Delay network startup to after complete initialization

bin/node-template/node/src/service.rs

@@ -83,7 +83,7 @@ pub fn new_full(config: Configuration) -> Result<TaskManager, ServiceError> {
 	let finality_proof_provider =
 		GrandpaFinalityProofProvider::new_for_service(backend.clone(), client.clone());
 
-	let (network, network_status_sinks, system_rpc_tx) =
+	let (network, network_status_sinks, system_rpc_tx, network_starter) =
 		sc_service::build_network(sc_service::BuildNetworkParams {
 			config: &config,
 			client: client.clone(),
@@ -215,6 +215,7 @@ pub fn new_full(config: Configuration) -> Result<TaskManager, ServiceError> {
 		)?;
 	}
 
+	network_starter.start_network();
 	Ok(task_manager)
 }
 
@@ -253,7 +254,7 @@ pub fn new_light(config: Configuration) -> Result<TaskManager, ServiceError> {
 	let finality_proof_provider =
 		GrandpaFinalityProofProvider::new_for_service(backend.clone(), client.clone());
 
-	let (network, network_status_sinks, system_rpc_tx) =
+	let (network, network_status_sinks, system_rpc_tx, network_starter) =
 		sc_service::build_network(sc_service::BuildNetworkParams {
 			config: &config,
 			client: client.clone(),
@@ -288,5 +289,7 @@ pub fn new_light(config: Configuration) -> Result<TaskManager, ServiceError> {
 		system_rpc_tx,
 	 })?;
 
+	 network_starter.start_network();
+
 	 Ok(task_manager)
 }

bin/node/cli/src/service.rs

@@ -164,8 +164,8 @@ pub fn new_full_base(
 
 	let finality_proof_provider =
 		GrandpaFinalityProofProvider::new_for_service(backend.clone(), client.clone());
-	
-	let (network, network_status_sinks, system_rpc_tx) =
+
+	let (network, network_status_sinks, system_rpc_tx, network_starter) =
 		sc_service::build_network(sc_service::BuildNetworkParams {
 			config: &config,
 			client: client.clone(),
@@ -206,7 +206,7 @@ pub fn new_full_base(
 		network_status_sinks,
 		system_rpc_tx,
 	})?;
-	
+
 	let (block_import, grandpa_link, babe_link) = import_setup;
 	let shared_voter_state = rpc_setup;
 
@@ -322,6 +322,7 @@ pub fn new_full_base(
 		)?;
 	}
 
+	network_starter.start_network();
 	Ok((task_manager, inherent_data_providers, client, network, transaction_pool))
 }
 
@@ -383,7 +384,7 @@ pub fn new_light_base(config: Configuration) -> Result<(
 	let finality_proof_provider =
 		GrandpaFinalityProofProvider::new_for_service(backend.clone(), client.clone());
 
-	let (network, network_status_sinks, system_rpc_tx) =
+	let (network, network_status_sinks, system_rpc_tx, network_starter) =
 		sc_service::build_network(sc_service::BuildNetworkParams {
 			config: &config,
 			client: client.clone(),
@@ -395,7 +396,8 @@ pub fn new_light_base(config: Configuration) -> Result<(
 			finality_proof_request_builder: Some(finality_proof_request_builder),
 			finality_proof_provider: Some(finality_proof_provider),
 		})?;
-
+	network_starter.start_network();
+
 	if config.offchain_worker.enabled {
 		sc_service::build_offchain_workers(
 			&config, backend.clone(), task_manager.spawn_handle(), client.clone(), network.clone(),
@@ -412,7 +414,7 @@ pub fn new_light_base(config: Configuration) -> Result<(
 	let rpc_extensions = node_rpc::create_light(light_deps);
 
 	let rpc_handlers =
-		sc_service::spawn_tasks(sc_service::SpawnTasksParams {	
+		sc_service::spawn_tasks(sc_service::SpawnTasksParams {
 			on_demand: Some(on_demand),
 			remote_blockchain: Some(backend.remote_blockchain()),
 			rpc_extensions_builder: Box::new(sc_service::NoopRpcExtensionBuilder(rpc_extensions)),
@@ -423,7 +425,7 @@ pub fn new_light_base(config: Configuration) -> Result<(
 			telemetry_connection_sinks: sc_service::TelemetryConnectionSinks::default(),
 			task_manager: &mut task_manager,
 		})?;
-	
+
 	Ok((task_manager, rpc_handlers, client, network, transaction_pool))
 }
 
