reenvision the subsystem requests as an extension trait

This works within `util.rs`, but fails in `core/backing/src/lib.rs`,
because we don't actually create the struct soon enough. Continuing
down this path would imply substantial rewriting.

node/core/backing/src/lib.rs

@@ -43,13 +43,12 @@ use polkadot_subsystem::{
 	messages::{
 		AllMessages, AvailabilityStoreMessage, CandidateBackingMessage, CandidateSelectionMessage,
 		CandidateValidationMessage, NewBackedCandidate, PoVDistributionMessage, ProvisionableData,
-		ProvisionerMessage, RuntimeApiMessage, StatementDistributionMessage, ValidationFailed,
+		ProvisionerMessage, RuntimeApiMessage, RuntimeApiRequest, StatementDistributionMessage, ValidationFailed,
 	},
 	util::{
 		self,
-		request_signing_context,
-		request_validator_groups,
-		request_validators,
+		JobTrait,
+		JobTraitExt,
 		Validator,
 	},
 };
@@ -198,23 +197,6 @@ impl From<FromJob> for AllMessages {
 	}
 }
 
-impl TryFrom<AllMessages> for FromJob {
-	type Error = &'static str;
-
-	fn try_from(f: AllMessages) -> Result<Self, Self::Error> {
-		match f {
-			AllMessages::AvailabilityStore(msg) => Ok(FromJob::AvailabilityStore(msg)),
-			AllMessages::RuntimeApi(msg) => Ok(FromJob::RuntimeApiMessage(msg)),
-			AllMessages::CandidateValidation(msg) => Ok(FromJob::CandidateValidation(msg)),
-			AllMessages::CandidateSelection(msg) => Ok(FromJob::CandidateSelection(msg)),
-			AllMessages::StatementDistribution(msg) => Ok(FromJob::StatementDistribution(msg)),
-			AllMessages::PoVDistribution(msg) => Ok(FromJob::PoVDistribution(msg)),
-			AllMessages::Provisioner(msg) => Ok(FromJob::Provisioner(msg)),
-			_ => Err("can't convert this AllMessages variant to FromJob"),
-		}
-	}
-}
-
 // It looks like it's not possible to do an `impl From` given the current state of
 // the code. So this does the necessary conversion.
 fn primitive_statement_to_table(s: &SignedFullStatement) -> TableSignedStatement {
@@ -547,38 +529,6 @@ impl CandidateBackingJob {
 		Ok(())
 	}
 
-	async fn request_pov_from_distribution(
-		&mut self,
-		descriptor: CandidateDescriptor,
-	) -> Result<Arc<PoV>, Error> {
-		let (tx, rx) = oneshot::channel();
-
-		self.tx_from.send(FromJob::PoVDistribution(
-			PoVDistributionMessage::FetchPoV(self.parent, descriptor, tx)
-		)).await?;
-
-		Ok(rx.await?)
-	}
-
-	async fn request_candidate_validation(
-		&mut self,
-		candidate: CandidateDescriptor,
-		pov: Arc<PoV>,
-	) -> Result<ValidationResult, Error> {
-		let (tx, rx) = oneshot::channel();
-
-		self.tx_from.send(FromJob::CandidateValidation(
-				CandidateValidationMessage::ValidateFromChainState(
-					candidate,
-					pov,
-					tx,
-				)
-			)
-		).await?;
-
-		Ok(rx.await??)
-	}
-
 	async fn store_chunk(
 		&mut self,
 		id: ValidatorIndex,
@@ -656,7 +606,7 @@ impl CandidateBackingJob {
 	}
 }
 
-impl util::JobTrait for CandidateBackingJob {
+impl JobTrait for CandidateBackingJob {
 	type ToJob = ToJob;
 	type FromJob = FromJob;
 	type Error = Error;
@@ -672,9 +622,9 @@ impl util::JobTrait for CandidateBackingJob {
 	) -> Pin<Box<dyn Future<Output = Result<(), Self::Error>> + Send>> {
 		async move {
 			let (validators, roster, signing_context) = futures::try_join!(
-				request_validators(parent, &mut tx_from).await?,
-				request_validator_groups(parent, &mut tx_from).await?,
-				request_signing_context(parent, &mut tx_from).await?,
+				self.request_validators().await?,
+				self.request_validator_groups().await?,
+				self.request_signing_context().await?,
 			)?;
 
 			let validator = Validator::construct(&validators, signing_context, keystore.clone())?;
@@ -724,6 +674,16 @@ impl util::JobTrait for CandidateBackingJob {
 	}
 }
 
+impl JobTraitExt for CandidateBackingJob {
+	fn sender(&self) -> mpsc::Sender<Self::FromJob> {
+		self.tx_from.clone()
+	}
+
+	fn relay_parent(&self) -> Hash {
+		self.parent
+	}
+}
+
 /// An implementation of the Candidate Backing subsystem.
 pub type CandidateBackingSubsystem<Spawner, Context> =
 	util::JobManager<Spawner, Context, CandidateBackingJob>;