service.rs

// This file is part of Substrate.

// Copyright (C) 2017-2020 Parity Technologies (UK) Ltd.
// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0

// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.

// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this program. If not, see <https://www.gnu.org/licenses/>.

//! Main entry point of the sc-network crate.
//!
//! There are two main structs in this module: [`NetworkWorker`] and [`NetworkService`].
//! The [`NetworkWorker`] *is* the network and implements the `Future` trait. It must be polled in
//! order for the network to advance.
//! The [`NetworkService`] is merely a shared version of the [`NetworkWorker`]. You can obtain an
//! `Arc<NetworkService>` by calling [`NetworkWorker::service`].
//!
//! The methods of the [`NetworkService`] are implemented by sending a message over a channel,
//! which is then processed by [`NetworkWorker::poll`].

use crate::{
	ExHashT, NetworkStateInfo, NetworkStatus,
	behaviour::{self, Behaviour, BehaviourOut},
	config::{parse_str_addr, NonReservedPeerMode, Params, Role, TransportConfig},
	DhtEvent,
	discovery::DiscoveryConfig,
	error::Error,
	network_state::{
		NetworkState, NotConnectedPeer as NetworkStateNotConnectedPeer, Peer as NetworkStatePeer,
	},
	on_demand_layer::AlwaysBadChecker,
	light_client_handler, block_requests, finality_requests,
	protocol::{self, event::Event, NotifsHandlerError, NotificationsSink, Ready, sync::SyncState, PeerInfo, Protocol},
	transport, ReputationChange,
};
use futures::{channel::oneshot, prelude::*};
use libp2p::{PeerId, multiaddr, Multiaddr};
use libp2p::core::{ConnectedPoint, Executor, connection::{ConnectionError, PendingConnectionError}, either::EitherError};
use libp2p::kad::record;
use libp2p::ping::handler::PingFailure;
use libp2p::swarm::{NetworkBehaviour, SwarmBuilder, SwarmEvent, protocols_handler::NodeHandlerWrapperError};
use log::{error, info, trace, warn};
use metrics::{Metrics, MetricSources, Histogram, HistogramVec};
use parking_lot::Mutex;
use sc_peerset::PeersetHandle;
use sp_consensus::import_queue::{BlockImportError, BlockImportResult, ImportQueue, Link};
use sp_runtime::traits::{Block as BlockT, NumberFor};
use sp_utils::mpsc::{tracing_unbounded, TracingUnboundedReceiver, TracingUnboundedSender};
use std::{
	borrow::Cow,
	collections::{HashMap, HashSet},
	fs,
	marker::PhantomData,
	num:: NonZeroUsize,
	pin::Pin,
	str,
	sync::{
		atomic::{AtomicBool, AtomicUsize, Ordering},
		Arc,
	},
	task::Poll,
};
use wasm_timer::Instant;

pub use behaviour::{ResponseFailure, InboundFailure, RequestFailure, OutboundFailure};

mod metrics;
mod out_events;
#[cfg(test)]
mod tests;

/// Substrate network service. Handles network IO and manages connectivity.
pub struct NetworkService<B: BlockT + 'static, H: ExHashT> {
	/// Number of peers we're connected to.
	num_connected: Arc<AtomicUsize>,
	/// The local external addresses.
	external_addresses: Arc<Mutex<Vec<Multiaddr>>>,
	/// Are we actively catching up with the chain?
	is_major_syncing: Arc<AtomicBool>,
	/// Local copy of the `PeerId` of the local node.
	local_peer_id: PeerId,
	/// Bandwidth logging system. Can be queried to know the average bandwidth consumed.
	bandwidth: Arc<transport::BandwidthSinks>,
	/// Peerset manager (PSM); manages the reputation of nodes and indicates the network which
	/// nodes it should be connected to or not.
	peerset: PeersetHandle,
	/// Channel that sends messages to the actual worker.
	to_worker: TracingUnboundedSender<ServiceToWorkerMsg<B, H>>,
	/// For each peer and protocol combination, an object that allows sending notifications to
	/// that peer. Updated by the [`NetworkWorker`].
	peers_notifications_sinks: Arc<Mutex<HashMap<(PeerId, Cow<'static, str>), NotificationsSink>>>,
	/// Field extracted from the [`Metrics`] struct and necessary to report the
	/// notifications-related metrics.
	notifications_sizes_metric: Option<HistogramVec>,
	/// Marker to pin the `H` generic. Serves no purpose except to not break backwards
	/// compatibility.
	_marker: PhantomData<H>,
}

impl<B: BlockT + 'static, H: ExHashT> NetworkWorker<B, H> {
	/// Creates the network service.
	///
	/// Returns a `NetworkWorker` that implements `Future` and must be regularly polled in order
	/// for the network processing to advance. From it, you can extract a `NetworkService` using
	/// `worker.service()`. The `NetworkService` can be shared through the codebase.
	pub fn new(params: Params<B, H>) -> Result<NetworkWorker<B, H>, Error> {
		// Ensure the listen addresses are consistent with the transport.
		ensure_addresses_consistent_with_transport(
			params.network_config.listen_addresses.iter(),
			&params.network_config.transport,
		)?;
		ensure_addresses_consistent_with_transport(
			params.network_config.boot_nodes.iter().map(|x| &x.multiaddr),
			&params.network_config.transport,
		)?;
		ensure_addresses_consistent_with_transport(
			params.network_config.reserved_nodes.iter().map(|x| &x.multiaddr),
			&params.network_config.transport,
		)?;
		ensure_addresses_consistent_with_transport(
			params.network_config.public_addresses.iter(),
			&params.network_config.transport,
		)?;

		let (to_worker, from_service) = tracing_unbounded("mpsc_network_worker");

		if let Some(path) = params.network_config.net_config_path {
			fs::create_dir_all(&path)?;
		}

		// List of multiaddresses that we know in the network.
		let mut known_addresses = Vec::new();
		let mut bootnodes = Vec::new();
		let mut boot_node_ids = HashSet::new();

		// Process the bootnodes.
		for bootnode in params.network_config.boot_nodes.iter() {
			bootnodes.push(bootnode.peer_id.clone());
			boot_node_ids.insert(bootnode.peer_id.clone());
			known_addresses.push((bootnode.peer_id.clone(), bootnode.multiaddr.clone()));
		}

		let boot_node_ids = Arc::new(boot_node_ids);

		// Check for duplicate bootnodes.
		known_addresses.iter()
			.try_for_each(|(peer_id, addr)|
				if let Some(other) = known_addresses
					.iter()
					.find(|o| o.1 == *addr && o.0 != *peer_id)
				{
					Err(Error::DuplicateBootnode {
						address: addr.clone(),
						first_id: peer_id.clone(),
						second_id: other.0.clone(),
					})
				} else {
					Ok(())
				}
			)?;

		// Initialize the peers we should always be connected to.
		let priority_groups = {
			let mut reserved_nodes = HashSet::new();
			for reserved in params.network_config.reserved_nodes.iter() {
				reserved_nodes.insert(reserved.peer_id.clone());
				known_addresses.push((reserved.peer_id.clone(), reserved.multiaddr.clone()));
			}

			let print_deprecated_message = match &params.role {
				Role::Sentry { .. } => true,
				Role::Authority { sentry_nodes } if !sentry_nodes.is_empty() => true,
				_ => false,
			};
			if print_deprecated_message {
				log::warn!(
					"🙇 Sentry nodes are deprecated, and the `--sentry` and  `--sentry-nodes` \
					CLI options will eventually be removed in a future version. The Substrate \
					and Polkadot networking protocol require validators to be \
					publicly-accessible. Please do not block access to your validator nodes. \
					For details, see https://github.com/paritytech/substrate/issues/6845."
				);
			}

			let mut sentries_and_validators = HashSet::new();
			match &params.role {
				Role::Sentry { validators } => {
					for validator in validators {
						sentries_and_validators.insert(validator.peer_id.clone());
						reserved_nodes.insert(validator.peer_id.clone());
						known_addresses.push((validator.peer_id.clone(), validator.multiaddr.clone()));
					}
				}
				Role::Authority { sentry_nodes } => {
					for sentry_node in sentry_nodes {
						sentries_and_validators.insert(sentry_node.peer_id.clone());
						reserved_nodes.insert(sentry_node.peer_id.clone());
						known_addresses.push((sentry_node.peer_id.clone(), sentry_node.multiaddr.clone()));
					}
				}
				_ => {}
			}

			vec![
				("reserved".to_owned(), reserved_nodes),
				("sentries_and_validators".to_owned(), sentries_and_validators),
			]
		};

		let peerset_config = sc_peerset::PeersetConfig {
			in_peers: params.network_config.in_peers,
			out_peers: params.network_config.out_peers,
			bootnodes,
			reserved_only: params.network_config.non_reserved_mode == NonReservedPeerMode::Deny,
			priority_groups,
		};

