Availability recovery subsystem (#2122)

* Adds message types

* Add code skeleton

* Adds subsystem code.

* Adds a first test

* Adds interaction result to availability_lru

* Use LruCache instead of a HashMap

* Whitespaces to tabs

* Do not ignore errors

* Change error type

* Add a timeout to chunk requests

* Add custom errors and log them

* Adds replace_availability_recovery method

* recovery_threshold computed by erasure crate

* change core to std

* adds docs to error type

* Adds a test for invalid reconstruction

* refactors interaction run into multiple methods

* Cleanup AwaitedChunks

* Even more fixes

* Test that recovery with wrong root is an error

* Break to launch another requests

* Styling fixes

* Add SessionIndex to API

* Proper relay parents for MakeRequest

* Remove validator_discovery and use message

* Remove a stream on exhaustion

* On cleanup free the request streams

* Fix merge and refactor

polkadot/node/network/protocol/src/lib.rs

@@ -282,7 +282,7 @@ impl View {
 pub mod v1 {
 	use polkadot_primitives::v1::{
 		Hash, CollatorId, Id as ParaId, ErasureChunk, CandidateReceipt,
-		SignedAvailabilityBitfield, PoV, CandidateHash,
+		SignedAvailabilityBitfield, PoV, CandidateHash, ValidatorIndex,
 	};
 	use polkadot_node_primitives::SignedFullStatement;
 	use parity_scale_codec::{Encode, Decode};
@@ -297,6 +297,16 @@ pub mod v1 {
 		Chunk(CandidateHash, ErasureChunk),
 	}
 
+	/// Network messages used by the availability recovery subsystem.
+	#[derive(Debug, Clone, Encode, Decode, PartialEq)]
+	pub enum AvailabilityRecoveryMessage {
+		/// Request a chunk for a given candidate hash and validator index.
+		RequestChunk(RequestId, CandidateHash, ValidatorIndex),
+		/// Respond with chunk for a given candidate hash and validator index.
+		/// The response may be `None` if the requestee does not have the chunk.
+		Chunk(RequestId, Option<ErasureChunk>),
+	}
+
 	/// Network messages used by the bitfield distribution subsystem.
 	#[derive(Debug, Clone, Encode, Decode, PartialEq)]
 	pub enum BitfieldDistributionMessage {
@@ -359,6 +369,9 @@ pub mod v1 {
 		/// Statement distribution messages
 		#[codec(index = "3")]
 		StatementDistribution(StatementDistributionMessage),
+		/// Availability recovery messages
+		#[codec(index = "4")]
+		AvailabilityRecovery(AvailabilityRecoveryMessage),
 	}
 
 	impl_try_from!(ValidationProtocol, AvailabilityDistribution, AvailabilityDistributionMessage);