Offence implementations can disable offenders independently from slashing (#10201)

* Offence implementations can disable offenders independently from slashing

* Fix build on CI

* Run cargo fmt

* Fixes based on review comments

* Make parameter naming consistent

* Fix migration and some English

* Fix migration - again

* cargo fmt

* Cover 2 new cases with a test

substrate/primitives/staking/src/offence.rs

@@ -37,6 +37,29 @@ pub type Kind = [u8; 16];
 /// so that we can slash it accordingly.
 pub type OffenceCount = u32;
 
+/// In case of an offence, which conditions get an offending validator disabled.
+#[derive(
+	Clone,
+	Copy,
+	PartialEq,
+	Eq,
+	Hash,
+	PartialOrd,
+	Ord,
+	Encode,
+	Decode,
+	sp_runtime::RuntimeDebug,
+	scale_info::TypeInfo,
+)]
+pub enum DisableStrategy {
+	/// Independently of slashing, this offence will not disable the offender.
+	Never,
+	/// Only disable the offender if it is also slashed.
+	WhenSlashed,
+	/// Independently of slashing, this offence will always disable the offender.
+	Always,
+}
+
 /// A trait implemented by an offence report.
 ///
 /// This trait assumes that the offence is legitimate and was validated already.
@@ -79,6 +102,11 @@ pub trait Offence<Offender> {
 	/// number. Note that for GRANDPA the round number is reset each epoch.
 	fn time_slot(&self) -> Self::TimeSlot;
 
+	/// In which cases this offence needs to disable offenders until the next era starts.
+	fn disable_strategy(&self) -> DisableStrategy {
+		DisableStrategy::WhenSlashed
+	}
+
 	/// A slash fraction of the total exposure that should be slashed for this
 	/// particular offence kind for the given parameters that happened at a singular `TimeSlot`.
 	///
@@ -150,12 +178,15 @@ pub trait OnOffenceHandler<Reporter, Offender, Res> {
 	///
 	/// The `session` parameter is the session index of the offence.
 	///
+	/// The `disable_strategy` parameter decides if the offenders need to be disabled immediately.
+	///
 	/// The receiver might decide to not accept this offence. In this case, the call site is
 	/// responsible for queuing the report and re-submitting again.
 	fn on_offence(
 		offenders: &[OffenceDetails<Reporter, Offender>],
 		slash_fraction: &[Perbill],
 		session: SessionIndex,
+		disable_strategy: DisableStrategy,
 	) -> Res;
 }
 
@@ -164,6 +195,7 @@ impl<Reporter, Offender, Res: Default> OnOffenceHandler<Reporter, Offender, Res>
 		_offenders: &[OffenceDetails<Reporter, Offender>],
 		_slash_fraction: &[Perbill],
 		_session: SessionIndex,
+		_disable_strategy: DisableStrategy,
 	) -> Res {
 		Default::default()
 	}