		// Private and public keys configuration.
		let local_identity = params.network_config.node_key.clone().into_keypair()?;
		let local_public = local_identity.public();
		let local_peer_id = local_public.clone().into_peer_id();
		info!(
			target: "sub-libp2p",
			"🏷 Local node identity is: {}",
			local_peer_id.to_base58(),
		);

		let checker = params.on_demand.as_ref()
			.map(|od| od.checker().clone())
			.unwrap_or_else(|| Arc::new(AlwaysBadChecker));

		let num_connected = Arc::new(AtomicUsize::new(0));
		let is_major_syncing = Arc::new(AtomicBool::new(false));
		let (protocol, peerset_handle) = Protocol::new(
			protocol::ProtocolConfig {
				roles: From::from(&params.role),
				max_parallel_downloads: params.network_config.max_parallel_downloads,
			},
			local_peer_id.clone(),
			params.chain.clone(),
			params.transaction_pool,
			params.finality_proof_request_builder,
			params.protocol_id.clone(),
			peerset_config,
			params.block_announce_validator,
			params.metrics_registry.as_ref(),
			boot_node_ids.clone(),
		)?;

		// Build the swarm.
		let (mut swarm, bandwidth): (Swarm<B, H>, _) = {
			let user_agent = format!(
				"{} ({})",
				params.network_config.client_version,
				params.network_config.node_name
			);
			let block_requests = {
				let config = block_requests::Config::new(&params.protocol_id);
				block_requests::BlockRequests::new(config, params.chain.clone())
			};
			let finality_proof_requests = {
				let config = finality_requests::Config::new(&params.protocol_id);
				finality_requests::FinalityProofRequests::new(config, params.finality_proof_provider.clone())
			};
			let light_client_handler = {
				let config = light_client_handler::Config::new(&params.protocol_id);
				light_client_handler::LightClientHandler::new(
					config,
					params.chain,
					checker,
					peerset_handle.clone(),
				)
			};

			let discovery_config = {
				let mut config = DiscoveryConfig::new(local_public.clone());
				config.with_user_defined(known_addresses);
				config.discovery_limit(u64::from(params.network_config.out_peers) + 15);
				config.add_protocol(params.protocol_id.clone());
				config.allow_non_globals_in_dht(params.network_config.allow_non_globals_in_dht);
				config.use_kademlia_disjoint_query_paths(params.network_config.kademlia_disjoint_query_paths);

				match params.network_config.transport {
					TransportConfig::MemoryOnly => {
						config.with_mdns(false);
						config.allow_private_ipv4(false);
					}
					TransportConfig::Normal { enable_mdns, allow_private_ipv4, .. } => {
						config.with_mdns(enable_mdns);
						config.allow_private_ipv4(allow_private_ipv4);
					}
				}

				config
			};

			let mut behaviour = {
				let result = Behaviour::new(
					protocol,
					params.role,
					user_agent,
					local_public,
					block_requests,
					finality_proof_requests,
					light_client_handler,
					discovery_config,
					params.network_config.request_response_protocols,
				);

				match result {
					Ok(b) => b,
					Err(crate::request_responses::RegisterError::DuplicateProtocol(proto)) => {
						return Err(Error::DuplicateRequestResponseProtocol {
							protocol: proto,
						})
					},
				}
			};

			for protocol in &params.network_config.notifications_protocols {
				behaviour.register_notifications_protocol(protocol.clone());
			}
			let (transport, bandwidth) = {
				let (config_mem, config_wasm) = match params.network_config.transport {
					TransportConfig::MemoryOnly => (true, None),
					TransportConfig::Normal { wasm_external_transport, .. } =>
						(false, wasm_external_transport)
				};
				transport::build_transport(local_identity, config_mem, config_wasm)
			};
			let mut builder = SwarmBuilder::new(transport, behaviour, local_peer_id.clone())
				.peer_connection_limit(crate::MAX_CONNECTIONS_PER_PEER)
				.notify_handler_buffer_size(NonZeroUsize::new(32).expect("32 != 0; qed"))
				.connection_event_buffer_size(1024);
			if let Some(spawner) = params.executor {
				struct SpawnImpl<F>(F);
				impl<F: Fn(Pin<Box<dyn Future<Output = ()> + Send>>)> Executor for SpawnImpl<F> {
					fn exec(&self, f: Pin<Box<dyn Future<Output = ()> + Send>>) {
						(self.0)(f)
					}
				}
				builder = builder.executor(Box::new(SpawnImpl(spawner)));
			}
			(builder.build(), bandwidth)
		};

		// Initialize the metrics.
		let metrics = match &params.metrics_registry {
			Some(registry) => {
				Some(metrics::register(registry, MetricSources {
					bandwidth: bandwidth.clone(),
					major_syncing: is_major_syncing.clone(),
					connected_peers: num_connected.clone(),
				})?)
			}
			None => None
		};

		// Listen on multiaddresses.
		for addr in &params.network_config.listen_addresses {
			if let Err(err) = Swarm::<B, H>::listen_on(&mut swarm, addr.clone()) {
				warn!(target: "sub-libp2p", "Can't listen on {} because: {:?}", addr, err)
			}
		}

		// Add external addresses.
		for addr in &params.network_config.public_addresses {
			Swarm::<B, H>::add_external_address(&mut swarm, addr.clone());
		}

		let external_addresses = Arc::new(Mutex::new(Vec::new()));
		let peers_notifications_sinks = Arc::new(Mutex::new(HashMap::new()));

		let service = Arc::new(NetworkService {
			bandwidth,
			external_addresses: external_addresses.clone(),
			num_connected: num_connected.clone(),
			is_major_syncing: is_major_syncing.clone(),
			peerset: peerset_handle,
			local_peer_id,
			to_worker,
			peers_notifications_sinks: peers_notifications_sinks.clone(),
			notifications_sizes_metric:
				metrics.as_ref().map(|metrics| metrics.notifications_sizes.clone()),
			_marker: PhantomData,
		});

		Ok(NetworkWorker {
			external_addresses,
			num_connected,
			is_major_syncing,
			network_service: swarm,
			service,
			import_queue: params.import_queue,
			from_service,
			light_client_rqs: params.on_demand.and_then(|od| od.extract_receiver()),
			event_streams: out_events::OutChannels::new(params.metrics_registry.as_ref())?,
			peers_notifications_sinks,
			metrics,
			boot_node_ids,
			pending_requests: HashMap::with_capacity(128),
		})
	}

	/// High-level network status information.
	pub fn status(&self) -> NetworkStatus<B> {
		NetworkStatus {
			sync_state: self.sync_state(),
			best_seen_block: self.best_seen_block(),
			num_sync_peers: self.num_sync_peers(),
			num_connected_peers: self.num_connected_peers(),
			num_active_peers: self.num_active_peers(),
			total_bytes_inbound: self.total_bytes_inbound(),
			total_bytes_outbound: self.total_bytes_outbound(),
		}
	}

	/// Returns the total number of bytes received so far.
	pub fn total_bytes_inbound(&self) -> u64 {
		self.service.bandwidth.total_inbound()
	}

	/// Returns the total number of bytes sent so far.
	pub fn total_bytes_outbound(&self) -> u64 {
		self.service.bandwidth.total_outbound()
	}

	/// Returns the number of peers we're connected to.
	pub fn num_connected_peers(&self) -> usize {
		self.network_service.user_protocol().num_connected_peers()
	}

	/// Returns the number of peers we're connected to and that are being queried.
	pub fn num_active_peers(&self) -> usize {
		self.network_service.user_protocol().num_active_peers()
	}

	/// Current global sync state.
	pub fn sync_state(&self) -> SyncState {
		self.network_service.user_protocol().sync_state()
	}

	/// Target sync block number.
	pub fn best_seen_block(&self) -> Option<NumberFor<B>> {
		self.network_service.user_protocol().best_seen_block()
	}

	/// Number of peers participating in syncing.
	pub fn num_sync_peers(&self) -> u32 {
		self.network_service.user_protocol().num_sync_peers()
	}

	/// Number of blocks in the import queue.
	pub fn num_queued_blocks(&self) -> u32 {
		self.network_service.user_protocol().num_queued_blocks()
	}

	/// Returns the number of downloaded blocks.
	pub fn num_downloaded_blocks(&self) -> usize {
		self.network_service.user_protocol().num_downloaded_blocks()
	}

	/// Number of active sync requests.
	pub fn num_sync_requests(&self) -> usize {
		self.network_service.user_protocol().num_sync_requests()
	}

	/// Adds an address for a node.
	pub fn add_known_address(&mut self, peer_id: PeerId, addr: Multiaddr) {
		self.network_service.add_known_address(peer_id, addr);
	}

	/// Return a `NetworkService` that can be shared through the code base and can be used to
	/// manipulate the worker.
	pub fn service(&self) -> &Arc<NetworkService<B, H>> {
		&self.service
	}

	/// You must call this when a new block is finalized by the client.
	pub fn on_block_finalized(&mut self, hash: B::Hash, header: B::Header) {
		self.network_service.user_protocol_mut().on_block_finalized(hash, &header);
	}

