backing: move the min votes threshold to the runtime (#1200)

* move min backing votes const to runtime

also cache it per-session in the backing subsystem

Signed-off-by: alindima <alin@parity.io>

* add runtime migration

* introduce api versioning for min_backing votes

also enable it for rococo/versi for testing

* also add min_backing_votes runtime calls to statement-distribution

this dependency has been recently introduced by async backing

* remove explicit version runtime API call

this is not needed, as the RuntimeAPISubsystem already takes care
of versioning and will return NotSupported if the version is not
right.

* address review comments

- parametrise backing votes runtime API with session index
- remove RuntimeInfo usage in backing subsystem, as runtime API
caches the min backing votes by session index anyway.
- move the logic for adjusting the configured needed backing votes with the size of the backing group
to a primitives helper.
- move the legacy min backing votes value to a primitives helper.
- mark JoinMultiple error as fatal, since the Canceled (non-multiple) counterpart is also fatal.
- make backing subsystem handle fatal errors for new leaves update.
- add HostConfiguration consistency check for zeroed backing votes threshold
- add cumulus accompanying change

* fix cumulus test compilation

* fix tests

* more small fixes

* fix merge

* bump runtime api version for westend and rollback version for rococo

---------

Signed-off-by: alindima <alin@parity.io>
Co-authored-by: Javier Viola <javier@parity.io>

polkadot/node/core/runtime-api/src/cache.rs

@@ -65,6 +65,7 @@ pub(crate) struct RequestResultCache {
 		LruMap<Hash, Vec<(SessionIndex, CandidateHash, vstaging::slashing::PendingSlashes)>>,
 	key_ownership_proof:
 		LruMap<(Hash, ValidatorId), Option<vstaging::slashing::OpaqueKeyOwnershipProof>>,
+	minimum_backing_votes: LruMap<SessionIndex, u32>,
 
 	staging_para_backing_state: LruMap<(Hash, ParaId), Option<vstaging::BackingState>>,
 	staging_async_backing_params: LruMap<Hash, vstaging::AsyncBackingParams>,
@@ -97,6 +98,7 @@ impl Default for RequestResultCache {
 			disputes: LruMap::new(ByLength::new(DEFAULT_CACHE_CAP)),
 			unapplied_slashes: LruMap::new(ByLength::new(DEFAULT_CACHE_CAP)),
 			key_ownership_proof: LruMap::new(ByLength::new(DEFAULT_CACHE_CAP)),
+			minimum_backing_votes: LruMap::new(ByLength::new(DEFAULT_CACHE_CAP)),
 
 			staging_para_backing_state: LruMap::new(ByLength::new(DEFAULT_CACHE_CAP)),
 			staging_async_backing_params: LruMap::new(ByLength::new(DEFAULT_CACHE_CAP)),
@@ -434,6 +436,18 @@ impl RequestResultCache {
 		None
 	}
 
+	pub(crate) fn minimum_backing_votes(&mut self, session_index: SessionIndex) -> Option<u32> {
+		self.minimum_backing_votes.get(&session_index).copied()
+	}
+
+	pub(crate) fn cache_minimum_backing_votes(
+		&mut self,
+		session_index: SessionIndex,
+		minimum_backing_votes: u32,
+	) {
+		self.minimum_backing_votes.insert(session_index, minimum_backing_votes);
+	}
+
 	pub(crate) fn staging_para_backing_state(
 		&mut self,
 		key: (Hash, ParaId),
@@ -469,6 +483,7 @@ pub(crate) enum RequestResult {
 	// The structure of each variant is (relay_parent, [params,]*, result)
 	Authorities(Hash, Vec<AuthorityDiscoveryId>),
 	Validators(Hash, Vec<ValidatorId>),
+	MinimumBackingVotes(Hash, SessionIndex, u32),
 	ValidatorGroups(Hash, (Vec<Vec<ValidatorIndex>>, GroupRotationInfo)),
 	AvailabilityCores(Hash, Vec<CoreState>),
 	PersistedValidationData(Hash, ParaId, OccupiedCoreAssumption, Option<PersistedValidationData>),