub fn dispatch_error_to_invalid(error: DispatchError) -> InvalidTransaction {

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

@@ -297,7 +297,7 @@ impl onchain::Config for OnChainSeqPhragmen {
 }
 
 pub struct MockFallback;
-impl ElectionProvider for MockFallback {
+impl ElectionProviderBase for MockFallback {
 	type AccountId = AccountId;
 	type BlockNumber = u64;
 	type Error = &'static str;
@@ -306,7 +306,8 @@ impl ElectionProvider for MockFallback {
 	fn ongoing() -> bool {
 		false
 	}
-
+}
+impl ElectionProvider for MockFallback {
 	fn elect() -> Result<Supports<AccountId>, Self::Error> {
 		Self::elect_with_bounds(Bounded::max_value(), Bounded::max_value())
 	}

frame/election-provider-support/src/lib.rs

@@ -20,10 +20,11 @@
 //! This crate provides two traits that could interact to enable extensible election functionality
 //! within FRAME pallets.
 //!
-//! Something that will provide the functionality of election will implement [`ElectionProvider`],
-//! whilst needing an associated [`ElectionProvider::DataProvider`], which needs to be fulfilled by
-//! an entity implementing [`ElectionDataProvider`]. Most often, *the data provider is* the receiver
-//! of the election, resulting in a diagram as below:
+//! Something that will provide the functionality of election will implement
+//! [`ElectionProvider`] and its parent-trait [`ElectionProviderBase`], whilst needing an
+//! associated [`ElectionProviderBase::DataProvider`], which needs to be
+//! fulfilled by an entity implementing [`ElectionDataProvider`]. Most often, *the data provider is*
+//! the receiver of the election, resulting in a diagram as below:
 //!
 //! ```ignore
 //!                                         ElectionDataProvider
@@ -131,12 +132,16 @@
 //!         type DataProvider: ElectionDataProvider<AccountId=AccountId, BlockNumber = BlockNumber>;
 //!     }
 //!
-//!     impl<T: Config> ElectionProvider for GenericElectionProvider<T> {
+//!     impl<T: Config> ElectionProviderBase for GenericElectionProvider<T> {
 //!         type AccountId = AccountId;
 //!         type BlockNumber = BlockNumber;
 //!         type Error = &'static str;
 //!         type DataProvider = T::DataProvider;
 //! 	        fn ongoing() -> bool { false }
+//!        
+//!     }
+//!
+//!     impl<T: Config> ElectionProvider for GenericElectionProvider<T> {
 //!         fn elect() -> Result<Supports<AccountId>, Self::Error> {
 //!             Self::DataProvider::electable_targets(None)
 //!                 .map_err(|_| "failed to elect")
@@ -177,8 +182,8 @@ pub use frame_support::{traits::Get, weights::Weight, BoundedVec, RuntimeDebug};
 /// Re-export some type as they are used in the interface.
 pub use sp_arithmetic::PerThing;
 pub use sp_npos_elections::{
-	Assignment, BalancingConfig, ElectionResult, Error, ExtendedBalance, IdentifierT, PerThing128,
-	Support, Supports, VoteWeight,
+	Assignment, BalancingConfig, BoundedSupports, ElectionResult, Error, ExtendedBalance,
+	IdentifierT, PerThing128, Support, Supports, VoteWeight,
 };
 pub use traits::NposSolution;
 
@@ -349,12 +354,12 @@ pub trait ElectionDataProvider {
 	fn clear() {}
 }
 
-/// Something that can compute the result of an election and pass it back to the caller.
+/// Base trait for [`ElectionProvider`] and [`BoundedElectionProvider`]. It is
+/// meant to be used only with an extension trait that adds an election
+/// functionality.
 ///
-/// This trait only provides an interface to _request_ an election, i.e.
-/// [`ElectionProvider::elect`]. That data required for the election need to be passed to the
-/// implemented of this trait through [`ElectionProvider::DataProvider`].
-pub trait ElectionProvider {
+/// Data can be bounded or unbounded and is fetched from [`Self::DataProvider`].
+pub trait ElectionProviderBase {
 	/// The account identifier type.
 	type AccountId;
 
@@ -372,24 +377,39 @@ pub trait ElectionProvider {
 
 	/// Indicate if this election provider is currently ongoing an asynchronous election or not.
 	fn ongoing() -> bool;
+}
 