	/// This should be called when blocks are added to the
	/// chain by something other than the import queue.
	/// Currently this is only useful for tests.
	pub fn update_chain(&mut self) {
		self.network_service.user_protocol_mut().update_chain();
	}

	/// Returns the local `PeerId`.
	pub fn local_peer_id(&self) -> &PeerId {
		Swarm::<B, H>::local_peer_id(&self.network_service)
	}

	/// Returns the list of addresses we are listening on.
	///
	/// Does **NOT** include a trailing `/p2p/` with our `PeerId`.
	pub fn listen_addresses(&self) -> impl Iterator<Item = &Multiaddr> {
		Swarm::<B, H>::listeners(&self.network_service)
	}

	/// Get network state.
	///
	/// **Note**: Use this only for debugging. This API is unstable. There are warnings literally
	/// everywhere about this. Please don't use this function to retrieve actual information.
	pub fn network_state(&mut self) -> NetworkState {
		let swarm = &mut self.network_service;
		let open = swarm.user_protocol().open_peers().cloned().collect::<Vec<_>>();

		let connected_peers = {
			let swarm = &mut *swarm;
			open.iter().filter_map(move |peer_id| {
				let known_addresses = NetworkBehaviour::addresses_of_peer(&mut **swarm, peer_id)
					.into_iter().collect();

				let endpoint = if let Some(e) = swarm.node(peer_id).map(|i| i.endpoint()) {
					e.clone().into()
				} else {
					error!(target: "sub-libp2p", "Found state inconsistency between custom protocol \
						and debug information about {:?}", peer_id);
					return None
				};

				Some((peer_id.to_base58(), NetworkStatePeer {
					endpoint,
					version_string: swarm.node(peer_id)
						.and_then(|i| i.client_version().map(|s| s.to_owned())),
					latest_ping_time: swarm.node(peer_id).and_then(|i| i.latest_ping()),
					enabled: swarm.user_protocol().is_enabled(&peer_id),
					open: swarm.user_protocol().is_open(&peer_id),
					known_addresses,
				}))
			}).collect()
		};

		let not_connected_peers = {
			let swarm = &mut *swarm;
			swarm.known_peers().into_iter()
				.filter(|p| open.iter().all(|n| n != p))
				.map(move |peer_id| {
					(peer_id.to_base58(), NetworkStateNotConnectedPeer {
						version_string: swarm.node(&peer_id)
							.and_then(|i| i.client_version().map(|s| s.to_owned())),
						latest_ping_time: swarm.node(&peer_id).and_then(|i| i.latest_ping()),
						known_addresses: NetworkBehaviour::addresses_of_peer(&mut **swarm, &peer_id)
							.into_iter().collect(),
					})
				})
				.collect()
		};

		NetworkState {
			peer_id: Swarm::<B, H>::local_peer_id(&swarm).to_base58(),
			listened_addresses: Swarm::<B, H>::listeners(&swarm).cloned().collect(),
			external_addresses: Swarm::<B, H>::external_addresses(&swarm).cloned().collect(),
			connected_peers,
			not_connected_peers,
			peerset: swarm.user_protocol_mut().peerset_debug_info(),
		}
	}

	/// Get currently connected peers.
	pub fn peers_debug_info(&mut self) -> Vec<(PeerId, PeerInfo<B>)> {
		self.network_service.user_protocol_mut()
			.peers_info()
			.map(|(id, info)| (id.clone(), info.clone()))
			.collect()
	}

	/// Removes a `PeerId` from the list of reserved peers.
	pub fn remove_reserved_peer(&self, peer: PeerId) {
		self.service.remove_reserved_peer(peer);
	}

	/// Adds a `PeerId` and its address as reserved. The string should encode the address
	/// and peer ID of the remote node.
	pub fn add_reserved_peer(&self, peer: String) -> Result<(), String> {
		self.service.add_reserved_peer(peer)
	}
}

impl<B: BlockT + 'static, H: ExHashT> NetworkService<B, H> {
	/// Returns the local `PeerId`.
	pub fn local_peer_id(&self) -> &PeerId {
		&self.local_peer_id
	}

	/// Set authorized peers.
	///
	/// Need a better solution to manage authorized peers, but now just use reserved peers for
	/// prototyping.
	pub fn set_authorized_peers(&self, peers: HashSet<PeerId>) {
		self.peerset.set_reserved_peers(peers)
	}

	/// Set authorized_only flag.
	///
	/// Need a better solution to decide authorized_only, but now just use reserved_only flag for
	/// prototyping.
	pub fn set_authorized_only(&self, reserved_only: bool) {
		self.peerset.set_reserved_only(reserved_only)
	}

	/// Appends a notification to the buffer of pending outgoing notifications with the given peer.
	/// Has no effect if the notifications channel with this protocol name is not open.
	///
	/// If the buffer of pending outgoing notifications with that peer is full, the notification
	/// is silently dropped and the connection to the remote will start being shut down. This
	/// happens if you call this method at a higher rate than the rate at which the peer processes
	/// these notifications, or if the available network bandwidth is too low.
	///
	/// For this reason, this method is considered soft-deprecated. You are encouraged to use
	/// [`NetworkService::notification_sender`] instead.
	///
	/// > **Note**: The reason why this is a no-op in the situation where we have no channel is
	/// >			that we don't guarantee message delivery anyway. Networking issues can cause
	/// >			connections to drop at any time, and higher-level logic shouldn't differentiate
	/// >			between the remote voluntarily closing a substream or a network error
	/// >			preventing the message from being delivered.
	///
	/// The protocol must have been registered with `register_notifications_protocol` or
	/// [`NetworkConfiguration::notifications_protocols`](crate::config::NetworkConfiguration::notifications_protocols).
	///
	pub fn write_notification(&self, target: PeerId, protocol: Cow<'static, str>, message: Vec<u8>) {
		// We clone the `NotificationsSink` in order to be able to unlock the network-wide
		// `peers_notifications_sinks` mutex as soon as possible.
		let sink = {
			let peers_notifications_sinks = self.peers_notifications_sinks.lock();
			if let Some(sink) = peers_notifications_sinks.get(&(target, protocol.clone())) {
				sink.clone()
			} else {
				// Notification silently discarded, as documented.
				log::error!(
					target: "sub-libp2p",
					"Attempted to send notification on unknown protocol: {:?}",
					protocol,
				);
				return;
			}
		};

		if let Some(notifications_sizes_metric) = self.notifications_sizes_metric.as_ref() {
			notifications_sizes_metric
				.with_label_values(&["out", &protocol])
				.observe(message.len() as f64);
		}

		// Sending is communicated to the `NotificationsSink`.
		sink.send_sync_notification(protocol, message);
	}

	/// Obtains a [`NotificationSender`] for a connected peer, if it exists.
	///
	/// A `NotificationSender` is scoped to a particular connection to the peer that holds
	/// a receiver. With a `NotificationSender` at hand, sending a notification is done in two steps:
	///
	/// 1.  [`NotificationSender::ready`] is used to wait for the sender to become ready
	/// for another notification, yielding a [`NotificationSenderReady`] token.
	/// 2.  [`NotificationSenderReady::send`] enqueues the notification for sending. This operation
	/// can only fail if the underlying notification substream or connection has suddenly closed.
	///
	/// An error is returned by [`NotificationSenderReady::send`] if there exists no open
	/// notifications substream with that combination of peer and protocol, or if the remote
	/// has asked to close the notifications substream. If that happens, it is guaranteed that an
	/// [`Event::NotificationStreamClosed`] has been generated on the stream returned by
	/// [`NetworkService::event_stream`].
	///
	/// If the remote requests to close the notifications substream, all notifications successfully
	/// enqueued using [`NotificationSenderReady::send`] will finish being sent out before the
	/// substream actually gets closed, but attempting to enqueue more notifications will now
	/// return an error. It is however possible for the entire connection to be abruptly closed,
	/// in which case enqueued notifications will be lost.
	///
	/// The protocol must have been registered with `register_notifications_protocol` or
	/// [`NetworkConfiguration::notifications_protocols`](crate::config::NetworkConfiguration::notifications_protocols).
	///
	/// # Usage
	///
	/// This method returns a struct that allows waiting until there is space available in the
	/// buffer of messages towards the given peer. If the peer processes notifications at a slower
	/// rate than we send them, this buffer will quickly fill up.
	///
	/// As such, you should never do something like this:
	///
	/// ```ignore
	/// // Do NOT do this
	/// for peer in peers {
	/// 	if let Ok(n) = network.notification_sender(peer, ...) {
	///			if let Ok(s) = n.ready().await {
	///				let _ = s.send(...);
	///			}
	/// 	}
	/// }
	/// ```
	///
	/// Doing so would slow down all peers to the rate of the slowest one. A malicious or
	/// malfunctioning peer could intentionally process notifications at a very slow rate.
	///
	/// Instead, you are encouraged to maintain your own buffer of notifications on top of the one
	/// maintained by `sc-network`, and use `notification_sender` to progressively send out
	/// elements from your buffer. If this additional buffer is full (which will happen at some
	/// point if the peer is too slow to process notifications), appropriate measures can be taken,
	/// such as removing non-critical notifications from the buffer or disconnecting the peer
	/// using [`NetworkService::disconnect_peer`].
	///
	///
	/// Notifications              Per-peer buffer
	///   broadcast    +------->   of notifications   +-->  `notification_sender`  +-->  Internet
	///                    ^       (not covered by
	///                    |         sc-network)
	///                    +
	///      Notifications should be dropped
	///             if buffer is full
	///
	///
	/// See also the [`gossip`](crate::gossip) module for a higher-level way to send
	/// notifications.
	///
	pub fn notification_sender(
		&self,
		target: PeerId,
		protocol: Cow<'static, str>,
	) -> Result<NotificationSender, NotificationSenderError> {
		// We clone the `NotificationsSink` in order to be able to unlock the network-wide
		// `peers_notifications_sinks` mutex as soon as possible.
		let sink = {
			let peers_notifications_sinks = self.peers_notifications_sinks.lock();
			if let Some(sink) = peers_notifications_sinks.get(&(target, protocol.clone())) {
				sink.clone()
			} else {
				return Err(NotificationSenderError::Closed);
			}
		};

