Request based PoV distribution (#2640)

* Indentation fix.

* Prepare request-response for PoV fetching.

* Drop old PoV distribution.

* WIP: Fetch PoV directly from backing.

* Backing compiles.

* Runtime access and connection management for PoV distribution.

* Get rid of seemingly dead code.

* Implement PoV fetching.

Backing does not yet use it.

* Don't send `ConnectToValidators` for empty list.

* Even better - no need to check over and over again.

* PoV fetching implemented.

+ Typechecks
+ Should work

Missing:

- Guide
- Tests
- Do fallback fetching in case fetching from seconding validator fails.

* Check PoV hash upon reception.

* Implement retry of PoV fetching in backing.

* Avoid pointless validation spawning.

* Add jaeger span to pov requesting.

* Add back tracing.

* Review remarks.

* Whitespace.

* Whitespace again.

* Cleanup + fix tests.

* Log to log target in overseer.

* Fix more tests.

* Don't fail if group cannot be found.

* Simple test for PoV fetcher.

* Handle missing group membership better.

* Add test for retry functionality.

* Fix flaky test.

* Spaces again.

* Guide updates.

* Spaces.

polkadot/node/network/protocol/src/request_response/v1.rs

@@ -114,6 +114,29 @@ impl IsRequest for CollationFetchingRequest {
 	const PROTOCOL: Protocol = Protocol::CollationFetching;
 }
 
+/// Request the advertised collation at that relay-parent.
+#[derive(Debug, Clone, Encode, Decode)]
+pub struct PoVFetchingRequest {
+	/// Candidate we want a PoV for.
+	pub candidate_hash: CandidateHash,
+}
+
+/// Responses to `PoVFetchingRequest`.
+#[derive(Debug, Clone, Encode, Decode)]
+pub enum PoVFetchingResponse {
+	/// Deliver requested PoV.
+	#[codec(index = 0)]
+	PoV(CompressedPoV),
+	/// PoV was not found in store.
+	#[codec(index = 1)]
+	NoSuchPoV,
+}
+
+impl IsRequest for PoVFetchingRequest {
+	type Response = PoVFetchingResponse;
+	const PROTOCOL: Protocol = Protocol::PoVFetching;
+}
+
 /// Request the entire available data for a candidate.
 #[derive(Debug, Clone, Encode, Decode)]
 pub struct AvailableDataFetchingRequest {