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/lib.rs

@@ -259,7 +259,7 @@ pub mod request_responses;
 pub mod types;
 pub mod utils;
 
-pub use event::{DhtEvent, Event};
+pub use event::{DhtEvent, Event, SyncEvent};
 #[doc(inline)]
 pub use libp2p::{multiaddr, Multiaddr, PeerId};
 pub use request_responses::{IfDisconnected, RequestFailure, RequestResponseConfig};
@@ -278,8 +278,8 @@ pub use service::{
 		NetworkStatusProvider, NetworkSyncForkRequest, NotificationSender as NotificationSenderT,
 		NotificationSenderError, NotificationSenderReady,
 	},
-	DecodingError, Keypair, NetworkService, NetworkWorker, NotificationSender, OutboundFailure,
-	PublicKey,
+	DecodingError, Keypair, NetworkService, NetworkWorker, NotificationSender, NotificationsSink,
+	OutboundFailure, PublicKey,
 };
 pub use types::ProtocolName;