		let notification_size_metric = self.notifications_sizes_metric.as_ref().map(|histogram| {
			histogram.with_label_values(&["out", &protocol])
		});

		Ok(NotificationSender {
			sink,
			protocol_name: protocol,
			notification_size_metric,
		})
	}

	/// Returns a stream containing the events that happen on the network.
	///
	/// If this method is called multiple times, the events are duplicated.
	///
	/// The stream never ends (unless the `NetworkWorker` gets shut down).
	///
	/// The name passed is used to identify the channel in the Prometheus metrics. Note that the
	/// parameter is a `&'static str`, and not a `String`, in order to avoid accidentally having
	/// an unbounded set of Prometheus metrics, which would be quite bad in terms of memory
	pub fn event_stream(&self, name: &'static str) -> impl Stream<Item = Event> {
		let (tx, rx) = out_events::channel(name);
		let _ = self.to_worker.unbounded_send(ServiceToWorkerMsg::EventStream(tx));
		rx
	}

	/// Sends a single targeted request to a specific peer. On success, returns the response of
	/// the peer.
	///
	/// Request-response protocols are a way to complement notifications protocols, but
	/// notifications should remain the default ways of communicating information. For example, a
	/// peer can announce something through a notification, after which the recipient can obtain
	/// more information by performing a request.
	/// As such, this function is meant to be called only with peers we are already connected to.
	/// Calling this method with a `target` we are not connected to will *not* attempt to connect
	/// to said peer.
	///
	/// No limit or throttling of concurrent outbound requests per peer and protocol are enforced.
	/// Such restrictions, if desired, need to be enforced at the call site(s).
	///
	/// The protocol must have been registered through
	/// [`NetworkConfiguration::request_response_protocols`](
	/// crate::config::NetworkConfiguration::request_response_protocols).
	pub async fn request(
		&self,
		target: PeerId,
		protocol: impl Into<Cow<'static, str>>,
		request: Vec<u8>
	) -> Result<Vec<u8>, RequestFailure> {
		let (tx, rx) = oneshot::channel();
		let _ = self.to_worker.unbounded_send(ServiceToWorkerMsg::Request {
			target,
			protocol: protocol.into(),
			request,
			pending_response: tx
		});

		match rx.await {
			Ok(v) => v,
			// The channel can only be closed if the network worker no longer exists. If the
			// network worker no longer exists, then all connections to `target` are necessarily
			// closed, and we legitimately report this situation as a "ConnectionClosed".
			Err(_) => Err(RequestFailure::Network(OutboundFailure::ConnectionClosed)),
		}
	}

	/// Registers a new notifications protocol.
	///
	/// After a protocol has been registered, you can call `write_notifications`.
	///
	/// **Important**: This method is a work-around, and you are instead strongly encouraged to
	/// pass the protocol in the `NetworkConfiguration::notifications_protocols` list instead.
	/// If you have no other choice but to use this method, you are very strongly encouraged to
	/// call it very early on. Any connection open will retain the protocols that were registered
	/// then, and not any new one.
	///
	/// Please call `event_stream` before registering a protocol, otherwise you may miss events
	/// about the protocol that you have registered.
	// TODO: remove this method after https://github.com/paritytech/substrate/issues/4587
	pub fn register_notifications_protocol(
		&self,
		protocol_name: impl Into<Cow<'static, str>>,
	) {
		let _ = self.to_worker.unbounded_send(ServiceToWorkerMsg::RegisterNotifProtocol {
			protocol_name: protocol_name.into(),
		});
	}

	/// You may call this when new transactons are imported by the transaction pool.
	///
	/// All transactions will be fetched from the `TransactionPool` that was passed at
	/// initialization as part of the configuration and propagated to peers.
	pub fn trigger_repropagate(&self) {
		let _ = self.to_worker.unbounded_send(ServiceToWorkerMsg::PropagateTransactions);
	}

	/// You must call when new transaction is imported by the transaction pool.
	///
	/// This transaction will be fetched from the `TransactionPool` that was passed at
	/// initialization as part of the configuration and propagated to peers.
	pub fn propagate_transaction(&self, hash: H) {
		let _ = self.to_worker.unbounded_send(ServiceToWorkerMsg::PropagateTransaction(hash));
	}

	/// Make sure an important block is propagated to peers.
	///
	/// In chain-based consensus, we often need to make sure non-best forks are
	/// at least temporarily synced. This function forces such an announcement.
	pub fn announce_block(&self, hash: B::Hash, data: Vec<u8>) {
		let _ = self.to_worker.unbounded_send(ServiceToWorkerMsg::AnnounceBlock(hash, data));
	}

	/// Report a given peer as either beneficial (+) or costly (-) according to the
	/// given scalar.
	pub fn report_peer(&self, who: PeerId, cost_benefit: ReputationChange) {
		self.peerset.report_peer(who, cost_benefit);
	}

	/// Disconnect from a node as soon as possible.
	///
	/// This triggers the same effects as if the connection had closed itself spontaneously.
	pub fn disconnect_peer(&self, who: PeerId) {
		let _ = self.to_worker.unbounded_send(ServiceToWorkerMsg::DisconnectPeer(who));
	}

	/// Request a justification for the given block from the network.
	///
	/// On success, the justification will be passed to the import queue that was part at
	/// initialization as part of the configuration.
	pub fn request_justification(&self, hash: &B::Hash, number: NumberFor<B>) {
		let _ = self
			.to_worker
			.unbounded_send(ServiceToWorkerMsg::RequestJustification(*hash, number));
	}

	/// Are we in the process of downloading the chain?
	pub fn is_major_syncing(&self) -> bool {
		self.is_major_syncing.load(Ordering::Relaxed)
	}

	/// Start getting a value from the DHT.
	///
	/// This will generate either a `ValueFound` or a `ValueNotFound` event and pass it as an
	/// item on the [`NetworkWorker`] stream.
	pub fn get_value(&self, key: &record::Key) {
		let _ = self
			.to_worker
			.unbounded_send(ServiceToWorkerMsg::GetValue(key.clone()));
	}

	/// Start putting a value in the DHT.
	///
	/// This will generate either a `ValuePut` or a `ValuePutFailed` event and pass it as an
	/// item on the [`NetworkWorker`] stream.
	pub fn put_value(&self, key: record::Key, value: Vec<u8>) {
		let _ = self
			.to_worker
			.unbounded_send(ServiceToWorkerMsg::PutValue(key, value));
	}

	/// Connect to unreserved peers and allow unreserved peers to connect.
	pub fn accept_unreserved_peers(&self) {
		self.peerset.set_reserved_only(false);
	}

	/// Disconnect from unreserved peers and deny new unreserved peers to connect.
	pub fn deny_unreserved_peers(&self) {
		self.peerset.set_reserved_only(true);
	}

	/// Removes a `PeerId` from the list of reserved peers.
	pub fn remove_reserved_peer(&self, peer: PeerId) {
		self.peerset.remove_reserved_peer(peer);
	}

	/// Adds a `PeerId` and its address as reserved. The string should encode the address
	/// and peer ID of the remote node.
	///
	/// Returns an `Err` if the given string is not a valid multiaddress
	/// or contains an invalid peer ID (which includes the local peer ID).
	pub fn add_reserved_peer(&self, peer: String) -> Result<(), String> {
		let (peer_id, addr) = parse_str_addr(&peer).map_err(|e| format!("{:?}", e))?;
		// Make sure the local peer ID is never added to the PSM.
		if peer_id == self.local_peer_id {
			return Err("Local peer ID cannot be added as a reserved peer.".to_string())
		}
		self.peerset.add_reserved_peer(peer_id.clone());
		let _ = self
			.to_worker
			.unbounded_send(ServiceToWorkerMsg::AddKnownAddress(peer_id, addr));
		Ok(())
	}

