Offences reporting and slashing (#3322)

* Remove offline slashing logic from staking.

* Initial version of reworked offence module, can report offences

* Clean up staking example.

* Commit SlashingOffence

* Force new era on slash.

* Add offenders in the SlashingOffence trait.

* Introduce the ReportOffence trait.

* Rename `Offence`.

* Add on_before_session_ending handler.

* Move offence related stuff under sr-primitives.

* Fix cargo check.

* Import new im-online implementation.

* Adding validator count to historical session storage as it's needed for slash calculations

* Add a comment about offence.

* Add BabeEquivocationOffence

* GrandpaEquivocationOffence

* slash_fraction and fix

* current_era_start_session_index

* UnresponsivnessOffence

* Finalise OnOffenceHandler traits, and stub impl for staking.

* slash_fraction doesn't really need &self

* Note that offenders count is greater than 0

* Add a test to ensure that I got the math right

* Use FullIdentification in offences.

* Use FullIndentification.

* Hook up the offences module.

* Report unresponsive validators

* Make sure eras have the same length.

* Slashing and rewards.

* Fix compilation.

* Distribute rewards.

* Supply validators_count

* Use identificationTuple in Unresponsivness report

* Fix merge.

* Make sure we don't slash if amount is zero.

* We don't return an error from report_offence anymo

* We actually can use vec!

* Prevent division by zero if the reporters is empty

* offence_forces_new_era/nominators_also_get_slashed

* advance_session

* Fix tests.

* Update srml/staking/src/lib.rs

Co-Authored-By: Robert Habermeier <rphmeier@gmail.com>

* slashing_performed_according_exposure

* Check that reporters receive their slice.

* Small clean-up.

* invulnerables_are_not_slashed

* Minor clean ups.

* Improve docs.

* dont_slash_if_fraction_is_zero

* Remove session dependency from offences.

* Introduce sr-staking-primitives

* Move offence under sr_staking_primitives

* rename session_index

* Resolves todos re using SessionIndex

* Fix staking tests.

* Properly scale denominator.

* Fix UnresponsivnessOffence

* Fix compilation.

* Tests for offences.

* Clean offences tests.

* Fix staking doc test.

* Bump spec version

* Fix aura tests.

* Fix node_executor

* Deposit an event on offence.

* Fix compilation of node-runtime

* Remove aura slashing logic.

* Remove HandleReport

* Update docs for timeslot.

* rename with_on_offence_fractions

* Add should_properly_count_offences

* Replace ValidatorIdByIndex with CurrentElectedSet

ValidatorIdByIndex was querying the current_elected set in each call, doing loading (even though its from cache), deserializing and cloning of element.

Instead of this it is more efficient to use `CurrentElectedSet`. As a small bonus, the invariant became a little bit easier: now we just rely on the fact that `keys` and `current_elected` set are of the same length rather than relying on the fact that `validator_id_by_index` would work similar to `<[T]>::get`.

* Clarify babe equivocation

* Fix offences.

* Rename validators_count to validator_set_count

* Fix squaring.

* Update core/sr-staking-primitives/src/offence.rs

Co-Authored-By: Gavin Wood <gavin@parity.io>

* Docs for CurrentElectedSet.

* Don't punish only invulnerables

* Use `get/insert` instead of `mutate`.

* Fix compilation

* Update core/sr-staking-primitives/src/offence.rs

Co-Authored-By: Gavin Wood <gavin@parity.io>

* Update srml/offences/src/lib.rs

Co-Authored-By: Robert Habermeier <rphmeier@gmail.com>

* Update srml/im-online/src/lib.rs

Co-Authored-By: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

* Update srml/im-online/src/lib.rs

Co-Authored-By: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

* Update srml/im-online/src/lib.rs

Co-Authored-By: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

* Update srml/babe/src/lib.rs

Co-Authored-By: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

* Update core/sr-staking-primitives/src/offence.rs

Co-Authored-By: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

* Update core/sr-staking-primitives/src/offence.rs

Co-Authored-By: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

* Update core/sr-staking-primitives/src/offence.rs

Co-Authored-By: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

* Update core/sr-staking-primitives/src/offence.rs

Co-Authored-By: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

* Update core/sr-staking-primitives/src/offence.rs

Co-Authored-By: joe petrowski <25483142+joepetrowski@users.noreply.github.com>

* Add aura todo.

