Integrate prospective parachains subsystem into backing: Part 1 (#5557)

* BEGIN ASYNC candidate-backing CHANGES

* rename & document modes

* answer prospective validation data requests

* GetMinimumRelayParents request is now plural

* implement an implicit view utility for backing subsystems

* implicit-view: get allowed relay parents

* refactorings and improvements to implicit view

* add some TODOs for tests

* split implicit view updates into 2 functions

* backing: define State to prepare for functional refactor

* add some docs

* backing: implement bones of new leaf activation logic

* backing: create per-relay-parent-states

* use new handle_active_leaves_update

* begin extracting logic from CandidateBackingJob

* mostly extract statement import from job logic

* handle statement imports outside of job logic

* do some TODO planning for prospective parachains integration

* finish rewriting backing subsystem in functional style

* add prospective parachains mode to relay parent entries

* fmt

* add a RejectedByProspectiveParachains error

* notify prospective parachains of seconded and backed candidates

* always validate candidates exhaustively in backing.

* return persisted_validation_data from validation

* handle rejections by prospective parachains

* implement seconding sanity check

* invoke validate_and_second

* Alter statement table to allow multiple seconded messages per validator

* refactor backing to have statements carry PVD

* clean up all warnings

* Add tests for implicit view

* Improve doc comments

* Prospective parachains mode based on Runtime API version

* Add a TODO

* Rework seconding_sanity_check

* Iterate over responses

* Update backing tests

* collator-protocol: load PVD from runtime

* Fix validator side tests

* Update statement-distribution to fetch PVD

* Fix statement-distribution tests

* Backing tests with prospective paras #1

* fix per_relay_parent pruning in backing

* Test multiple leaves

* Test seconding sanity check

* Import statement order

Before creating an entry in `PerCandidateState` map
wait for the approval from the prospective parachains

* Add a test for correct state updates

* Second multiple candidates per relay parent test

* Add backing tests with prospective paras

* Second more than one test without prospective paras

* Add a test for prospective para blocks

* Update malus

* typos

Co-authored-by: Chris Sosnin <[email protected]>

node/core/backing/src/error.rs

@@ -17,9 +17,9 @@
 use fatality::Nested;
 use futures::channel::{mpsc, oneshot};
 
-use polkadot_node_subsystem::{messages::ValidationFailed, SubsystemError};
+use polkadot_node_subsystem::{messages::ValidationFailed, RuntimeApiError, SubsystemError};
 use polkadot_node_subsystem_util::Error as UtilError;
-use polkadot_primitives::v2::BackedCandidate;
+use polkadot_primitives::v2::{BackedCandidate, ValidationCodeHash};
 
 use crate::LOG_TARGET;
 
@@ -42,16 +42,31 @@ pub enum Error {
 	#[error("FetchPoV failed")]
 	FetchPoV,
 
+	#[error("Fetching validation code by hash failed {0:?}, {1:?}")]
+	FetchValidationCode(ValidationCodeHash, RuntimeApiError),
+
+	#[error("Fetching Runtime API version failed {0:?}")]
+	FetchRuntimeApiVersion(RuntimeApiError),
+
+	#[error("No validation code {0:?}")]
+	NoValidationCode(ValidationCodeHash),
+
+	#[error("Candidate rejected by prospective parachains subsystem")]
+	RejectedByProspectiveParachains,
+
 	#[fatal]
 	#[error("Failed to spawn background task")]
 	FailedToSpawnBackgroundTask,
 
-	#[error("ValidateFromChainState channel closed before receipt")]
-	ValidateFromChainState(#[source] oneshot::Canceled),
+	#[error("ValidateFromExhaustive channel closed before receipt")]
+	ValidateFromExhaustive(#[source] oneshot::Canceled),
 
 	#[error("StoreAvailableData channel closed before receipt")]
 	StoreAvailableData(#[source] oneshot::Canceled),
 
+	#[error("RuntimeAPISubsystem channel closed before receipt")]
+	RuntimeApiUnavailable(#[source] oneshot::Canceled),
+
 	#[error("a channel was closed before receipt in try_join!")]
 	JoinMultiple(#[source] oneshot::Canceled),