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/subsystem-types/src/messages.rs

@@ -691,6 +691,8 @@ pub enum RuntimeApiRequest {
 		slashing::OpaqueKeyOwnershipProof,
 		RuntimeApiSender<Option<()>>,
 	),
+	/// Get the minimum required backing votes.
+	MinimumBackingVotes(SessionIndex, RuntimeApiSender<u32>),
 
 	/// Get the backing state of the given para.
 	/// This is a staging API that will not be available on production runtimes.
@@ -719,6 +721,9 @@ impl RuntimeApiRequest {
 	/// `SubmitReportDisputeLost`
 	pub const SUBMIT_REPORT_DISPUTE_LOST_RUNTIME_REQUIREMENT: u32 = 5;
 
+	/// `MinimumBackingVotes`
+	pub const MINIMUM_BACKING_VOTES_RUNTIME_REQUIREMENT: u32 = 6;
+
 	/// Minimum version for backing state, required for async backing.
 	///
 	/// 99 for now, should be adjusted to VSTAGING/actual runtime version once released.

polkadot/node/subsystem-types/src/runtime_client.rs

@@ -232,6 +232,14 @@ pub trait RuntimeApiSubsystemClient {
 		session_index: SessionIndex,
 	) -> Result<Option<ExecutorParams>, ApiError>;
 
+	// === STAGING v6 ===
+	/// Get the minimum number of backing votes.
+	async fn minimum_backing_votes(
+		&self,
+		at: Hash,
+		session_index: SessionIndex,
+	) -> Result<u32, ApiError>;
+
 	// === Asynchronous backing API ===
 
 	/// Returns candidate's acceptance limitations for asynchronous backing for a relay parent.
@@ -473,6 +481,14 @@ where
 		runtime_api.submit_report_dispute_lost(at, dispute_proof, key_ownership_proof)
 	}
 
+	async fn minimum_backing_votes(
+		&self,
+		at: Hash,
+		_session_index: SessionIndex,
+	) -> Result<u32, ApiError> {
+		self.client.runtime_api().minimum_backing_votes(at)
+	}
+
 	async fn staging_para_backing_state(
 		&self,
 		at: Hash,