-	/// Elect a new set of winners, without specifying any bounds on the amount of data fetched from
-	/// [`Self::DataProvider`]. An implementation could nonetheless impose its own custom limits.
-	///
-	/// The result is returned in a target major format, namely as *vector of supports*.
-	///
-	/// This should be implemented as a self-weighing function. The implementor should register its
-	/// appropriate weight at the end of execution with the system pallet directly.
+/// Elect a new set of winners, bounded by `MaxWinners`.
+///
+/// Returns a result in bounded, target major format, namely as
+/// *BoundedVec<(AccountId, Vec<Support>), MaxWinners>*.   
+pub trait BoundedElectionProvider: ElectionProviderBase {
+	/// The upper bound on election winners.
+	type MaxWinners: Get<u32>;
+	/// Performs the election. This should be implemented as a self-weighing function. The
+	/// implementor should register its appropriate weight at the end of execution with the
+	/// system pallet directly.
+	fn elect() -> Result<BoundedSupports<Self::AccountId, Self::MaxWinners>, Self::Error>;
+}
+
+/// Same a [`BoundedElectionProvider`], but no bounds are imposed on the number
+/// of winners.
+///
+/// The result is returned in a target major format, namely as
+///*Vec<(AccountId, Vec<Support>)>*.
+pub trait ElectionProvider: ElectionProviderBase {
+	/// Performs the election. This should be implemented as a self-weighing
+	/// function, similar to [`BoundedElectionProvider::elect()`].
 	fn elect() -> Result<Supports<Self::AccountId>, Self::Error>;
 }
 
-/// A sub-trait of the [`ElectionProvider`] for cases where we need to be sure an election needs to
-/// happen instantly, not asynchronously.
+/// A sub-trait of the [`ElectionProvider`] for cases where we need to be sure
+/// an election needs to happen instantly, not asynchronously.
 ///
 /// The same `DataProvider` is assumed to be used.
 ///
-/// Consequently, allows for control over the amount of data that is being fetched from the
-/// [`ElectionProvider::DataProvider`].
+/// Consequently, allows for control over the amount of data that is being
+/// fetched from the [`ElectionProviderBase::DataProvider`].
 pub trait InstantElectionProvider: ElectionProvider {
 	/// Elect a new set of winners, but unlike [`ElectionProvider::elect`] which cannot enforce
 	/// bounds, this trait method can enforce bounds on the amount of data provided by the
@@ -410,7 +430,7 @@ pub trait InstantElectionProvider: ElectionProvider {
 pub struct NoElection<X>(sp_std::marker::PhantomData<X>);
 
 #[cfg(feature = "std")]
-impl<AccountId, BlockNumber, DataProvider> ElectionProvider
+impl<AccountId, BlockNumber, DataProvider> ElectionProviderBase
 	for NoElection<(AccountId, BlockNumber, DataProvider)>
 where
 	DataProvider: ElectionDataProvider<AccountId = AccountId, BlockNumber = BlockNumber>,
@@ -420,15 +440,22 @@ where
 	type Error = &'static str;
 	type DataProvider = DataProvider;
 
-	fn elect() -> Result<Supports<AccountId>, Self::Error> {
-		Err("<NoElection as ElectionProvider> cannot do anything.")
-	}
-
 	fn ongoing() -> bool {
 		false
 	}
 }
 
+#[cfg(feature = "std")]
+impl<AccountId, BlockNumber, DataProvider> ElectionProvider
+	for NoElection<(AccountId, BlockNumber, DataProvider)>
+where
+	DataProvider: ElectionDataProvider<AccountId = AccountId, BlockNumber = BlockNumber>,
+{
+	fn elect() -> Result<Supports<AccountId>, Self::Error> {
+		Err("<NoElection as ElectionProvider> cannot do anything.")
+	}
+}
+
 /// A utility trait for something to implement `ElectionDataProvider` in a sensible way.
 ///
 /// This is generic over `AccountId` and it can represent a validator, a nominator, or any other

frame/election-provider-support/src/onchain.rs

@@ -20,7 +20,8 @@
 //! careful when using it onchain.
 
 use crate::{
-	Debug, ElectionDataProvider, ElectionProvider, InstantElectionProvider, NposSolver, WeightInfo,
+	Debug, ElectionDataProvider, ElectionProvider, ElectionProviderBase, InstantElectionProvider,
+	NposSolver, WeightInfo,
 };
 use frame_support::{dispatch::DispatchClass, traits::Get};
 use sp_npos_elections::*;
@@ -133,15 +134,6 @@ fn elect_with<T: Config>(
 }
 
 impl<T: Config> ElectionProvider for UnboundedExecution<T> {
-	type AccountId = <T::System as frame_system::Config>::AccountId;
-	type BlockNumber = <T::System as frame_system::Config>::BlockNumber;
-	type Error = Error;
-	type DataProvider = T::DataProvider;
-
-	fn ongoing() -> bool {
-		false
-	}
-
 	fn elect() -> Result<Supports<Self::AccountId>, Self::Error> {
 		// This should not be called if not in `std` mode (and therefore neither in genesis nor in
 		// testing)
@@ -156,6 +148,17 @@ impl<T: Config> ElectionProvider for UnboundedExecution<T> {
 	}
 }
 