	/// Configure an explicit fork sync request.
	/// Note that this function should not be used for recent blocks.
	/// Sync should be able to download all the recent forks normally.
	/// `set_sync_fork_request` should only be used if external code detects that there's
	/// a stale fork missing.
	/// Passing empty `peers` set effectively removes the sync request.
	pub fn set_sync_fork_request(&self, peers: Vec<PeerId>, hash: B::Hash, number: NumberFor<B>) {
		let _ = self
			.to_worker
			.unbounded_send(ServiceToWorkerMsg::SyncFork(peers, hash, number));
	}

	/// Modify a peerset priority group.
	///
	/// Each `Multiaddr` must end with a `/p2p/` component containing the `PeerId`.
	///
	/// Returns an `Err` if one of the given addresses is invalid or contains an
	/// invalid peer ID (which includes the local peer ID).
	//
	// NOTE: even though this function is currently sync, it's marked as async for
	// future-proofing, see https://github.com/paritytech/substrate/pull/7247#discussion_r502263451.
	pub async fn set_priority_group(&self, group_id: String, peers: HashSet<Multiaddr>) -> Result<(), String> {
		let peers = self.split_multiaddr_and_peer_id(peers)?;

		let peer_ids = peers.iter().map(|(peer_id, _addr)| peer_id.clone()).collect();

		self.peerset.set_priority_group(group_id, peer_ids);

		for (peer_id, addr) in peers.into_iter() {
			let _ = self
				.to_worker
				.unbounded_send(ServiceToWorkerMsg::AddKnownAddress(peer_id, addr));
		}

		Ok(())
	}

	/// Add peers to a peerset priority group.
	///
	/// Each `Multiaddr` must end with a `/p2p/` component containing the `PeerId`.
	///
	/// Returns an `Err` if one of the given addresses is invalid or contains an
	/// invalid peer ID (which includes the local peer ID).
	//
	// NOTE: even though this function is currently sync, it's marked as async for
	// future-proofing, see https://github.com/paritytech/substrate/pull/7247#discussion_r502263451.
	pub async fn add_to_priority_group(&self, group_id: String, peers: HashSet<Multiaddr>) -> Result<(), String> {
		let peers = self.split_multiaddr_and_peer_id(peers)?;

		for (peer_id, addr) in peers.into_iter() {
			self.peerset.add_to_priority_group(group_id.clone(), peer_id.clone());

			let _ = self
				.to_worker
				.unbounded_send(ServiceToWorkerMsg::AddKnownAddress(peer_id, addr));
		}

		Ok(())
	}

	/// Remove peers from a peerset priority group.
	///
	/// Each `Multiaddr` must end with a `/p2p/` component containing the `PeerId`.
	///
	/// Returns an `Err` if one of the given addresses is invalid or contains an
	/// invalid peer ID (which includes the local peer ID).
	//
	// NOTE: even though this function is currently sync, it's marked as async for
	// future-proofing, see https://github.com/paritytech/substrate/pull/7247#discussion_r502263451.
	// NOTE: technically, this function only needs `Vec<PeerId>`, but we use `Multiaddr` here for convenience.
	pub async fn remove_from_priority_group(&self, group_id: String, peers: HashSet<Multiaddr>) -> Result<(), String> {
		let peers = self.split_multiaddr_and_peer_id(peers)?;
		for (peer_id, _) in peers.into_iter() {
			self.peerset.remove_from_priority_group(group_id.clone(), peer_id);
		}
		Ok(())
	}

	/// Returns the number of peers we're connected to.
	pub fn num_connected(&self) -> usize {
		self.num_connected.load(Ordering::Relaxed)
	}

	/// This function should be called when blocks are added to the chain by something other
	/// than the import queue.
	///
	/// > **Important**: This function is a hack and can be removed at any time. Do **not** use it.
	pub fn update_chain(&self) {
		let _ = self
			.to_worker
			.unbounded_send(ServiceToWorkerMsg::UpdateChain);
	}

	/// Inform the network service about an own imported block.
	pub fn own_block_imported(&self, hash: B::Hash, number: NumberFor<B>) {
		let _ = self
			.to_worker
			.unbounded_send(ServiceToWorkerMsg::OwnBlockImported(hash, number));
	}

	/// Utility function to extract `PeerId` from each `Multiaddr` for priority group updates.
	///
	/// Returns an `Err` if one of the given addresses is invalid or contains an
	/// invalid peer ID (which includes the local peer ID).
	fn split_multiaddr_and_peer_id(&self, peers: HashSet<Multiaddr>) -> Result<Vec<(PeerId, Multiaddr)>, String> {
		peers.into_iter()
			.map(|mut addr| {
				let peer = match addr.pop() {
					Some(multiaddr::Protocol::P2p(key)) => PeerId::from_multihash(key)
						.map_err(|_| "Invalid PeerId format".to_string())?,
					_ => return Err("Missing PeerId from address".to_string()),
				};

				// Make sure the local peer ID is never added to the PSM
				// or added as a "known address", even if given.
				if peer == self.local_peer_id {
					Err("Local peer ID in priority group.".to_string())
				} else {
					Ok((peer, addr))
				}
			})
			.collect::<Result<Vec<(PeerId, Multiaddr)>, String>>()
	}
}

impl<B: BlockT + 'static, H: ExHashT> sp_consensus::SyncOracle
	for NetworkService<B, H>
{
	fn is_major_syncing(&mut self) -> bool {
		NetworkService::is_major_syncing(self)
	}

	fn is_offline(&mut self) -> bool {
		self.num_connected.load(Ordering::Relaxed) == 0
	}
}

impl<'a, B: BlockT + 'static, H: ExHashT> sp_consensus::SyncOracle
	for &'a NetworkService<B, H>
{
	fn is_major_syncing(&mut self) -> bool {
		NetworkService::is_major_syncing(self)
	}

	fn is_offline(&mut self) -> bool {
		self.num_connected.load(Ordering::Relaxed) == 0
	}
}

impl<B, H> NetworkStateInfo for NetworkService<B, H>
	where
		B: sp_runtime::traits::Block,
		H: ExHashT,
{
	/// Returns the local external addresses.
	fn external_addresses(&self) -> Vec<Multiaddr> {
		self.external_addresses.lock().clone()
	}

	/// Returns the local Peer ID.
	fn local_peer_id(&self) -> PeerId {
		self.local_peer_id.clone()
	}
}

/// A `NotificationSender` allows for sending notifications to a peer with a chosen protocol.
#[must_use]
pub struct NotificationSender {
	sink: NotificationsSink,

	/// Name of the protocol on the wire.
	protocol_name: Cow<'static, str>,

	/// Field extracted from the [`Metrics`] struct and necessary to report the
	/// notifications-related metrics.
	notification_size_metric: Option<Histogram>,
}

impl NotificationSender {
	/// Returns a future that resolves when the `NotificationSender` is ready to send a notification.
	pub async fn ready<'a>(&'a self) -> Result<NotificationSenderReady<'a>, NotificationSenderError> {
		Ok(NotificationSenderReady {
			ready: match self.sink.reserve_notification(self.protocol_name.clone()).await {
				Ok(r) => r,
				Err(()) => return Err(NotificationSenderError::Closed),
			},
			notification_size_metric: self.notification_size_metric.clone(),
		})
	}
}

/// Reserved slot in the notifications buffer, ready to accept data.
#[must_use]
pub struct NotificationSenderReady<'a> {
	ready: Ready<'a>,

	/// Field extracted from the [`Metrics`] struct and necessary to report the
	/// notifications-related metrics.
	notification_size_metric: Option<Histogram>,
}

impl<'a> NotificationSenderReady<'a> {
	/// Consumes this slots reservation and actually queues the notification.
	pub fn send(self, notification: impl Into<Vec<u8>>) -> Result<(), NotificationSenderError> {
		let notification = notification.into();

		if let Some(notification_size_metric) = &self.notification_size_metric {
			notification_size_metric.observe(notification.len() as f64);
		}

		self.ready
			.send(notification)
			.map_err(|()| NotificationSenderError::Closed)
	}
}

/// Error returned by [`NetworkService::send_notification`].
#[derive(Debug, derive_more::Display, derive_more::Error)]
pub enum NotificationSenderError {
	/// The notification receiver has been closed, usually because the underlying connection closed.
	///
	/// Some of the notifications most recently sent may not have been received. However,
	/// the peer may still be connected and a new `NotificationSender` for the same
	/// protocol obtained from [`NetworkService::notification_sender`].
	Closed,
	/// Protocol name hasn't been registered.
	BadProtocol,
}

