Add spawn_blocking to SubsystemContext (#1570)

* subsystem: add spawn_blocking to SubsystemContext

* candidate-validation: use spawn_blocking for exhaustive tasks

polkadot/node/core/candidate-validation/src/lib.rs

@@ -318,7 +318,7 @@ async fn spawn_validate_exhaustive(
 		let _ = tx.send(res);
 	};
 
-	ctx.spawn("blocking-candidate-validation-task", fut.boxed()).await?;
+	ctx.spawn_blocking("blocking-candidate-validation-task", fut.boxed()).await?;
 	rx.await.map_err(Into::into)
 }
 

polkadot/node/overseer/src/lib.rs

@@ -112,6 +112,13 @@ enum ToOverseer {
 		name: &'static str,
 		s: BoxFuture<'static, ()>,
 	},
+
+	/// Same as `SpawnJob` but for blocking tasks to be executed on a
+	/// dedicated thread pool.
+	SpawnBlockingJob {
+		name: &'static str,
+		s: BoxFuture<'static, ()>,
+	},
 }
 
 /// An event telling the `Overseer` on the particular block
@@ -238,7 +245,8 @@ impl Debug for ToOverseer {
 			ToOverseer::SubsystemMessage(msg) => {
 				write!(f, "OverseerMessage::SubsystemMessage({:?})", msg)
 			}
-			ToOverseer::SpawnJob { .. } => write!(f, "OverseerMessage::Spawn(..)")
+			ToOverseer::SpawnJob { .. } => write!(f, "OverseerMessage::Spawn(..)"),
+			ToOverseer::SpawnBlockingJob { .. } => write!(f, "OverseerMessage::SpawnBlocking(..)")
 		}
 	}
 }
@@ -290,6 +298,17 @@ impl<M: Send + 'static> SubsystemContext for OverseerSubsystemContext<M> {
 		Ok(())
 	}
 
+	async fn spawn_blocking(&mut self, name: &'static str, s: Pin<Box<dyn Future<Output = ()> + Send>>)
+		-> SubsystemResult<()>
+	{
+		self.tx.send(ToOverseer::SpawnBlockingJob {
+			name,
+			s,
+		}).await?;
+
+		Ok(())
+	}
+
 	async fn send_message(&mut self, msg: AllMessages) -> SubsystemResult<()> {
 		self.tx.send(ToOverseer::SubsystemMessage(msg)).await?;
 
@@ -803,6 +822,9 @@ where
 					ToOverseer::SpawnJob { name, s } => {
 						self.spawn_job(name, s);
 					}
+					ToOverseer::SpawnBlockingJob { name, s } => {
+						self.spawn_blocking_job(name, s);
+					}
 				}
 			}
 
@@ -992,6 +1014,10 @@ where
 	fn spawn_job(&mut self, name: &'static str, j: BoxFuture<'static, ()>) {
 		self.s.spawn(name, j);
 	}
+
+	fn spawn_blocking_job(&mut self, name: &'static str, j: BoxFuture<'static, ()>) {
+		self.s.spawn_blocking(name, j);
+	}
 }
 
 fn spawn<S: SpawnNamed, M: Send + 'static>(

polkadot/node/subsystem-test-helpers/src/lib.rs

@@ -177,6 +177,13 @@ impl<M: Send + 'static, S: SpawnNamed + Send + 'static> SubsystemContext for Tes
 		Ok(())
 	}
 
+	async fn spawn_blocking(&mut self, name: &'static str, s: Pin<Box<dyn Future<Output = ()> + Send>>)
+		-> SubsystemResult<()>
+	{
+		self.spawn.spawn_blocking(name, s);
+		Ok(())
+	}
+
 	async fn send_message(&mut self, msg: AllMessages) -> SubsystemResult<()> {
 		self.tx.send(msg).await.expect("test overseer no longer live");
 		Ok(())

polkadot/node/subsystem/src/lib.rs

@@ -178,6 +178,13 @@ pub trait SubsystemContext: Send + 'static {
 	/// Spawn a child task on the executor.
 	async fn spawn(&mut self, name: &'static str, s: Pin<Box<dyn Future<Output = ()> + Send>>) -> SubsystemResult<()>;
 
+	/// Spawn a blocking child task on the executor's dedicated thread pool.
+	async fn spawn_blocking(
+		&mut self,
+		name: &'static str,
+		s: Pin<Box<dyn Future<Output = ()> + Send>>,
+	) -> SubsystemResult<()>;
+
 	/// Send a direct message to some other `Subsystem`, routed based on message type.
 	async fn send_message(&mut self, msg: AllMessages) -> SubsystemResult<()>;