Implementation of the new validator disabling strategy (#2226)

Closes https://github.com/paritytech/polkadot-sdk/issues/1966,
https://github.com/paritytech/polkadot-sdk/issues/1963 and
https://github.com/paritytech/polkadot-sdk/issues/1962.

Disabling strategy specification
[here](https://github.com/paritytech/polkadot-sdk/pull/2955). (Updated
13/02/2024)

Implements:
* validator disabling for a whole era instead of just a session
* no more than 1/3 of the validators in the active set are disabled
Removes:
* `DisableStrategy` enum - now each validator committing an offence is
disabled.
* New era is not forced if too many validators are disabled.

Before this PR not all offenders were disabled. A decision was made
based on [`enum
DisableStrategy`](https://github.com/paritytech/polkadot-sdk/blob/bbb6631641f9adba30c0ee6f4d11023a424dd362/substrate/primitives/staking/src/offence.rs#L54).
Some offenders were disabled for a whole era, some just for a session,
some were not disabled at all.

This PR changes the disabling behaviour. Now a validator committing an
offense is disabled immediately till the end of the current era.

Some implementation notes:
* `OffendingValidators` in pallet session keeps all offenders (this is
not changed). However its type is changed from `Vec<(u32, bool)>` to
`Vec<u32>`. The reason is simple - each offender is getting disabled so
the bool doesn't make sense anymore.
* When a validator is disabled it is first added to
`OffendingValidators` and then to `DisabledValidators`. This is done in
[`add_offending_validator`](https://github.com/paritytech/polkadot-sdk/blob/bbb6631641f9adba30c0ee6f4d11023a424dd362/substrate/frame/staking/src/slashing.rs#L325)
from staking pallet.
* In
[`rotate_session`](https://github.com/paritytech/polkadot-sdk/blob/bdbe98297032e21a553bf191c530690b1d591405/substrate/frame/session/src/lib.rs#L623)
the `end_session` also calls
[`end_era`](https://github.com/paritytech/polkadot-sdk/blob/bbb6631641f9adba30c0ee6f4d11023a424dd362/substrate/frame/staking/src/pallet/impls.rs#L490)
when an era ends. In this case `OffendingValidators` are cleared
**(1)**.
* Then in
[`rotate_session`](https://github.com/paritytech/polkadot-sdk/blob/bdbe98297032e21a553bf191c530690b1d591405/substrate/frame/session/src/lib.rs#L623)
`DisabledValidators` are cleared **(2)**
* And finally (still in `rotate_session`) a call to
[`start_session`](https://github.com/paritytech/polkadot-sdk/blob/bbb6631641f9adba30c0ee6f4d11023a424dd362/substrate/frame/staking/src/pallet/impls.rs#L430)
repopulates the disabled validators **(3)**.
* The reason for this complication is that session pallet knows nothing
abut eras. To overcome this on each new session the disabled list is
repopulated (points 2 and 3). Staking pallet knows when a new era starts
so with point 1 it ensures that the offenders list is cleared.

---------

Co-authored-by: ordian <noreply@reusable.software>
Co-authored-by: ordian <write@reusable.software>
Co-authored-by: Maciej <maciej.zyszkiewicz@parity.io>
Co-authored-by: Gonçalo Pestana <g6pestana@gmail.com>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: command-bot <>
Co-authored-by: Ankan <10196091+Ank4n@users.noreply.github.com>

substrate/frame/offences/benchmarking/src/mock.rs

@@ -174,7 +174,6 @@ impl pallet_staking::Config for Test {
 	type EraPayout = pallet_staking::ConvertCurve<RewardCurve>;
 	type NextNewSession = Session;
 	type MaxExposurePageSize = ConstU32<64>;
-	type OffendingValidatorsThreshold = ();
 	type ElectionProvider = onchain::OnChainExecution<OnChainSeqPhragmen>;
 	type GenesisElectionProvider = Self::ElectionProvider;
 	type VoterList = pallet_staking::UseNominatorsAndValidatorsMap<Self>;
@@ -186,6 +185,7 @@ impl pallet_staking::Config for Test {
 	type EventListeners = ();
 	type BenchmarkingConfig = pallet_staking::TestBenchmarkingConfig;
 	type WeightInfo = ();
+	type DisablingStrategy = pallet_staking::UpToLimitDisablingStrategy;
 }
 
 impl pallet_im_online::Config for Test {

substrate/frame/offences/src/lib.rs

@@ -132,7 +132,6 @@ where
 			&concurrent_offenders,
 			&slash_perbill,
 			offence.session_index(),
-			offence.disable_strategy(),
 		);
 
 		// Deposit the event.

substrate/frame/offences/src/migration.rs

@@ -23,7 +23,7 @@ use frame_support::{
 	weights::Weight,
 	Twox64Concat,
 };
-use sp_staking::offence::{DisableStrategy, OnOffenceHandler};
+use sp_staking::offence::OnOffenceHandler;
 use sp_std::vec::Vec;
 
 #[cfg(feature = "try-runtime")]
@@ -106,12 +106,7 @@ pub fn remove_deferred_storage<T: Config>() -> Weight {
 	let deferred = <DeferredOffences<T>>::take();
 	log::info!(target: LOG_TARGET, "have {} deferred offences, applying.", deferred.len());
 	for (offences, perbill, session) in deferred.iter() {
-		let consumed = T::OnOffenceHandler::on_offence(
-			offences,
-			perbill,
-			*session,
-			DisableStrategy::WhenSlashed,
-		);
+		let consumed = T::OnOffenceHandler::on_offence(offences, perbill, *session);
 		weight = weight.saturating_add(consumed);
 	}
 

substrate/frame/offences/src/mock.rs

@@ -33,7 +33,7 @@ use sp_runtime::{
 	BuildStorage, Perbill,
 };
 use sp_staking::{
-	offence::{self, DisableStrategy, Kind, OffenceDetails},
+	offence::{self, Kind, OffenceDetails},
 	SessionIndex,
 };
 
@@ -51,7 +51,6 @@ impl<Reporter, Offender> offence::OnOffenceHandler<Reporter, Offender, Weight>
 		_offenders: &[OffenceDetails<Reporter, Offender>],
 		slash_fraction: &[Perbill],
 		_offence_session: SessionIndex,
-		_disable_strategy: DisableStrategy,
 	) -> Weight {
 		OnOffencePerbill::mutate(|f| {
 			*f = slash_fraction.to_vec();