/// Messages sent from the `NetworkService` to the `NetworkWorker`.
///
/// Each entry corresponds to a method of `NetworkService`.
enum ServiceToWorkerMsg<B: BlockT, H: ExHashT> {
	PropagateTransaction(H),
	PropagateTransactions,
	RequestJustification(B::Hash, NumberFor<B>),
	AnnounceBlock(B::Hash, Vec<u8>),
	GetValue(record::Key),
	PutValue(record::Key, Vec<u8>),
	AddKnownAddress(PeerId, Multiaddr),
	SyncFork(Vec<PeerId>, B::Hash, NumberFor<B>),
	EventStream(out_events::Sender),
	Request {
		target: PeerId,
		protocol: Cow<'static, str>,
		request: Vec<u8>,
		pending_response: oneshot::Sender<Result<Vec<u8>, RequestFailure>>,
	},
	RegisterNotifProtocol {
		protocol_name: Cow<'static, str>,
	},
	DisconnectPeer(PeerId),
	UpdateChain,
	OwnBlockImported(B::Hash, NumberFor<B>),
}

/// Main network worker. Must be polled in order for the network to advance.
///
/// You are encouraged to poll this in a separate background thread or task.
#[must_use = "The NetworkWorker must be polled in order for the network to advance"]
pub struct NetworkWorker<B: BlockT + 'static, H: ExHashT> {
	/// Updated by the `NetworkWorker` and loaded by the `NetworkService`.
	external_addresses: Arc<Mutex<Vec<Multiaddr>>>,
	/// Updated by the `NetworkWorker` and loaded by the `NetworkService`.
	num_connected: Arc<AtomicUsize>,
	/// Updated by the `NetworkWorker` and loaded by the `NetworkService`.
	is_major_syncing: Arc<AtomicBool>,
	/// The network service that can be extracted and shared through the codebase.
	service: Arc<NetworkService<B, H>>,
	/// The *actual* network.
	network_service: Swarm<B, H>,
	/// The import queue that was passed at initialization.
	import_queue: Box<dyn ImportQueue<B>>,
	/// Messages from the [`NetworkService`] that must be processed.
	from_service: TracingUnboundedReceiver<ServiceToWorkerMsg<B, H>>,
	/// Receiver for queries from the light client that must be processed.
	light_client_rqs: Option<TracingUnboundedReceiver<light_client_handler::Request<B>>>,
	/// Senders for events that happen on the network.
	event_streams: out_events::OutChannels,
	/// Prometheus network metrics.
	metrics: Option<Metrics>,
	/// The `PeerId`'s of all boot nodes.
	boot_node_ids: Arc<HashSet<PeerId>>,
	/// Requests started using [`NetworkService::request`]. Includes the channel to send back the
	/// response, when the request has started, and the name of the protocol for diagnostic
	/// purposes.
	pending_requests: HashMap<
		behaviour::RequestId,
		(oneshot::Sender<Result<Vec<u8>, RequestFailure>>, Instant, String)
	>,
	/// For each peer and protocol combination, an object that allows sending notifications to
	/// that peer. Shared with the [`NetworkService`].
	peers_notifications_sinks: Arc<Mutex<HashMap<(PeerId, Cow<'static, str>), NotificationsSink>>>,
}

impl<B: BlockT + 'static, H: ExHashT> Future for NetworkWorker<B, H> {
	type Output = ();

	fn poll(mut self: Pin<&mut Self>, cx: &mut std::task::Context) -> Poll<Self::Output> {
		let this = &mut *self;

		// Poll the import queue for actions to perform.
		this.import_queue.poll_actions(cx, &mut NetworkLink {
			protocol: &mut this.network_service,
		});

		// Check for new incoming light client requests.
		if let Some(light_client_rqs) = this.light_client_rqs.as_mut() {
			while let Poll::Ready(Some(rq)) = light_client_rqs.poll_next_unpin(cx) {
				// This can error if there are too many queued requests already.
				if this.network_service.light_client_request(rq).is_err() {
					log::warn!("Couldn't start light client request: too many pending requests");
				}
				if let Some(metrics) = this.metrics.as_ref() {
					metrics.issued_light_requests.inc();
				}
			}
		}

		// At the time of writing of this comment, due to a high volume of messages, the network
		// worker sometimes takes a long time to process the loop below. When that happens, the
		// rest of the polling is frozen. In order to avoid negative side-effects caused by this
		// freeze, a limit to the number of iterations is enforced below. If the limit is reached,
		// the task is interrupted then scheduled again.
		//
		// This allows for a more even distribution in the time taken by each sub-part of the
		// polling.
		let mut num_iterations = 0;
		loop {
			num_iterations += 1;
			if num_iterations >= 100 {
				cx.waker().wake_by_ref();
				break;
			}

			// Process the next message coming from the `NetworkService`.
			let msg = match this.from_service.poll_next_unpin(cx) {
				Poll::Ready(Some(msg)) => msg,
				Poll::Ready(None) => return Poll::Ready(()),
				Poll::Pending => break,
			};

			match msg {
				ServiceToWorkerMsg::AnnounceBlock(hash, data) =>
					this.network_service.user_protocol_mut().announce_block(hash, data),
				ServiceToWorkerMsg::RequestJustification(hash, number) =>
					this.network_service.user_protocol_mut().request_justification(&hash, number),
				ServiceToWorkerMsg::PropagateTransaction(hash) =>
					this.network_service.user_protocol_mut().propagate_transaction(&hash),
				ServiceToWorkerMsg::PropagateTransactions =>
					this.network_service.user_protocol_mut().propagate_transactions(),
				ServiceToWorkerMsg::GetValue(key) =>
					this.network_service.get_value(&key),
				ServiceToWorkerMsg::PutValue(key, value) =>
					this.network_service.put_value(key, value),
				ServiceToWorkerMsg::AddKnownAddress(peer_id, addr) =>
					this.network_service.add_known_address(peer_id, addr),
				ServiceToWorkerMsg::SyncFork(peer_ids, hash, number) =>
					this.network_service.user_protocol_mut().set_sync_fork_request(peer_ids, &hash, number),
				ServiceToWorkerMsg::EventStream(sender) =>
					this.event_streams.push(sender),
				ServiceToWorkerMsg::Request { target, protocol, request, pending_response } => {
					// Calling `send_request` can fail immediately in some circumstances.
					// This is handled by sending back an error on the channel.
					match this.network_service.send_request(&target, &protocol, request) {
						Ok(request_id) => {
							if let Some(metrics) = this.metrics.as_ref() {
								metrics.requests_out_started_total
									.with_label_values(&[&protocol])
									.inc();
							}
							this.pending_requests.insert(
								request_id,
								(pending_response, Instant::now(), protocol.to_string())
							);
						},
						Err(behaviour::SendRequestError::NotConnected) => {
							let err = RequestFailure::Network(OutboundFailure::ConnectionClosed);
							let _ = pending_response.send(Err(err));
						},
						Err(behaviour::SendRequestError::UnknownProtocol) => {
							let err = RequestFailure::Network(OutboundFailure::UnsupportedProtocols);
							let _ = pending_response.send(Err(err));
						},
					}
				},
				ServiceToWorkerMsg::RegisterNotifProtocol { protocol_name } =>
					this.network_service.register_notifications_protocol(protocol_name),
				ServiceToWorkerMsg::DisconnectPeer(who) =>
					this.network_service.user_protocol_mut().disconnect_peer(&who),
				ServiceToWorkerMsg::UpdateChain =>
					this.network_service.user_protocol_mut().update_chain(),
				ServiceToWorkerMsg::OwnBlockImported(hash, number) =>
					this.network_service.user_protocol_mut().own_block_imported(hash, number),
			}
		}

		// `num_iterations` serves the same purpose as in the previous loop.
		// See the previous loop for explanations.
		let mut num_iterations = 0;
		loop {
			num_iterations += 1;
			if num_iterations >= 1000 {
				cx.waker().wake_by_ref();
				break;
			}

			// Process the next action coming from the network.
			let next_event = this.network_service.next_event();
			futures::pin_mut!(next_event);
			let poll_value = next_event.poll_unpin(cx);

			match poll_value {
				Poll::Pending => break,
				Poll::Ready(SwarmEvent::Behaviour(BehaviourOut::BlockImport(origin, blocks))) => {
					if let Some(metrics) = this.metrics.as_ref() {
						metrics.import_queue_blocks_submitted.inc();
					}
					this.import_queue.import_blocks(origin, blocks);
				},
				Poll::Ready(SwarmEvent::Behaviour(BehaviourOut::JustificationImport(origin, hash, nb, justification))) => {
					if let Some(metrics) = this.metrics.as_ref() {
						metrics.import_queue_justifications_submitted.inc();
					}
					this.import_queue.import_justification(origin, hash, nb, justification);
				},
				Poll::Ready(SwarmEvent::Behaviour(BehaviourOut::FinalityProofImport(origin, hash, nb, proof))) => {
					if let Some(metrics) = this.metrics.as_ref() {
						metrics.import_queue_finality_proofs_submitted.inc();
					}
					this.import_queue.import_finality_proof(origin, hash, nb, proof);
				},
				Poll::Ready(SwarmEvent::Behaviour(BehaviourOut::InboundRequest { protocol, result, .. })) => {
					if let Some(metrics) = this.metrics.as_ref() {
						match result {
							Ok(serve_time) => {
								metrics.requests_in_success_total
									.with_label_values(&[&protocol])
									.observe(serve_time.as_secs_f64());
							}
							Err(err) => {
								let reason = match err {
									ResponseFailure::Busy => "busy",
									ResponseFailure::Network(InboundFailure::Timeout) => "timeout",
									ResponseFailure::Network(InboundFailure::UnsupportedProtocols) =>
										"unsupported",
									ResponseFailure::Network(InboundFailure::ConnectionClosed) =>
										"connection-closed",
								};

