Simplify subsystem jobs (#2037)

* Simplify subsystem jobs

This pr simplifies the subsystem jobs interface. Instead of requiring an
extra message that is used to signal that a job should be ended, a job
now ends when the receiver returns `None`. Besides that it changes the
interface to enforce that messages to a job provide a relay parent.

* Drop ToJobTrait

* Remove FromJob

We always convert this message to FromJobCommand anyway.

polkadot/node/subsystem/src/messages.rs

@@ -39,8 +39,13 @@ use polkadot_primitives::v1::{
 	ValidationCode, ValidatorId, ValidationData, CandidateHash,
 	ValidatorIndex, ValidatorSignature, InboundDownwardMessage, InboundHrmpMessage,
 };
-use std::sync::Arc;
-use std::collections::btree_map::BTreeMap;
+use std::{sync::Arc, collections::btree_map::BTreeMap};
+
+/// Subsystem messages where each message is always bound to a relay parent.
+pub trait BoundToRelayParent {
+	/// Returns the relay parent this message is bound to.
+	fn relay_parent(&self) -> Hash;
+}
 
 /// A notification of a new backed candidate.
 #[derive(Debug)]
@@ -56,12 +61,11 @@ pub enum CandidateSelectionMessage {
 	Invalid(Hash, CandidateReceipt),
 }
 
-impl CandidateSelectionMessage {
-	/// If the current variant contains the relay parent hash, return it.
-	pub fn relay_parent(&self) -> Option<Hash> {
+impl BoundToRelayParent for CandidateSelectionMessage {
+	fn relay_parent(&self) -> Hash {
 		match self {
-			Self::Collation(hash, ..) => Some(*hash),
-			Self::Invalid(hash, _) => Some(*hash),
+			Self::Collation(hash, ..) => *hash,
+			Self::Invalid(hash, _) => *hash,
 		}
 	}
 }
@@ -86,13 +90,12 @@ pub enum CandidateBackingMessage {
 	Statement(Hash, SignedFullStatement),
 }
 
-impl CandidateBackingMessage {
-	/// If the current variant contains the relay parent hash, return it.
-	pub fn relay_parent(&self) -> Option<Hash> {
+impl BoundToRelayParent for CandidateBackingMessage {
+	fn relay_parent(&self) -> Hash {
 		match self {
-			Self::GetBackedCandidates(hash, _) => Some(*hash),
-			Self::Second(hash, _, _) => Some(*hash),
-			Self::Statement(hash, _) => Some(*hash),
+			Self::GetBackedCandidates(hash, _) => *hash,
+			Self::Second(hash, _, _) => *hash,
+			Self::Statement(hash, _) => *hash,
 		}
 	}
 }
@@ -273,10 +276,9 @@ impl BitfieldDistributionMessage {
 #[derive(Debug)]
 pub enum BitfieldSigningMessage {}
 
-impl BitfieldSigningMessage {
-	/// If the current variant contains the relay parent hash, return it.
-	pub fn relay_parent(&self) -> Option<Hash> {
-		None
+impl BoundToRelayParent for BitfieldSigningMessage {
+	fn relay_parent(&self) -> Hash {
+		match *self {}
 	}
 }
 
@@ -525,13 +527,12 @@ pub enum ProvisionerMessage {
 	ProvisionableData(Hash, ProvisionableData),
 }
 
-impl ProvisionerMessage {
-	/// If the current variant contains the relay parent hash, return it.
-	pub fn relay_parent(&self) -> Option<Hash> {
+impl BoundToRelayParent for ProvisionerMessage {
+	fn relay_parent(&self) -> Hash {
 		match self {
-			Self::RequestBlockAuthorshipData(hash, _) => Some(*hash),
-			Self::RequestInherentData(hash, _) => Some(*hash),
-			Self::ProvisionableData(hash, _) => Some(*hash),
+			Self::RequestBlockAuthorshipData(hash, _) => *hash,
+			Self::RequestInherentData(hash, _) => *hash,
+			Self::ProvisionableData(hash, _) => *hash,
 		}
 	}
 }