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/primitives/src/lib.rs

@@ -34,19 +34,19 @@ pub mod runtime_api;
 // Current primitives not requiring versioning are exported here.
 // Primitives requiring versioning must not be exported and must be referred by an exact version.
 pub use v5::{
-	byzantine_threshold, check_candidate_backing, collator_signature_payload, metric_definitions,
-	slashing, supermajority_threshold, well_known_keys, AbridgedHostConfiguration,
-	AbridgedHrmpChannel, AccountId, AccountIndex, AccountPublic, ApprovalVote, AssignmentId,
-	AuthorityDiscoveryId, AvailabilityBitfield, BackedCandidate, Balance, BlakeTwo256, Block,
-	BlockId, BlockNumber, CandidateCommitments, CandidateDescriptor, CandidateEvent, CandidateHash,
-	CandidateIndex, CandidateReceipt, CheckedDisputeStatementSet, CheckedMultiDisputeStatementSet,
-	CollatorId, CollatorSignature, CommittedCandidateReceipt, CompactStatement, ConsensusLog,
-	CoreIndex, CoreOccupied, CoreState, DisputeState, DisputeStatement, DisputeStatementSet,
-	DownwardMessage, EncodeAs, ExecutorParam, ExecutorParams, ExecutorParamsHash,
-	ExplicitDisputeStatement, GroupIndex, GroupRotationInfo, Hash, HashT, HeadData, Header,
-	HrmpChannelId, Id, InboundDownwardMessage, InboundHrmpMessage, IndexedVec, InherentData,
-	InvalidDisputeStatementKind, Moment, MultiDisputeStatementSet, Nonce, OccupiedCore,
-	OccupiedCoreAssumption, OutboundHrmpMessage, ParathreadClaim, ParathreadEntry,
+	byzantine_threshold, check_candidate_backing, collator_signature_payload,
+	effective_minimum_backing_votes, metric_definitions, slashing, supermajority_threshold,
+	well_known_keys, AbridgedHostConfiguration, AbridgedHrmpChannel, AccountId, AccountIndex,
+	AccountPublic, ApprovalVote, AssignmentId, AuthorityDiscoveryId, AvailabilityBitfield,
+	BackedCandidate, Balance, BlakeTwo256, Block, BlockId, BlockNumber, CandidateCommitments,
+	CandidateDescriptor, CandidateEvent, CandidateHash, CandidateIndex, CandidateReceipt,
+	CheckedDisputeStatementSet, CheckedMultiDisputeStatementSet, CollatorId, CollatorSignature,
+	CommittedCandidateReceipt, CompactStatement, ConsensusLog, CoreIndex, CoreOccupied, CoreState,
+	DisputeState, DisputeStatement, DisputeStatementSet, DownwardMessage, EncodeAs, ExecutorParam,
+	ExecutorParams, ExecutorParamsHash, ExplicitDisputeStatement, GroupIndex, GroupRotationInfo,
+	Hash, HashT, HeadData, Header, HrmpChannelId, Id, InboundDownwardMessage, InboundHrmpMessage,
+	IndexedVec, InherentData, InvalidDisputeStatementKind, Moment, MultiDisputeStatementSet, Nonce,
+	OccupiedCore, OccupiedCoreAssumption, OutboundHrmpMessage, ParathreadClaim, ParathreadEntry,
 	PersistedValidationData, PvfCheckStatement, PvfExecTimeoutKind, PvfPrepTimeoutKind,
 	RuntimeMetricLabel, RuntimeMetricLabelValue, RuntimeMetricLabelValues, RuntimeMetricLabels,
 	RuntimeMetricOp, RuntimeMetricUpdate, ScheduledCore, ScrapedOnChainVotes, SessionIndex,
@@ -55,9 +55,9 @@ pub use v5::{
 	UncheckedSignedAvailabilityBitfields, UncheckedSignedStatement, UpgradeGoAhead,
 	UpgradeRestriction, UpwardMessage, ValidDisputeStatementKind, ValidationCode,
 	ValidationCodeHash, ValidatorId, ValidatorIndex, ValidatorSignature, ValidityAttestation,
-	ValidityError, ASSIGNMENT_KEY_TYPE_ID, LOWEST_PUBLIC_ID, MAX_CODE_SIZE, MAX_HEAD_DATA_SIZE,
-	MAX_POV_SIZE, ON_DEMAND_DEFAULT_QUEUE_MAX_SIZE, PARACHAINS_INHERENT_IDENTIFIER,
-	PARACHAIN_KEY_TYPE_ID,
+	ValidityError, ASSIGNMENT_KEY_TYPE_ID, LEGACY_MIN_BACKING_VOTES, LOWEST_PUBLIC_ID,
+	MAX_CODE_SIZE, MAX_HEAD_DATA_SIZE, MAX_POV_SIZE, ON_DEMAND_DEFAULT_QUEUE_MAX_SIZE,
+	PARACHAINS_INHERENT_IDENTIFIER, PARACHAIN_KEY_TYPE_ID,
 };
 
 #[cfg(feature = "std")]

polkadot/primitives/src/runtime_api.rs

@@ -240,6 +240,13 @@ sp_api::decl_runtime_apis! {
 			key_ownership_proof: vstaging::slashing::OpaqueKeyOwnershipProof,
 		) -> Option<()>;
 
+		/***** Staging *****/
+
+		/// Get the minimum number of backing votes for a parachain candidate.
+		/// This is a staging method! Do not use on production runtimes!
+		#[api_version(6)]
+		fn minimum_backing_votes() -> u32;
+
 		/***** Asynchronous backing *****/
 
 		/// Returns the state of parachain backing for a given para.

polkadot/primitives/src/v5/mod.rs

@@ -390,6 +390,10 @@ pub const MAX_POV_SIZE: u32 = 5 * 1024 * 1024;
 /// Can be adjusted in configuration.
 pub const ON_DEMAND_DEFAULT_QUEUE_MAX_SIZE: u32 = 10_000;
 
+/// Backing votes threshold used from the host prior to runtime API version 6 and from the runtime
+/// prior to v9 configuration migration.
+pub const LEGACY_MIN_BACKING_VOTES: u32 = 2;
+
 // The public key of a keypair used by a validator for determining assignments
 /// to approve included parachain candidates.
 mod assignment_app {
@@ -1695,6 +1699,14 @@ pub const fn supermajority_threshold(n: usize) -> usize {
 	n - byzantine_threshold(n)
 }
 
+/// Adjust the configured needed backing votes with the size of the backing group.
+pub fn effective_minimum_backing_votes(
+	group_len: usize,
+	configured_minimum_backing_votes: u32,
+) -> usize {
+	sp_std::cmp::min(group_len, configured_minimum_backing_votes as usize)
+}
+
 /// Information about validator sets of a session.
 ///
 /// NOTE: `SessionInfo` is frozen. Do not include new fields, consider creating a separate runtime