								metrics.requests_in_failure_total
									.with_label_values(&[&protocol, reason])
									.inc();
							}
						}
					}
				},
				Poll::Ready(SwarmEvent::Behaviour(BehaviourOut::RequestFinished { request_id, result })) => {
					if let Some((send_back, started, protocol)) = this.pending_requests.remove(&request_id) {
						if let Some(metrics) = this.metrics.as_ref() {
							match &result {
								Ok(_) => {
									metrics.requests_out_success_total
										.with_label_values(&[&protocol])
										.observe(started.elapsed().as_secs_f64());
								}
								Err(err) => {
									let reason = match err {
										RequestFailure::Refused => "refused",
										RequestFailure::Network(OutboundFailure::DialFailure) =>
											"dial-failure",
										RequestFailure::Network(OutboundFailure::Timeout) =>
											"timeout",
										RequestFailure::Network(OutboundFailure::ConnectionClosed) =>
											"connection-closed",
										RequestFailure::Network(OutboundFailure::UnsupportedProtocols) =>
											"unsupported",
									};

									metrics.requests_out_failure_total
										.with_label_values(&[&protocol, reason])
										.inc();
								}
							}
						}
						let _ = send_back.send(result);
					} else {
						error!("Request not in pending_requests");
					}
				},
				Poll::Ready(SwarmEvent::Behaviour(BehaviourOut::OpaqueRequestStarted { protocol, .. })) => {
					if let Some(metrics) = this.metrics.as_ref() {
						metrics.requests_out_started_total
							.with_label_values(&[&protocol])
							.inc();
					}
				},
				Poll::Ready(SwarmEvent::Behaviour(BehaviourOut::OpaqueRequestFinished { protocol, request_duration, .. })) => {
					if let Some(metrics) = this.metrics.as_ref() {
						metrics.requests_out_success_total
							.with_label_values(&[&protocol])
							.observe(request_duration.as_secs_f64());
					}
				},
				Poll::Ready(SwarmEvent::Behaviour(BehaviourOut::RandomKademliaStarted(protocol))) => {
					if let Some(metrics) = this.metrics.as_ref() {
						metrics.kademlia_random_queries_total
							.with_label_values(&[&protocol.as_ref()])
							.inc();
					}
				},
				Poll::Ready(SwarmEvent::Behaviour(BehaviourOut::NotificationStreamOpened {
					remote, protocol, notifications_sink, role
				})) => {
					if let Some(metrics) = this.metrics.as_ref() {
						metrics.notifications_streams_opened_total
							.with_label_values(&[&protocol]).inc();
					}
					{
						let mut peers_notifications_sinks = this.peers_notifications_sinks.lock();
						peers_notifications_sinks.insert((remote.clone(), protocol.clone()), notifications_sink);
					}
					this.event_streams.send(Event::NotificationStreamOpened {
						remote,
						protocol,
						role,
					});
				},
				Poll::Ready(SwarmEvent::Behaviour(BehaviourOut::NotificationStreamReplaced {
					remote, protocol, notifications_sink
				})) => {
					let mut peers_notifications_sinks = this.peers_notifications_sinks.lock();
					if let Some(s) = peers_notifications_sinks.get_mut(&(remote, protocol)) {
						*s = notifications_sink;
					} else {
						log::error!(
							target: "sub-libp2p",
							"NotificationStreamReplaced for non-existing substream"
						);
					}

					// TODO: Notifications might have been lost as a result of the previous
					// connection being dropped, and as a result it would be preferable to notify
					// the users of this fact by simulating the substream being closed then
					// reopened.
					// The code below doesn't compile because `role` is unknown. Propagating the
					// handshake of the secondary connections is quite an invasive change and
					// would conflict with https://github.com/paritytech/substrate/issues/6403.
					// Considering that dropping notifications is generally regarded as
					// acceptable, this bug is at the moment intentionally left there and is
					// intended to be fixed at the same time as
					// https://github.com/paritytech/substrate/issues/6403.
					/*this.event_streams.send(Event::NotificationStreamClosed {
						remote,
						protocol,
					});
					this.event_streams.send(Event::NotificationStreamOpened {
						remote,
						protocol,
						role,
					});*/
				},
				Poll::Ready(SwarmEvent::Behaviour(BehaviourOut::NotificationStreamClosed { remote, protocol })) => {
					if let Some(metrics) = this.metrics.as_ref() {
						metrics.notifications_streams_closed_total
							.with_label_values(&[&protocol[..]]).inc();
					}
					this.event_streams.send(Event::NotificationStreamClosed {
						remote: remote.clone(),
						protocol: protocol.clone(),
					});
					{
						let mut peers_notifications_sinks = this.peers_notifications_sinks.lock();
						peers_notifications_sinks.remove(&(remote.clone(), protocol));
					}
				},
				Poll::Ready(SwarmEvent::Behaviour(BehaviourOut::NotificationsReceived { remote, messages })) => {
					if let Some(metrics) = this.metrics.as_ref() {
						for (protocol, message) in &messages {
							metrics.notifications_sizes
								.with_label_values(&["in", protocol])
								.observe(message.len() as f64);
						}
					}
					this.event_streams.send(Event::NotificationsReceived {
						remote,
						messages,
					});
				},
				Poll::Ready(SwarmEvent::Behaviour(BehaviourOut::Dht(event, duration))) => {
					if let Some(metrics) = this.metrics.as_ref() {
						let query_type = match event {
							DhtEvent::ValueFound(_) => "value-found",
							DhtEvent::ValueNotFound(_) => "value-not-found",
							DhtEvent::ValuePut(_) => "value-put",
							DhtEvent::ValuePutFailed(_) => "value-put-failed",
						};
						metrics.kademlia_query_duration.with_label_values(&[query_type])
							.observe(duration.as_secs_f64());
					}

					this.event_streams.send(Event::Dht(event));
				},
				Poll::Ready(SwarmEvent::ConnectionEstablished { peer_id, endpoint, num_established }) => {
					trace!(target: "sub-libp2p", "Libp2p => Connected({:?})", peer_id);

					if let Some(metrics) = this.metrics.as_ref() {
						let direction = match endpoint {
							ConnectedPoint::Dialer { .. } => "out",
							ConnectedPoint::Listener { .. } => "in",
						};
						metrics.connections_opened_total.with_label_values(&[direction]).inc();

						if num_established.get() == 1 {
							metrics.distinct_peers_connections_opened_total.inc();
						}
					}
				},
				Poll::Ready(SwarmEvent::ConnectionClosed { peer_id, cause, endpoint, num_established }) => {
					trace!(target: "sub-libp2p", "Libp2p => Disconnected({:?}, {:?})", peer_id, cause);
					if let Some(metrics) = this.metrics.as_ref() {
						let direction = match endpoint {
							ConnectedPoint::Dialer { .. } => "out",
							ConnectedPoint::Listener { .. } => "in",
						};
						let reason = match cause {
							Some(ConnectionError::IO(_)) => "transport-error",
							Some(ConnectionError::Handler(NodeHandlerWrapperError::Handler(EitherError::A(EitherError::A(
								EitherError::A(EitherError::A(EitherError::A(EitherError::B(
								EitherError::A(PingFailure::Timeout)))))))))) => "ping-timeout",
							Some(ConnectionError::Handler(NodeHandlerWrapperError::Handler(EitherError::A(EitherError::A(
								EitherError::A(EitherError::A(EitherError::A(EitherError::A(
								NotifsHandlerError::SyncNotificationsClogged))))))))) => "sync-notifications-clogged",
							Some(ConnectionError::Handler(NodeHandlerWrapperError::Handler(_))) => "protocol-error",
							Some(ConnectionError::Handler(NodeHandlerWrapperError::KeepAliveTimeout)) => "keep-alive-timeout",
							None => "actively-closed",
						};
						metrics.connections_closed_total.with_label_values(&[direction, reason]).inc();

