Remove request multiplexer (#3624)

* WIP: Get rid of request multiplexer.

* WIP

* Receiver for handling of incoming requests.

* Get rid of useless `Fault` abstraction.

The things the type system let us do are not worth getting abstracted in
its own type. Instead error handling is going to be merely a pattern.

* Make most things compile again.

* Port availability distribution away from request multiplexer.

* Formatting.

* Port dispute distribution over.

* Fixup statement distribution.

* Handle request directly in collator protocol.

+ Only allow fatal errors at top level.

* Use direct request channel for availability recovery.

* Finally get rid of request multiplexer

Fixes #2842 and paves the way for more back pressure possibilities.

* Fix overseer and statement distribution tests.

* Fix collator protocol and network bridge tests.

* Fix tests in availability recovery.

* Fix availability distribution tests.

* Fix dispute distribution tests.

* Add missing dependency

* Typos.

* Review remarks.

* More remarks.

polkadot/node/network/statement-distribution/src/responder.rs

@@ -22,9 +22,9 @@ use futures::{
 
 use polkadot_node_network_protocol::{
 	request_response::{
-		request::OutgoingResponse,
+		incoming::{self, OutgoingResponse},
 		v1::{StatementFetchingRequest, StatementFetchingResponse},
-		IncomingRequest, MAX_PARALLEL_STATEMENT_REQUESTS,
+		IncomingRequestReceiver, MAX_PARALLEL_STATEMENT_REQUESTS,
 	},
 	PeerId, UnifiedReputationChange as Rep,
 };
@@ -51,7 +51,7 @@ pub enum ResponderMessage {
 /// `CommittedCandidateReceipt` from peers, whether this can be used to re-assemble one ore
 /// many `SignedFullStatement`s needs to be verified by the caller.
 pub async fn respond(
-	mut receiver: mpsc::Receiver<sc_network::config::IncomingRequest>,
+	mut receiver: IncomingRequestReceiver<StatementFetchingRequest>,
 	mut sender: mpsc::Sender<ResponderMessage>,
 ) {
 	let mut pending_out = FuturesUnordered::new();
@@ -74,23 +74,16 @@ pub async fn respond(
 			pending_out.next().await;
 		}
 
-		let raw = match receiver.next().await {
-			None => {
-				tracing::debug!(target: LOG_TARGET, "Shutting down request responder");
+		let req = match receiver.recv(|| vec![COST_INVALID_REQUEST]).await {
+			Err(incoming::Error::Fatal(f)) => {
+				tracing::debug!(target: LOG_TARGET, error = ?f, "Shutting down request responder");
 				return
 			},
-			Some(v) => v,
-		};
-
-		let req = match IncomingRequest::<StatementFetchingRequest>::try_from_raw(
-			raw,
-			vec![COST_INVALID_REQUEST],
-		) {
-			Err(err) => {
+			Err(incoming::Error::NonFatal(err)) => {
 				tracing::debug!(target: LOG_TARGET, ?err, "Decoding request failed");
 				continue
 			},
-			Ok(payload) => payload,
+			Ok(v) => v,
 		};
 
 		let (tx, rx) = oneshot::channel();