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/im-online/src/lib.rs

@@ -104,7 +104,7 @@ use sp_runtime::{
 	PerThing, Perbill, Permill, RuntimeDebug, SaturatedConversion,
 };
 use sp_staking::{
-	offence::{DisableStrategy, Kind, Offence, ReportOffence},
+	offence::{Kind, Offence, ReportOffence},
 	SessionIndex,
 };
 use sp_std::prelude::*;
@@ -847,10 +847,6 @@ impl<Offender: Clone> Offence<Offender> for UnresponsivenessOffence<Offender> {
 		self.session_index
 	}
 
-	fn disable_strategy(&self) -> DisableStrategy {
-		DisableStrategy::Never
-	}
-
 	fn slash_fraction(&self, offenders: u32) -> Perbill {
 		// the formula is min((3 * (k - (n / 10 + 1))) / n, 1) * 0.07
 		// basically, 10% can be offline with no slash, but after that, it linearly climbs up to 7%

substrate/frame/im-online/src/tests.rs

@@ -50,9 +50,6 @@ fn test_unresponsiveness_slash_fraction() {
 		dummy_offence.slash_fraction(17),
 		Perbill::from_parts(46200000), // 4.62%
 	);
-
-	// Offline offences should never lead to being disabled.
-	assert_eq!(dummy_offence.disable_strategy(), DisableStrategy::Never);
 }
 
 #[test]