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/frame/babe/src/lib.rs

@@ -19,8 +19,8 @@
 
 #![cfg_attr(not(feature = "std"), no_std)]
 #![forbid(unused_must_use, unsafe_code, unused_variables, unused_must_use)]
-#![deny(unused_imports)]
-pub use pallet_timestamp;
+
+use pallet_timestamp;
 
 use sp_std::{result, prelude::*};
 use frame_support::{
@@ -29,7 +29,7 @@ use frame_support::{
 };
 use sp_timestamp::OnTimestampSet;
 use sp_runtime::{generic::DigestItem, ConsensusEngineId, Perbill, PerThing};
-use sp_runtime::traits::{IsMember, SaturatedConversion, Saturating, Hash};
+use sp_runtime::traits::{IsMember, SaturatedConversion, Saturating, Hash, One};
 use sp_staking::{
 	SessionIndex,
 	offence::{Offence, Kind},
@@ -307,12 +307,34 @@ impl<T: Trait> Module<T> {
 		// epoch 0 as having started at the slot of block 1. We want to use
 		// the same randomness and validator set as signalled in the genesis,
 		// so we don't rotate the epoch.
-		now != sp_runtime::traits::One::one() && {
+		now != One::one() && {
 			let diff = CurrentSlot::get().saturating_sub(Self::current_epoch_start());
 			diff >= T::EpochDuration::get()
 		}
 	}
 
+	/// Return the _best guess_ block number, at which the next epoch change is predicted to happen.
+	///
+	/// Returns None if the prediction is in the past; This implies an error internally in the Babe
+	/// and should not happen under normal circumstances.
+	///
+	/// In other word, this is only accurate if no slots are missed. Given missed slots, the slot
+	/// number will grow while the block number will not. Hence, the result can be interpreted as an
+	/// upper bound.
+	// -------------- IMPORTANT NOTE --------------
+	// This implementation is linked to how [`should_epoch_change`] is working. This might need to
+	// be updated accordingly, if the underlying mechanics of slot and epochs change.
+	pub fn next_expected_epoch_change(now: T::BlockNumber) -> Option<T::BlockNumber> {
+		let next_slot = Self::current_epoch_start().saturating_add(T::EpochDuration::get());
+		next_slot
+			.checked_sub(CurrentSlot::get())
+			.map(|slots_remaining| {
+				// This is a best effort guess. Drifts in the slot/block ratio will cause errors here.
+				let blocks_remaining: T::BlockNumber = slots_remaining.saturated_into();
+				now.saturating_add(blocks_remaining)
+			})
+	}
+
 	/// DANGEROUS: Enact an epoch change. Should be done on every block where `should_epoch_change` has returned `true`,
 	/// and the caller is the only caller of this function.
 	///
@@ -324,10 +346,7 @@ impl<T: Trait> Module<T> {
 	) {
 		// PRECONDITION: caller has done initialization and is guaranteed
 		// by the session module to be called before this.
-		#[cfg(debug_assertions)]
-		{
-			assert!(Self::initialized().is_some())
-		}
+		debug_assert!(Self::initialized().is_some());
 
 		// Update epoch index
 		let epoch_index = EpochIndex::get()
@@ -473,6 +492,12 @@ impl<T: Trait> OnTimestampSet<T::Moment> for Module<T> {
 	fn on_timestamp_set(_moment: T::Moment) { }
 }
 
+impl<T: Trait> frame_support::traits::EstimateNextSessionRotation<T::BlockNumber> for Module<T> {
+	fn estimate_next_session_rotation(now: T::BlockNumber) -> Option<T::BlockNumber> {
+		Self::next_expected_epoch_change(now)
+	}
+}
+
 impl<T: Trait> sp_runtime::BoundToRuntimeAppPublic for Module<T> {
 	type Public = AuthorityId;
 }