Attempt to relieve pressure on mpsc_network_worker (#13725)

* Attempt to relieve pressure on `mpsc_network_worker`

`SyncingEngine` interacting with `NetworkWorker` can put a lot of strain
on the channel if the number of inbound connections is high. This is
because `SyncingEngine` is notified of each inbound substream which it
then can either accept or reject and this causes a lot of message
exchange on the already busy channel.

Use a direct channel pair between `Protocol` and `SyncingEngine`
to exchange notification events. It is a temporary change to alleviate
the problems caused by syncing being an independent protocol and the
fix will be removed once `NotificationService` is implemented.

* Apply review comments

* fixes

* trigger ci

* Fix tests

Verify that both peers have a connection now that the validation goes
through `SyncingEngine`. Depending on how the tasks are scheduled,
one of them might not have the peer registered in `SyncingEngine` at which
point the test won't make any progress because block announcement received
from an unknown peer is discarded.

Move polling of `ChainSync` at the end of the function so that if a block
announcement causes a block request to be sent, that can be sent in the
same call to `SyncingEngine::poll()`.

---------

Co-authored-by: parity-processbot <>

substrate/client/network/src/config.rs

@@ -22,6 +22,7 @@
 //! See the documentation of [`Params`].
 
 pub use crate::{
+	protocol::NotificationsSink,
 	request_responses::{
 		IncomingRequest, OutgoingResponse, ProtocolConfig as RequestResponseConfig,
 	},
@@ -31,7 +32,12 @@ pub use crate::{
 use codec::Encode;
 use libp2p::{identity::Keypair, multiaddr, Multiaddr, PeerId};
 use prometheus_endpoint::Registry;
-pub use sc_network_common::{role::Role, sync::warp::WarpSyncProvider, ExHashT};
+pub use sc_network_common::{
+	role::{Role, Roles},
+	sync::warp::WarpSyncProvider,
+	ExHashT,
+};
+use sc_utils::mpsc::TracingUnboundedSender;
 use zeroize::Zeroize;
 
 use sp_runtime::traits::Block as BlockT;
@@ -714,6 +720,9 @@ pub struct Params<Block: BlockT> {
 	/// Block announce protocol configuration
 	pub block_announce_config: NonDefaultSetConfig,
 
+	/// TX channel for direct communication with `SyncingEngine` and `Protocol`.
+	pub tx: TracingUnboundedSender<crate::event::SyncEvent<Block>>,
+
 	/// Request response protocol configurations
 	pub request_response_protocol_configs: Vec<RequestResponseConfig>,
 }