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/core/sr-primitives/src/lib.rs

@@ -298,6 +298,18 @@ impl Perbill {
 
 		Perbill(part as u32)
 	}
+
+	/// Return the product of multiplication of this value by itself.
+	pub fn square(self) -> Self {
+		let p: u64 = self.0 as u64 * self.0 as u64;
+		let q: u64 = 1_000_000_000 * 1_000_000_000;
+		Self::from_rational_approximation(p, q)
+	}
+
+	/// Take out the raw parts-per-billions.
+	pub fn into_parts(self) -> u32 {
+		self.0
+	}
 }
 
 impl<N> ops::Mul<N> for Perbill
@@ -959,4 +971,23 @@ mod tests {
 			((Into::<U256>::into(std::u128::MAX) * 999_999u32) / 1_000_000u32).as_u128()
 		);
 	}
+
+	#[test]
+	fn per_bill_square() {
+		const FIXTURES: &[(u32, u32)] = &[
+			(0, 0),
+			(1250000, 1562), // (0.00125, 0.000001562)
+			(255300000, 65178090), // (0.2553, 0.06517809)
+			(500000000, 250000000), // (0.5, 0.25)
+			(999995000, 999990000), // (0.999995, 0.999990000, but ideally 0.99999000002)
+			(1000000000, 1000000000),
+		];
+
+		for &(x, r) in FIXTURES {
+			assert_eq!(
+				Perbill::from_parts(x).square(),
+				Perbill::from_parts(r),
+			);
+		}
+	}
 }