babe: report equivocations (#6362)

* slots: create primitives crate for consensus slots

* offences: add method to check if an offence is unknown

* babe: initial equivocation reporting implementation

* babe: organize imports

* babe: working equivocation reporting

* babe: add slot number to equivocation proof

* session: move duplicate traits to session primitives

* babe: move equivocation stuff to its own file

* offences: fix test

* session: don't have primitives depend on frame_support

* babe: use opaque type for key owner proof

* babe: cleanup client equivocation reporting

* babe: cleanup equivocation code in pallet

* babe: allow sending signed equivocation reports

* node: fix compilation

* fix test compilation

* babe: return bool on check_equivocation_proof

* babe: add test for equivocation reporting

* babe: add more tests

* babe: add test for validate unsigned

* babe: take slot number in generate_key_ownership_proof API

* babe: add benchmark for equivocation proof checking

* session: add benchmark for membership proof checking

* offences: fix babe benchmark

* babe: add weights based on benchmark results

* babe: adjust weights after benchmarking on reference hardware

* babe: reorder checks in check_and_report_equivocation

substrate/primitives/staking/src/offence.rs

@@ -117,10 +117,21 @@ impl sp_runtime::traits::Printable for OffenceError {
 pub trait ReportOffence<Reporter, Offender, O: Offence<Offender>> {
 	/// Report an `offence` and reward given `reporters`.
 	fn report_offence(reporters: Vec<Reporter>, offence: O) -> Result<(), OffenceError>;
+
+	/// Returns true iff all of the given offenders have been previously reported
+	/// at the given time slot. This function is useful to prevent the sending of
+	/// duplicate offence reports.
+	fn is_known_offence(offenders: &[Offender], time_slot: &O::TimeSlot) -> bool;
 }
 
 impl<Reporter, Offender, O: Offence<Offender>> ReportOffence<Reporter, Offender, O> for () {
-	fn report_offence(_reporters: Vec<Reporter>, _offence: O) -> Result<(), OffenceError> { Ok(()) }
+	fn report_offence(_reporters: Vec<Reporter>, _offence: O) -> Result<(), OffenceError> {
+		Ok(())
+	}
+
+	fn is_known_offence(_offenders: &[Offender], _time_slot: &O::TimeSlot) -> bool {
+		true
+	}
 }
 
 /// A trait to take action on an offence.