@@ -498,7 +500,7 @@ mod tests {
 							setup_handles = Some((block_import.clone(), babe_link.clone()));
 						}
 					)?;
-				
+
 				let node = sc_service_test::TestNetComponents::new(
 					keep_alive, client, network, transaction_pool
 				);

client/service/src/builder.rs

@@ -33,7 +33,7 @@ use sp_consensus::{
 	block_validation::{BlockAnnounceValidator, DefaultBlockAnnounceValidator, Chain},
 	import_queue::ImportQueue,
 };
-use futures::{FutureExt, StreamExt, future::ready};
+use futures::{FutureExt, StreamExt, future::ready, channel::oneshot};
 use jsonrpc_pubsub::manager::SubscriptionManager;
 use sc_keystore::Store as Keystore;
 use log::{info, warn, error};
@@ -668,7 +668,7 @@ fn build_telemetry<TBl: BlockT>(
 	let startup_time = SystemTime::UNIX_EPOCH.elapsed()
 		.map(|dur| dur.as_millis())
 		.unwrap_or(0);
-	
+
 	spawn_handle.spawn(
 		"telemetry-worker",
 		telemetry.clone()
@@ -822,6 +822,7 @@ pub fn build_network<TBl, TExPool, TImpQu, TCl>(
 		Arc<NetworkService<TBl, <TBl as BlockT>::Hash>>,
 		NetworkStatusSinks<TBl>,
 		TracingUnboundedSender<sc_rpc::system::Request<TBl>>,
+		NetworkStarter,
 	),
 	Error
 >
@@ -900,14 +901,55 @@ pub fn build_network<TBl, TExPool, TImpQu, TCl>(
 		config.announce_block,
 	);
 
+	// TODO: Normally, one is supposed to pass a list of notifications protocols supported by the
+	// node through the `NetworkConfiguration` struct. But because this function doesn't know in
+	// advance which components, such as GrandPa or Polkadot, will be plugged on top of the
+	// service, it is unfortunately not possible to do so without some deep refactoring. To bypass
+	// this problem, the `NetworkService` provides a `register_notifications_protocol` method that
+	// can be called even after the network has been initialized. However, we want to avoid the
+	// situation where `register_notifications_protocol` is called *after* the network actually
+	// connects to other peers. For this reason, we delay the process of the network future until
+	// the user calls `NetworkStarter::start_network`.
+	//
+	// This entire hack should eventually be removed in favour of passing the list of protocols
+	// through the configuration.
+	//
+	// See also https://github.com/paritytech/substrate/issues/6827
+	let (network_start_tx, network_start_rx) = oneshot::channel();
+
 	// The network worker is responsible for gathering all network messages and processing
 	// them. This is quite a heavy task, and at the time of the writing of this comment it
 	// frequently happens that this future takes several seconds or in some situations
 	// even more than a minute until it has processed its entire queue. This is clearly an
 	// issue, and ideally we would like to fix the network future to take as little time as
 	// possible, but we also take the extra harm-prevention measure to execute the networking
 	// future using `spawn_blocking`.
-	spawn_handle.spawn_blocking("network-worker", future);
+	spawn_handle.spawn_blocking("network-worker", async move {
+		if network_start_rx.await.is_err() {
+			debug_assert!(false);
+			log::warn!(
+				"The NetworkStart returned as part of `build_network` has been silently dropped"
+			);
+			// This `return` might seem unnecessary, but we don't want to make it look like
+			// everything is working as normal even though the user is clearly misusing the API.
+			return;
+		}
+
+		future.await
+	});
+
+	Ok((network, network_status_sinks, system_rpc_tx, NetworkStarter(network_start_tx)))
+}
 
-	Ok((network, network_status_sinks, system_rpc_tx))
+/// Object used to start the network.
+#[must_use]
+pub struct NetworkStarter(oneshot::Sender<()>);
+
+impl NetworkStarter {
+	/// Start the network. Call this after all sub-components have been initialized.
+	///
+	/// > **Note**: If you don't call this function, the networking will not work.
+	pub fn start_network(self) {
+		let _ = self.0.send(());
+	}
 }

client/service/src/lib.rs

@@ -53,7 +53,7 @@ use sp_utils::{status_sinks, mpsc::{tracing_unbounded, TracingUnboundedReceiver,
 pub use self::error::Error;
 pub use self::builder::{
 	new_full_client, new_client, new_full_parts, new_light_parts,
-	spawn_tasks, build_network, BuildNetworkParams, build_offchain_workers,
+	spawn_tasks, build_network, BuildNetworkParams, NetworkStarter, build_offchain_workers,
 	SpawnTasksParams, TFullClient, TLightClient, TFullBackend, TLightBackend,
 	TLightBackendWithHash, TLightClientWithBackend,
 	TFullCallExecutor, TLightCallExecutor, RpcExtensionBuilder, NoopRpcExtensionBuilder,