Offchain Phragmén BREAKING. (#4517)

* Initial skeleton for offchain phragmen

* Basic compact encoding decoding for results

* add compact files

* Bring back Self::ensure_storage_upgraded();

* Make staking use compact stuff.

* First seemingly working version of reduce, full of todos

* Everything phragmen related works again.

* Signing made easier, still issues.

* Signing from offchain compile fine 😎

* make compact work with staked asssignment

* Evaluation basics are in place.

* Move reduce into crate. Document stuff

* move reduce into no_std

* Add files

* Remove other std deps. Runtime compiles

* Seemingly it is al stable; cycle implemented but not integrated.

* Add fuzzing code.

* Cleanup reduce a bit more.

* a metric ton of tests for staking; wip 🔨

* Implement a lot more of the tests.

* wip getting the unsigned stuff to work

* A bit gleanup for unsigned debug

* Clean and finalize compact code.

* Document reduce.

* Still problems with signing

* We officaly duct taped the transaction submission stuff. 🤓

* Deadlock with keys again

* Runtime builds

* Unsigned test works 🙌

* Some cleanups

* Make all the tests compile and stuff

* Minor cleanup

* fix more merge stuff

* Most tests work again.

* a very nasty bug in reduce

* Fix all integrations

* Fix more todos

* Revamp everything and everything

* Remove bogus test

* Some review grumbles.

* Some fixes

* Fix doc test

* loop for submission

* Fix cli, keyring etc.

* some cleanup

* Fix staking tests again

* fix per-things; bring patches from benchmarking

* better score prediction

* Add fuzzer, more patches.

* Some fixes

* More docs

* Remove unused generics

* Remove max-nominator footgun

* Better fuzzer

* Disable it ❌

* Bump.

* Another round of self-review

* Refactor a lot

* More major fixes in perThing

* Add new fuzz file

* Update lock

* fix fuzzing code.

* Fix nominator retain test

* Add slashing check

* Update frame/staking/src/tests.rs

Co-Authored-By: Joshy Orndorff <JoshOrndorff@users.noreply.github.com>

* Some formatting nits

* Review comments.

* Fix cargo file

* Almost all tests work again

* Update frame/staking/src/tests.rs

Co-Authored-By: thiolliere <gui.thiolliere@gmail.com>

* Fix review comments

* More review stuff

* Some nits

* Fix new staking / session / babe relation

* Update primitives/phragmen/src/lib.rs

Co-Authored-By: thiolliere <gui.thiolliere@gmail.com>

* Update primitives/phragmen/src/lib.rs

Co-Authored-By: thiolliere <gui.thiolliere@gmail.com>

* Update primitives/phragmen/compact/src/lib.rs

Co-Authored-By: thiolliere <gui.thiolliere@gmail.com>

* Some doc updates to slashing

* Fix derive

* Remove imports

* Remove unimplemented tests

* nits

* Remove dbg

* Better fuzzing params

* Remove unused pref map

* Deferred Slashing/Offence for offchain Phragmen  (#5151)

* Some boilerplate

* Add test

* One more test

* Review comments

* Fix build

* review comments

* fix more

* fix build

* Some cleanups and self-reviews

* More minor self reviews

* Final nits

* Some merge fixes.

* opt comment

* Fix build

* Fix build again.

* Update frame/staking/fuzz/fuzz_targets/submit_solution.rs

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

* Update frame/staking/src/slashing.rs

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

* Update frame/staking/src/offchain_election.rs

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

* Fix review comments

* fix test

* === 🔑 Revamp without staking key.

* final round of changes.

* Fix cargo-deny

* Update frame/staking/src/lib.rs

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

Co-authored-by: Joshy Orndorff <JoshOrndorff@users.noreply.github.com>
Co-authored-by: thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Gavin Wood <gavin@parity.io>

substrate/primitives/staking/src/offence.rs

@@ -142,11 +142,20 @@ pub trait OnOffenceHandler<Reporter, Offender> {
 	/// Zero is a valid value for a fraction.
 	///
 	/// The `session` parameter is the session index of the offence.
+	///
+	/// 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,
-	);
+	) -> Result<(), ()>;
+
+	/// Can an offence be reported now or not. This is an method to short-circuit a call into
+	/// `on_offence`. Ideally, a correct implementation should return `false` if `on_offence` will
+	/// return `Err`. Nonetheless, this is up to the implementation and this trait cannot guarantee
+	/// it.
+	fn can_report() -> bool;
 }
 
 impl<Reporter, Offender> OnOffenceHandler<Reporter, Offender> for () {
@@ -154,7 +163,9 @@ impl<Reporter, Offender> OnOffenceHandler<Reporter, Offender> for () {
 		_offenders: &[OffenceDetails<Reporter, Offender>],
 		_slash_fraction: &[Perbill],
 		_session: SessionIndex,
-	) {}
+	) -> Result<(), ()> { Ok(()) }
+
+	fn can_report() -> bool { true }
 }
 
 /// A details about an offending authority for a particular kind of offence.