* Allow multiple reports for single offence report.

* Fix slash_fraction calculation.

* Fix typos.

* Fix compilation and tests.

* Fix staking tests.

* Update srml/im-online/src/lib.rs

Co-Authored-By: Logan Saether <x@logansaether.com>

* Fix doc on time_slot

* Allow slashing only on current era (#3411)

* only slash in current era

* prune journal for last era

* comment own_slash

* emit an event when old slashing events are discarded

* Pave the way for pruning

* Address issues.

* Try to refactor collect_offence_reports

* Other fixes.

* More fixes.

substrate/srml/babe/src/lib.rs

@@ -18,15 +18,22 @@
 //! from VRF outputs and manages epoch transitions.
 
 #![cfg_attr(not(feature = "std"), no_std)]
-#![forbid(unused_must_use, unsafe_code, unused_variables, dead_code)]
+#![forbid(unused_must_use, unsafe_code, unused_variables)]
+
+// TODO: @marcio uncomment this when BabeEquivocation is integrated.
+// #![forbid(dead_code)]
 
 pub use timestamp;
 
 use rstd::{result, prelude::*};
 use srml_support::{decl_storage, decl_module, StorageValue, StorageMap, traits::FindAuthor, traits::Get};
 use timestamp::{OnTimestampSet};
-use sr_primitives::{generic::DigestItem, ConsensusEngineId};
+use sr_primitives::{generic::DigestItem, ConsensusEngineId, Perbill};
 use sr_primitives::traits::{IsMember, SaturatedConversion, Saturating, RandomnessBeacon};
+use sr_staking_primitives::{
+	SessionIndex,
+	offence::{Offence, Kind},
+};
 use sr_primitives::weights::SimpleDispatchInfo;
 #[cfg(feature = "std")]
 use timestamp::TimestampInherentData;
@@ -282,6 +289,53 @@ impl<T: Trait> session::ShouldEndSession<T::BlockNumber> for Module<T> {
 	}
 }
 
+// TODO [slashing]: @marcio use this, remove the dead_code annotation.
+/// A BABE equivocation offence report.
+///
+/// When a validator released two or more blocks at the same slot.
+#[allow(dead_code)]
+struct BabeEquivocationOffence<FullIdentification> {
+	/// A babe slot number in which this incident happened.
+	slot: u64,
+	/// The session index in which the incident happened.
+	session_index: SessionIndex,
+	/// The size of the validator set at the time of the offence.
+	validator_set_count: u32,
+	/// The authority that produced the equivocation.
+	offender: FullIdentification,
+}
+
+impl<FullIdentification: Clone> Offence<FullIdentification> for BabeEquivocationOffence<FullIdentification> {
+	const ID: Kind = *b"babe:equivocatio";
+	type TimeSlot = u64;
+
+	fn offenders(&self) -> Vec<FullIdentification> {
+		vec![self.offender.clone()]
+	}
+
+	fn session_index(&self) -> SessionIndex {
+		self.session_index
+	}
+
+	fn validator_set_count(&self) -> u32 {
+		self.validator_set_count
+	}
+
+	fn time_slot(&self) -> Self::TimeSlot {
+		self.slot
+	}
+
+	fn slash_fraction(
+		offenders_count: u32,
+		validator_set_count: u32,
+	) -> Perbill {
+		// the formula is min((3k / n)^2, 1)
+		let x = Perbill::from_rational_approximation(3 * offenders_count, validator_set_count);
+		// _ ^ 2
+		x.square()
+	}
+}
+
 impl<T: Trait> Module<T> {
 	/// Determine the BABE slot duration based on the Timestamp module configuration.
 	pub fn slot_duration() -> T::Moment {