Integrate litep2p into Polkadot SDK (#2944)

[litep2p](https://github.com/altonen/litep2p) is a libp2p-compatible P2P
networking library. It supports all of the features of `rust-libp2p`
that are currently being utilized by Polkadot SDK.

Compared to `rust-libp2p`, `litep2p` has a quite different architecture
which is why the new `litep2p` network backend is only able to use a
little of the existing code in `sc-network`. The design has been mainly
influenced by how we'd wish to structure our networking-related code in
Polkadot SDK: independent higher-levels protocols directly communicating
with the network over links that support bidirectional backpressure. A
good example would be `NotificationHandle`/`RequestResponseHandle`
abstractions which allow, e.g., `SyncingEngine` to directly communicate
with peers to announce/request blocks.

I've tried running `polkadot --network-backend litep2p` with a few
different peer configurations and there is a noticeable reduction in
networking CPU usage. For high load (`--out-peers 200`), networking CPU
usage goes down from ~110% to ~30% (80 pp) and for normal load
(`--out-peers 40`), the usage goes down from ~55% to ~18% (37 pp).

These should not be taken as final numbers because:

a) there are still some low-hanging optimization fruits, such as
enabling [receive window
auto-tuning](https://github.com/libp2p/rust-yamux/pull/176), integrating
`Peerset` more closely with `litep2p` or improving memory usage of the
WebSocket transport
b) fixing bugs/instabilities that incorrectly cause `litep2p` to do less
work will increase the networking CPU usage
c) verification in a more diverse set of tests/conditions is needed

Nevertheless, these numbers should give an early estimate for CPU usage
of the new networking backend.

This PR consists of three separate changes:
* introduce a generic `PeerId` (wrapper around `Multihash`) so that we
don't have use `NetworkService::PeerId` in every part of the code that
uses a `PeerId`
* introduce `NetworkBackend` trait, implement it for the libp2p network
stack and make Polkadot SDK generic over `NetworkBackend`
  * implement `NetworkBackend` for litep2p

The new library should be considered experimental which is why
`rust-libp2p` will remain as the default option for the time being. This
PR currently depends on the master branch of `litep2p` but I'll cut a
new release for the library once all review comments have been
addresses.

---------

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Dmitry Markin <dmitry@markin.tech>
Co-authored-by: Alexandru Vasile <60601340+lexnv@users.noreply.github.com>
Co-authored-by: Alexandru Vasile <alexandru.vasile@parity.io>

substrate/client/network/statement/src/lib.rs

@@ -30,18 +30,23 @@ use crate::config::*;
 
 use codec::{Decode, Encode};
 use futures::{channel::oneshot, prelude::*, stream::FuturesUnordered, FutureExt};
-use libp2p::{multiaddr, PeerId};
 use prometheus_endpoint::{register, Counter, PrometheusError, Registry, U64};
 use sc_network::{
-	config::{NonDefaultSetConfig, NonReservedPeerMode, SetConfig},
-	error,
-	service::traits::{NotificationEvent, NotificationService, ValidationResult},
+	config::{NonReservedPeerMode, SetConfig},
+	error, multiaddr,
+	peer_store::PeerStoreProvider,
+	service::{
+		traits::{NotificationEvent, NotificationService, ValidationResult},
+		NotificationMetrics,
+	},
 	types::ProtocolName,
 	utils::{interval, LruHashSet},
-	NetworkEventStream, NetworkNotification, NetworkPeers,
+	NetworkBackend, NetworkEventStream, NetworkPeers,
 };
 use sc_network_common::role::ObservedRole;
 use sc_network_sync::{SyncEvent, SyncEventStream};
+use sc_network_types::PeerId;
+use sp_runtime::traits::Block as BlockT;
 use sp_statement_store::{
 	Hash, NetworkPriority, Statement, StatementSource, StatementStore, SubmitResult,
 };
@@ -107,17 +112,23 @@ pub struct StatementHandlerPrototype {
 
 impl StatementHandlerPrototype {
 	/// Create a new instance.
-	pub fn new<Hash: AsRef<[u8]>>(
+	pub fn new<
+		Hash: AsRef<[u8]>,
+		Block: BlockT,
+		Net: NetworkBackend<Block, <Block as BlockT>::Hash>,
+	>(
 		genesis_hash: Hash,
 		fork_id: Option<&str>,
-	) -> (Self, NonDefaultSetConfig) {
+		metrics: NotificationMetrics,
+		peer_store_handle: Arc<dyn PeerStoreProvider>,
+	) -> (Self, Net::NotificationProtocolConfig) {
 		let genesis_hash = genesis_hash.as_ref();
 		let protocol_name = if let Some(fork_id) = fork_id {
 			format!("/{}/{}/statement/1", array_bytes::bytes2hex("", genesis_hash), fork_id)
 		} else {
 			format!("/{}/statement/1", array_bytes::bytes2hex("", genesis_hash))
 		};
-		let (config, notification_service) = NonDefaultSetConfig::new(
+		let (config, notification_service) = Net::notification_config(
 			protocol_name.clone().into(),
 			Vec::new(),
 			MAX_STATEMENT_SIZE,
@@ -128,6 +139,8 @@ impl StatementHandlerPrototype {
 				reserved_nodes: Vec::new(),
 				non_reserved_mode: NonReservedPeerMode::Deny,
 			},
+			metrics,
+			peer_store_handle,
 		);
 
 		(Self { protocol_name: protocol_name.into(), notification_service }, config)
@@ -138,7 +151,7 @@ impl StatementHandlerPrototype {
 	/// Important: the statements handler is initially disabled and doesn't gossip statements.
 	/// Gossiping is enabled when major syncing is done.
 	pub fn build<
-		N: NetworkPeers + NetworkEventStream + NetworkNotification,
+		N: NetworkPeers + NetworkEventStream,
 		S: SyncEventStream + sp_consensus::SyncOracle,
 	>(
 		self,
@@ -201,7 +214,7 @@ impl StatementHandlerPrototype {
 
 /// Handler for statements. Call [`StatementHandler::run`] to start the processing.
 pub struct StatementHandler<
-	N: NetworkPeers + NetworkEventStream + NetworkNotification,
+	N: NetworkPeers + NetworkEventStream,
 	S: SyncEventStream + sp_consensus::SyncOracle,
 > {
 	protocol_name: ProtocolName,
@@ -241,7 +254,7 @@ struct Peer {
 
 impl<N, S> StatementHandler<N, S>
 where
-	N: NetworkPeers + NetworkEventStream + NetworkNotification,
+	N: NetworkPeers + NetworkEventStream,
 	S: SyncEventStream + sp_consensus::SyncOracle,
 {
 	/// Turns the [`StatementHandler`] into a future that should run forever and not be
@@ -459,8 +472,7 @@ where
 
 			if !to_send.is_empty() {
 				log::trace!(target: LOG_TARGET, "Sending {} statements to {}", to_send.len(), who);
-				self.network
-					.write_notification(*who, self.protocol_name.clone(), to_send.encode());
+				self.notification_service.send_sync_notification(who, to_send.encode());
 			}
 		}