						// `num_established` represents the number of *remaining* connections.
						if num_established == 0 {
							metrics.distinct_peers_connections_closed_total.inc();
						}
					}
				},
				Poll::Ready(SwarmEvent::NewListenAddr(addr)) => {
					trace!(target: "sub-libp2p", "Libp2p => NewListenAddr({})", addr);
					if let Some(metrics) = this.metrics.as_ref() {
						metrics.listeners_local_addresses.inc();
					}
				},
				Poll::Ready(SwarmEvent::ExpiredListenAddr(addr)) => {
					info!(target: "sub-libp2p", "📪 No longer listening on {}", addr);
					if let Some(metrics) = this.metrics.as_ref() {
						metrics.listeners_local_addresses.dec();
					}
				},
				Poll::Ready(SwarmEvent::UnreachableAddr { peer_id, address, error, .. }) => {
					trace!(
						target: "sub-libp2p", "Libp2p => Failed to reach {:?} through {:?}: {}",
						peer_id,
						address,
						error,
					);

					if this.boot_node_ids.contains(&peer_id) {
						if let PendingConnectionError::InvalidPeerId = error {
							error!(
								"💔 The bootnode you want to connect to at `{}` provided a different peer ID than the one you expect: `{}`.",
								address,
								peer_id,
							);
						}
					}

					if let Some(metrics) = this.metrics.as_ref() {
						match error {
							PendingConnectionError::ConnectionLimit(_) =>
								metrics.pending_connections_errors_total.with_label_values(&["limit-reached"]).inc(),
							PendingConnectionError::InvalidPeerId =>
								metrics.pending_connections_errors_total.with_label_values(&["invalid-peer-id"]).inc(),
							PendingConnectionError::Transport(_) | PendingConnectionError::IO(_) =>
								metrics.pending_connections_errors_total.with_label_values(&["transport-error"]).inc(),
						}
					}
				}
				Poll::Ready(SwarmEvent::Dialing(peer_id)) =>
					trace!(target: "sub-libp2p", "Libp2p => Dialing({:?})", peer_id),
				Poll::Ready(SwarmEvent::IncomingConnection { local_addr, send_back_addr }) => {
					trace!(target: "sub-libp2p", "Libp2p => IncomingConnection({},{}))",
						local_addr, send_back_addr);
					if let Some(metrics) = this.metrics.as_ref() {
						metrics.incoming_connections_total.inc();
					}
				},
				Poll::Ready(SwarmEvent::IncomingConnectionError { local_addr, send_back_addr, error }) => {
					trace!(target: "sub-libp2p", "Libp2p => IncomingConnectionError({},{}): {}",
						local_addr, send_back_addr, error);
					if let Some(metrics) = this.metrics.as_ref() {
						let reason = match error {
							PendingConnectionError::ConnectionLimit(_) => "limit-reached",
							PendingConnectionError::InvalidPeerId => "invalid-peer-id",
							PendingConnectionError::Transport(_) |
							PendingConnectionError::IO(_) => "transport-error",
						};

						metrics.incoming_connections_errors_total.with_label_values(&[reason]).inc();
					}
				},
				Poll::Ready(SwarmEvent::BannedPeer { peer_id, endpoint }) => {
					trace!(target: "sub-libp2p", "Libp2p => BannedPeer({}). Connected via {:?}.",
						peer_id, endpoint);
					if let Some(metrics) = this.metrics.as_ref() {
						metrics.incoming_connections_errors_total.with_label_values(&["banned"]).inc();
					}
				},
				Poll::Ready(SwarmEvent::UnknownPeerUnreachableAddr { address, error }) =>
					trace!(target: "sub-libp2p", "Libp2p => UnknownPeerUnreachableAddr({}): {}",
						address, error),
				Poll::Ready(SwarmEvent::ListenerClosed { reason, addresses }) => {
					if let Some(metrics) = this.metrics.as_ref() {
						metrics.listeners_local_addresses.sub(addresses.len() as u64);
					}
					let addrs = addresses.into_iter().map(|a| a.to_string())
						.collect::<Vec<_>>().join(", ");
					match reason {
						Ok(()) => error!(
							target: "sub-libp2p",
							"📪 Libp2p listener ({}) closed gracefully",
							addrs
						),
						Err(e) => error!(
							target: "sub-libp2p",
							"📪 Libp2p listener ({}) closed: {}",
							addrs, e
						),
					}
				},
				Poll::Ready(SwarmEvent::ListenerError { error }) => {
					trace!(target: "sub-libp2p", "Libp2p => ListenerError: {}", error);
					if let Some(metrics) = this.metrics.as_ref() {
						metrics.listeners_errors_total.inc();
					}
				},
			};
		}

		let num_connected_peers = this.network_service.user_protocol_mut().num_connected_peers();

		// Update the variables shared with the `NetworkService`.
		this.num_connected.store(num_connected_peers, Ordering::Relaxed);
		{
			let external_addresses = Swarm::<B, H>::external_addresses(&this.network_service).cloned().collect();
			*this.external_addresses.lock() = external_addresses;
		}

		let is_major_syncing = match this.network_service.user_protocol_mut().sync_state() {
			SyncState::Idle => false,
			SyncState::Downloading => true,
		};

		this.is_major_syncing.store(is_major_syncing, Ordering::Relaxed);

		if let Some(metrics) = this.metrics.as_ref() {
			for (proto, buckets) in this.network_service.num_entries_per_kbucket() {
				for (lower_ilog2_bucket_bound, num_entries) in buckets {
					metrics.kbuckets_num_nodes
						.with_label_values(&[&proto.as_ref(), &lower_ilog2_bucket_bound.to_string()])
						.set(num_entries as u64);
				}
			}
			for (proto, num_entries) in this.network_service.num_kademlia_records() {
				metrics.kademlia_records_count.with_label_values(&[&proto.as_ref()]).set(num_entries as u64);
			}
			for (proto, num_entries) in this.network_service.kademlia_records_total_size() {
				metrics.kademlia_records_sizes_total.with_label_values(&[&proto.as_ref()]).set(num_entries as u64);
			}
			metrics.peerset_num_discovered.set(this.network_service.user_protocol().num_discovered_peers() as u64);
			metrics.peerset_num_requested.set(this.network_service.user_protocol().requested_peers().count() as u64);
			metrics.pending_connections.set(Swarm::network_info(&this.network_service).num_connections_pending as u64);
		}

		Poll::Pending
	}
}

impl<B: BlockT + 'static, H: ExHashT> Unpin for NetworkWorker<B, H> {
}

/// The libp2p swarm, customized for our needs.
type Swarm<B, H> = libp2p::swarm::Swarm<Behaviour<B, H>>;

// Implementation of `import_queue::Link` trait using the available local variables.
struct NetworkLink<'a, B: BlockT, H: ExHashT> {
	protocol: &'a mut Swarm<B, H>,
}

impl<'a, B: BlockT, H: ExHashT> Link<B> for NetworkLink<'a, B, H> {
	fn blocks_processed(
		&mut self,
		imported: usize,
		count: usize,
		results: Vec<(Result<BlockImportResult<NumberFor<B>>, BlockImportError>, B::Hash)>
	) {
		self.protocol.user_protocol_mut().on_blocks_processed(imported, count, results)
	}
	fn justification_imported(&mut self, who: PeerId, hash: &B::Hash, number: NumberFor<B>, success: bool) {
		self.protocol.user_protocol_mut().justification_import_result(hash.clone(), number, success);
		if !success {
			info!("💔 Invalid justification provided by {} for #{}", who, hash);
			self.protocol.user_protocol_mut().disconnect_peer(&who);
			self.protocol.user_protocol_mut().report_peer(who, ReputationChange::new_fatal("Invalid justification"));
		}
	}
	fn request_justification(&mut self, hash: &B::Hash, number: NumberFor<B>) {
		self.protocol.user_protocol_mut().request_justification(hash, number)
	}
	fn request_finality_proof(&mut self, hash: &B::Hash, number: NumberFor<B>) {
		self.protocol.user_protocol_mut().request_finality_proof(hash, number)
	}
	fn finality_proof_imported(
		&mut self,
		who: PeerId,
		request_block: (B::Hash, NumberFor<B>),
		finalization_result: Result<(B::Hash, NumberFor<B>), ()>,
	) {
		let success = finalization_result.is_ok();
		self.protocol.user_protocol_mut().finality_proof_import_result(request_block, finalization_result);
		if !success {
			info!("💔 Invalid finality proof provided by {} for #{}", who, request_block.0);
			self.protocol.user_protocol_mut().disconnect_peer(&who);
			self.protocol.user_protocol_mut().report_peer(who, ReputationChange::new_fatal("Invalid finality proof"));
		}
	}
}

fn ensure_addresses_consistent_with_transport<'a>(
	addresses: impl Iterator<Item = &'a Multiaddr>,
	transport: &TransportConfig,
) -> Result<(), Error> {
	if matches!(transport, TransportConfig::MemoryOnly) {
		let addresses: Vec<_> = addresses
			.filter(|x| x.iter()
				.any(|y| !matches!(y, libp2p::core::multiaddr::Protocol::Memory(_)))
			)
			.cloned()
			.collect();

		if !addresses.is_empty() {
			return Err(Error::AddressesForAnotherTransport {
				transport: transport.clone(),
				addresses,
			});
		}
	} else {
		let addresses: Vec<_> = addresses
			.filter(|x| x.iter()
				.any(|y| matches!(y, libp2p::core::multiaddr::Protocol::Memory(_)))
			)
			.cloned()
			.collect();

		if !addresses.is_empty() {
			return Err(Error::AddressesForAnotherTransport {
				transport: transport.clone(),
				addresses,
			});
		}
	}

	Ok(())
}