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/runtime/parachains/src/configuration.rs

@@ -24,8 +24,8 @@ use frame_system::pallet_prelude::*;
 use parity_scale_codec::{Decode, Encode};
 use polkadot_parachain::primitives::{MAX_HORIZONTAL_MESSAGE_NUM, MAX_UPWARD_MESSAGE_NUM};
 use primitives::{
-	vstaging::AsyncBackingParams, Balance, ExecutorParams, SessionIndex, MAX_CODE_SIZE,
-	MAX_HEAD_DATA_SIZE, MAX_POV_SIZE, ON_DEMAND_DEFAULT_QUEUE_MAX_SIZE,
+	vstaging::AsyncBackingParams, Balance, ExecutorParams, SessionIndex, LEGACY_MIN_BACKING_VOTES,
+	MAX_CODE_SIZE, MAX_HEAD_DATA_SIZE, MAX_POV_SIZE, ON_DEMAND_DEFAULT_QUEUE_MAX_SIZE,
 };
 use sp_runtime::{traits::Zero, Perbill};
 use sp_std::prelude::*;
@@ -245,6 +245,9 @@ pub struct HostConfiguration<BlockNumber> {
 	/// This value should be greater than
 	/// [`paras_availability_period`](Self::paras_availability_period).
 	pub minimum_validation_upgrade_delay: BlockNumber,
+	/// The minimum number of valid backing statements required to consider a parachain candidate
+	/// backable.
+	pub minimum_backing_votes: u32,
 }
 
 impl<BlockNumber: Default + From<u32>> Default for HostConfiguration<BlockNumber> {
@@ -295,6 +298,7 @@ impl<BlockNumber: Default + From<u32>> Default for HostConfiguration<BlockNumber
 			on_demand_fee_variability: Perbill::from_percent(3),
 			on_demand_target_queue_utilization: Perbill::from_percent(25),
 			on_demand_ttl: 5u32.into(),
+			minimum_backing_votes: LEGACY_MIN_BACKING_VOTES,
 		}
 	}
 }
@@ -331,6 +335,8 @@ pub enum InconsistentError<BlockNumber> {
 	MaxHrmpOutboundChannelsExceeded,
 	/// Maximum number of HRMP inbound channels exceeded.
 	MaxHrmpInboundChannelsExceeded,
+	/// `minimum_backing_votes` is set to zero.
+	ZeroMinimumBackingVotes,
 }
 
 impl<BlockNumber> HostConfiguration<BlockNumber>
@@ -411,6 +417,10 @@ where
 			return Err(MaxHrmpInboundChannelsExceeded)
 		}
 
+		if self.minimum_backing_votes.is_zero() {
+			return Err(ZeroMinimumBackingVotes)
+		}
+
 		Ok(())
 	}
 
@@ -477,7 +487,8 @@ pub mod pallet {
 	/// v5-v6: <https://github.com/paritytech/polkadot/pull/6271> (remove UMP dispatch queue)
 	/// v6-v7: <https://github.com/paritytech/polkadot/pull/7396>
 	/// v7-v8: <https://github.com/paritytech/polkadot/pull/6969>
-	const STORAGE_VERSION: StorageVersion = StorageVersion::new(8);
+	/// v8-v9: <https://github.com/paritytech/polkadot/pull/7577>
+	const STORAGE_VERSION: StorageVersion = StorageVersion::new(9);
 
 	#[pallet::pallet]
 	#[pallet::storage_version(STORAGE_VERSION)]
@@ -1153,6 +1164,18 @@ pub mod pallet {
 				config.on_demand_ttl = new;
 			})
 		}
+		/// Set the minimum backing votes threshold.
+		#[pallet::call_index(52)]
+		#[pallet::weight((
+			T::WeightInfo::set_config_with_u32(),
+			DispatchClass::Operational
+		))]
+		pub fn set_minimum_backing_votes(origin: OriginFor<T>, new: u32) -> DispatchResult {
+			ensure_root(origin)?;
+			Self::schedule_config_update(|config| {
+				config.minimum_backing_votes = new;
+			})
+		}
 	}
 
 	#[pallet::hooks]