+impl<T: Config> ElectionProviderBase for UnboundedExecution<T> {
+	type AccountId = <T::System as frame_system::Config>::AccountId;
+	type BlockNumber = <T::System as frame_system::Config>::BlockNumber;
+	type Error = Error;
+	type DataProvider = T::DataProvider;
+
+	fn ongoing() -> bool {
+		false
+	}
+}
+
 impl<T: Config> InstantElectionProvider for UnboundedExecution<T> {
 	fn elect_with_bounds(
 		max_voters: usize,
@@ -165,7 +168,7 @@ impl<T: Config> InstantElectionProvider for UnboundedExecution<T> {
 	}
 }
 
-impl<T: BoundedConfig> ElectionProvider for BoundedExecution<T> {
+impl<T: BoundedConfig> ElectionProviderBase for BoundedExecution<T> {
 	type AccountId = <T::System as frame_system::Config>::AccountId;
 	type BlockNumber = <T::System as frame_system::Config>::BlockNumber;
 	type Error = Error;
@@ -174,7 +177,9 @@ impl<T: BoundedConfig> ElectionProvider for BoundedExecution<T> {
 	fn ongoing() -> bool {
 		false
 	}
+}
 
+impl<T: BoundedConfig> ElectionProvider for BoundedExecution<T> {
 	fn elect() -> Result<Supports<Self::AccountId>, Self::Error> {
 		elect_with::<T>(Some(T::VotersBound::get() as usize), Some(T::TargetsBound::get() as usize))
 	}

frame/fast-unstake/src/lib.rs

@@ -80,7 +80,7 @@ macro_rules! log {
 pub mod pallet {
 	use super::*;
 	use crate::types::*;
-	use frame_election_provider_support::ElectionProvider;
+	use frame_election_provider_support::ElectionProviderBase;
 	use frame_support::{
 		pallet_prelude::*,
 		traits::{Defensive, ReservableCurrency},
@@ -330,7 +330,8 @@ pub mod pallet {
 				}
 			}
 
-			if <T as pallet_staking::Config>::ElectionProvider::ongoing() {
+			if <<T as pallet_staking::Config>::ElectionProvider as ElectionProviderBase>::ongoing()
+			{
 				// NOTE: we assume `ongoing` does not consume any weight.
 				// there is an ongoing election -- we better not do anything. Imagine someone is not
 				// exposed anywhere in the last era, and the snapshot for the election is already

frame/fast-unstake/src/mock.rs

@@ -104,7 +104,7 @@ parameter_types! {
 }
 
 pub struct MockElection;
-impl frame_election_provider_support::ElectionProvider for MockElection {
+impl frame_election_provider_support::ElectionProviderBase for MockElection {
 	type AccountId = AccountId;
 	type BlockNumber = BlockNumber;
 	type DataProvider = Staking;
@@ -113,7 +113,9 @@ impl frame_election_provider_support::ElectionProvider for MockElection {
 	fn ongoing() -> bool {
 		Ongoing::get()
 	}
+}
 
+impl frame_election_provider_support::ElectionProvider for MockElection {
 	fn elect() -> Result<frame_election_provider_support::Supports<AccountId>, Self::Error> {
 		Err(())
 	}

primitives/npos-elections/src/lib.rs

@@ -74,17 +74,16 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 
+use codec::{Decode, Encode, MaxEncodedLen};
 use scale_info::TypeInfo;
+#[cfg(feature = "std")]
+use serde::{Deserialize, Serialize};
 use sp_arithmetic::{traits::Zero, Normalizable, PerThing, Rational128, ThresholdOrd};
-use sp_core::RuntimeDebug;
+use sp_core::{bounded::BoundedVec, RuntimeDebug};
 use sp_std::{
 	cell::RefCell, cmp::Ordering, collections::btree_map::BTreeMap, prelude::*, rc::Rc, vec,
 };
 
-use codec::{Decode, Encode, MaxEncodedLen};
-#[cfg(feature = "std")]
-use serde::{Deserialize, Serialize};
-
 #[cfg(test)]
 mod mock;
 #[cfg(test)]
@@ -451,6 +450,11 @@ impl<AccountId> Default for Support<AccountId> {
 /// The main advantage of this is that it is encodable.
 pub type Supports<A> = Vec<(A, Support<A>)>;
 
+/// Same as `Supports` bounded by `MaxWinners`.
+///
+/// To note, the inner `Support` is still unbounded.
+pub type BoundedSupports<A, MaxWinners> = BoundedVec<(A, Support<A>), MaxWinners>;
+
 /// Linkage from a winner to their [`Support`].
 ///
 /// This is more helpful than a normal [`Supports`] as it allows faster error checking.