Only maintain at most 1 UnlockChunk per era (#10670)

* Only maintain at most 1 `UnlockChunk` per era

* Bound `unlocking`

* Run cargo +nightly-2021-10-29 fmt

* Make benchmarks stuff compile

* Update frame/staking/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Remove DerefMut; Implement neccesary methods directly

* Doc comments for new BoundedVec methods

* Fix benchmarks

* wip bonded_vec macro

* Correct rust doc

* Apply suggestions from code review

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* Update staking::Config impls

* Add MaxUnlockingChunks to more places

* Use defensive saturating add

* FMT

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

substrate/frame/staking/src/pallet/impls.rs

@@ -928,7 +928,7 @@ impl<T: Config> ElectionDataProvider for Pallet<T> {
 				stash: voter.clone(),
 				active: stake,
 				total: stake,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			},
 		);
@@ -946,7 +946,7 @@ impl<T: Config> ElectionDataProvider for Pallet<T> {
 				stash: target.clone(),
 				active: stake,
 				total: stake,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: vec![],
 			},
 		);
@@ -983,7 +983,7 @@ impl<T: Config> ElectionDataProvider for Pallet<T> {
 					stash: v.clone(),
 					active: stake,
 					total: stake,
-					unlocking: vec![],
+					unlocking: Default::default(),
 					claimed_rewards: vec![],
 				},
 			);
@@ -1004,7 +1004,7 @@ impl<T: Config> ElectionDataProvider for Pallet<T> {
 					stash: v.clone(),
 					active: stake,
 					total: stake,
-					unlocking: vec![],
+					unlocking: Default::default(),
 					claimed_rewards: vec![],
 				},
 			);

substrate/frame/staking/src/pallet/mod.rs

@@ -21,8 +21,8 @@ use frame_election_provider_support::SortedListProvider;
 use frame_support::{
 	pallet_prelude::*,
 	traits::{
-		Currency, CurrencyToVote, EnsureOrigin, EstimateNextNewSession, Get, LockIdentifier,
-		LockableCurrency, OnUnbalanced, UnixTime,
+		Currency, CurrencyToVote, DefensiveSaturating, EnsureOrigin, EstimateNextNewSession, Get,
+		LockIdentifier, LockableCurrency, OnUnbalanced, UnixTime,
 	},
 	weights::Weight,
 };
@@ -40,12 +40,11 @@ pub use impls::*;
 
 use crate::{
 	log, slashing, weights::WeightInfo, ActiveEraInfo, BalanceOf, EraPayout, EraRewardPoints,
-	Exposure, Forcing, NegativeImbalanceOf, Nominations, PositiveImbalanceOf, Releases,
-	RewardDestination, SessionInterface, StakingLedger, UnappliedSlash, UnlockChunk,
+	Exposure, Forcing, MaxUnlockingChunks, NegativeImbalanceOf, Nominations, PositiveImbalanceOf,
+	Releases, RewardDestination, SessionInterface, StakingLedger, UnappliedSlash, UnlockChunk,
 	ValidatorPrefs,
 };
 
-pub const MAX_UNLOCKING_CHUNKS: usize = 32;
 const STAKING_ID: LockIdentifier = *b"staking ";
 
 #[frame_support::pallet]
@@ -157,6 +156,10 @@ pub mod pallet {
 		/// the bags-list is not desired, [`impls::UseNominatorsMap`] is likely the desired option.
 		type SortedListProvider: SortedListProvider<Self::AccountId>;
 
+		/// The maximum number of `unlocking` chunks a [`StakingLedger`] can have. Effectively
+		/// determines how many unique eras a staker may be unbonding in.
+		type MaxUnlockingChunks: Get<u32>;
+
 		/// Some parameters of the benchmarking.
 		type BenchmarkingConfig: BenchmarkingConfig;
 
@@ -772,7 +775,7 @@ pub mod pallet {
 				stash,
 				total: value,
 				active: value,
-				unlocking: vec![],
+				unlocking: Default::default(),
 				claimed_rewards: (last_reward_era..current_era).collect(),
 			};
 			Self::update_ledger(&controller, &item);
@@ -837,7 +840,7 @@ pub mod pallet {
 		/// Once the unlock period is done, you can call `withdraw_unbonded` to actually move
 		/// the funds out of management ready for transfer.
 		///
-		/// No more than a limited number of unlocking chunks (see `MAX_UNLOCKING_CHUNKS`)
+		/// No more than a limited number of unlocking chunks (see `MaxUnlockingChunks`)
 		/// can co-exists at the same time. In that case, [`Call::withdraw_unbonded`] need
 		/// to be called first to remove some of the chunks (if possible).
 		///
@@ -854,7 +857,10 @@ pub mod pallet {
 		) -> DispatchResult {
 			let controller = ensure_signed(origin)?;
 			let mut ledger = Self::ledger(&controller).ok_or(Error::<T>::NotController)?;
-			ensure!(ledger.unlocking.len() < MAX_UNLOCKING_CHUNKS, Error::<T>::NoMoreChunks,);
+			ensure!(
+				ledger.unlocking.len() < MaxUnlockingChunks::get() as usize,
+				Error::<T>::NoMoreChunks,
+			);
 
 			let mut value = value.min(ledger.active);
 
@@ -881,7 +887,19 @@ pub mod pallet {
 
 				// Note: in case there is no current era it is fine to bond one era more.
 				let era = Self::current_era().unwrap_or(0) + T::BondingDuration::get();
-				ledger.unlocking.push(UnlockChunk { value, era });
+				if let Some(mut chunk) =
+					ledger.unlocking.last_mut().filter(|chunk| chunk.era == era)
+				{
+					// To keep the chunk count down, we only keep one chunk per era. Since
+					// `unlocking` is a FiFo queue, if a chunk exists for `era` we know that it will
+					// be the last one.
+					chunk.value = chunk.value.defensive_saturating_add(value)
+				} else {
+					ledger
+						.unlocking
+						.try_push(UnlockChunk { value, era })
+						.map_err(|_| Error::<T>::NoMoreChunks)?;
+				};
 				// NOTE: ledger must be updated prior to calling `Self::weight_of`.
 				Self::update_ledger(&controller, &ledger);
 
@@ -1348,10 +1366,10 @@ pub mod pallet {
 		///
 		/// # <weight>
 		/// - Time complexity: O(L), where L is unlocking chunks
-		/// - Bounded by `MAX_UNLOCKING_CHUNKS`.
+		/// - Bounded by `MaxUnlockingChunks`.
 		/// - Storage changes: Can't increase storage, only decrease it.
 		/// # </weight>
-		#[pallet::weight(T::WeightInfo::rebond(MAX_UNLOCKING_CHUNKS as u32))]
+		#[pallet::weight(T::WeightInfo::rebond(MaxUnlockingChunks::get() as u32))]
 		pub fn rebond(
 			origin: OriginFor<T>,
 			#[pallet::compact] value: BalanceOf<T>,