node: spawn block authoring and grandpa voter as blocking tasks (#6446)

* service: add spawner for essential tasks

* node: spawn block authoring and grandpa voter as blocking tasks

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/client/service/src/lib.rs

@@ -82,7 +82,7 @@ pub use sc_network::config::{
 	TransactionImportFuture,
 };
 pub use sc_tracing::TracingReceiver;
-pub use task_manager::SpawnTaskHandle;
+pub use task_manager::{SpawnEssentialTaskHandle, SpawnTaskHandle};
 use task_manager::TaskManager;
 use sp_blockchain::{HeaderBackend, HeaderMetadata};
 use sp_api::{ApiExt, ConstructRuntimeApi, ApiErrorExt};
@@ -166,13 +166,19 @@ pub trait AbstractService: Future<Output = Result<(), Error>> + Send + Unpin + S
 	/// The task name is a `&'static str` as opposed to a `String`. The reason for that is that
 	/// in order to avoid memory consumption issues with the Prometheus metrics, the set of
 	/// possible task names has to be bounded.
+	#[deprecated(note = "Use `spawn_task_handle().spawn() instead.")]
 	fn spawn_task(&self, name: &'static str, task: impl Future<Output = ()> + Send + 'static);
 
 	/// Spawns a task in the background that runs the future passed as
 	/// parameter. The given task is considered essential, i.e. if it errors we
 	/// trigger a service exit.
+	#[deprecated(note = "Use `spawn_essential_task_handle().spawn() instead.")]
 	fn spawn_essential_task(&self, name: &'static str, task: impl Future<Output = ()> + Send + 'static);
 
+	/// Returns a handle for spawning essential tasks. Any task spawned through this handle is
+	/// considered essential, i.e. if it errors we trigger a service exit.
+	fn spawn_essential_task_handle(&self) -> SpawnEssentialTaskHandle;
+
 	/// Returns a handle for spawning tasks.
 	fn spawn_task_handle(&self) -> SpawnTaskHandle;
 
@@ -269,13 +275,20 @@ where
 				let _ = essential_failed.send(());
 			});
 
-		let _ = self.spawn_task(name, essential_task);
+		let _ = self.spawn_task_handle().spawn(name, essential_task);
 	}
 
 	fn spawn_task_handle(&self) -> SpawnTaskHandle {
 		self.task_manager.spawn_handle()
 	}
 
+	fn spawn_essential_task_handle(&self) -> SpawnEssentialTaskHandle {
+		SpawnEssentialTaskHandle::new(
+			self.essential_failed_tx.clone(),
+			self.task_manager.spawn_handle(),
+		)
+	}
+
 	fn rpc_query(&self, mem: &RpcSession, request: &str) -> Pin<Box<dyn Future<Output = Option<String>> + Send>> {
 		Box::pin(
 			self.rpc_handlers.handle_request(request, mem.metadata.clone())