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/statement-table/src/generic.rs

@@ -30,7 +30,10 @@ use std::{
 	hash::Hash,
 };
 
-use primitives::{ValidatorSignature, ValidityAttestation as PrimitiveValidityAttestation};
+use primitives::{
+	effective_minimum_backing_votes, ValidatorSignature,
+	ValidityAttestation as PrimitiveValidityAttestation,
+};
 
 use parity_scale_codec::{Decode, Encode};
 
@@ -57,8 +60,8 @@ pub trait Context {
 	/// Members are meant to submit candidates and vote on validity.
 	fn is_member_of(&self, authority: &Self::AuthorityId, group: &Self::GroupId) -> bool;
 
-	/// requisite number of votes for validity from a group.
-	fn requisite_votes(&self, group: &Self::GroupId) -> usize;
+	/// Get a validator group size.
+	fn get_group_size(&self, group: &Self::GroupId) -> Option<usize>;
 }
 
 /// Table configuration.
@@ -319,9 +322,12 @@ impl<Ctx: Context> Table<Ctx> {
 		&self,
 		digest: &Ctx::Digest,
 		context: &Ctx,
+		minimum_backing_votes: u32,
 	) -> Option<AttestedCandidate<Ctx::GroupId, Ctx::Candidate, Ctx::AuthorityId, Ctx::Signature>> {
 		self.candidate_votes.get(digest).and_then(|data| {
-			let v_threshold = context.requisite_votes(&data.group_id);
+			let v_threshold = context.get_group_size(&data.group_id).map_or(usize::MAX, |len| {
+				effective_minimum_backing_votes(len, minimum_backing_votes)
+			});
 			data.attested(v_threshold)
 		})
 	}
@@ -636,16 +642,13 @@ mod tests {
 			self.authorities.get(authority).map(|v| v == group).unwrap_or(false)
 		}
 
-		fn requisite_votes(&self, id: &GroupId) -> usize {
-			let mut total_validity = 0;
-
-			for validity in self.authorities.values() {
-				if validity == id {
-					total_validity += 1
-				}
+		fn get_group_size(&self, group: &Self::GroupId) -> Option<usize> {
+			let count = self.authorities.values().filter(|g| *g == group).count();
+			if count == 0 {
+				None
+			} else {
+				Some(count)
 			}
-
-			total_validity / 2 + 1
 		}
 	}
 
@@ -910,7 +913,7 @@ mod tests {
 		table.import_statement(&context, statement);
 
 		assert!(!table.detected_misbehavior.contains_key(&AuthorityId(1)));
-		assert!(table.attested_candidate(&candidate_digest, &context).is_none());
+		assert!(table.attested_candidate(&candidate_digest, &context, 2).is_none());
 
 		let vote = SignedStatement {
 			statement: Statement::Valid(candidate_digest),
@@ -920,7 +923,7 @@ mod tests {
 
 		table.import_statement(&context, vote);
 		assert!(!table.detected_misbehavior.contains_key(&AuthorityId(2)));
-		assert!(table.attested_candidate(&candidate_digest, &context).is_some());
+		assert!(table.attested_candidate(&candidate_digest, &context, 2).is_some());
 	}
 
 	#[test]