Request based availability distribution (#2423)

* WIP

* availability distribution, still very wip.

Work on the requesting side of things.

* Some docs on what I intend to do.

* Checkpoint of session cache implementation

as I will likely replace it with something smarter.

* More work, mostly on cache

and getting things to type check.

* Only derive MallocSizeOf and Debug for std.

* availability-distribution: Cache feature complete.

* Sketch out logic in `FetchTask` for actual fetching.

- Compile fixes.
- Cleanup.

* Format cleanup.

* More format fixes.

* Almost feature complete `fetch_task`.

Missing:

- Check for cancel
- Actual querying of peer ids.

* Finish FetchTask so far.

* Directly use AuthorityDiscoveryId in protocol and cache.

* Resolve `AuthorityDiscoveryId` on sending requests.

* Rework fetch_task

- also make it impossible to check the wrong chunk index.
- Export needed function in validator_discovery.

* From<u32> implementation for `ValidatorIndex`.

* Fixes and more integration work.

* Make session cache proper lru cache.

* Use proper lru cache.

* Requester finished.

* ProtocolState -> Requester

Also make sure to not fetch our own chunk.

* Cleanup + fixes.

* Remove unused functions

- FetchTask::is_finished
- SessionCache::fetch_session_info

* availability-distribution responding side.

* Cleanup + Fixes.

* More fixes.

* More fixes.

adder-collator is running!

* Some docs.

* Docs.

* Fix reporting of bad guys.

* Fix tests

* Make all tests compile.

* Fix test.

* Cleanup + get rid of some warnings.

* state -> requester

* Mostly doc fixes.

* Fix test suite.

* Get rid of now redundant message types.

* WIP

* Rob's review remarks.

* Fix test suite.

* core.relay_parent -> leaf for session request.

* Style fix.

* Decrease request timeout.

* Cleanup obsolete errors.

* Metrics + don't fail on non fatal errors.

* requester.rs -> requester/mod.rs

* Panic on invalid BadValidator report.

* Fix indentation.

* Use typed default timeout constant.

* Make channel size 0, as each sender gets one slot anyways.

* Fix incorrect metrics initialization.

* Fix build after merge.

* More fixes.

* Hopefully valid metrics names.

* Better metrics names.

* Some tests that already work.

* Slightly better docs.

* Some more tests.

* Fix network bridge test.

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

@@ -24,18 +24,54 @@ use super::request::IsRequest;
 use super::Protocol;
 
 /// Request an availability chunk.
-#[derive(Debug, Clone, Encode, Decode)]
+#[derive(Debug, Copy, Clone, Encode, Decode)]
 pub struct AvailabilityFetchingRequest {
-	candidate_hash: CandidateHash,
-	index: ValidatorIndex,
+	/// Hash of candidate we want a chunk for.
+	pub candidate_hash: CandidateHash,
+	/// The index of the chunk to fetch.
+	pub index: ValidatorIndex,
 }
 
 /// Receive a rqeuested erasure chunk.
 #[derive(Debug, Clone, Encode, Decode)]
 pub enum AvailabilityFetchingResponse {
-	/// The requested chunk.
+	/// The requested chunk data.
 	#[codec(index = 0)]
-	Chunk(ErasureChunk),
+	Chunk(ChunkResponse),
+	/// Node was not in possession of the requested chunk.
+	#[codec(index = 1)]
+	NoSuchChunk,
+}
+
+/// Skimmed down variant of `ErasureChunk`.
+///
+/// Instead of transmitting a full `ErasureChunk` we transmit `ChunkResponse` in
+/// `AvailabilityFetchingResponse`, which omits the chunk's index. The index is already known by
+/// the requester and by not transmitting it, we ensure the requester is going to use his index
+/// value for validating the response, thus making sure he got what he requested.
+#[derive(Debug, Clone, Encode, Decode)]
+pub struct ChunkResponse {
+	/// The erasure-encoded chunk of data belonging to the candidate block.
+	pub chunk: Vec<u8>,
+	/// Proof for this chunk's branch in the Merkle tree.
+	pub proof: Vec<Vec<u8>>,
+}
+
+impl From<ErasureChunk> for ChunkResponse {
+	fn from(ErasureChunk {chunk, index: _, proof}: ErasureChunk) -> Self {
+		ChunkResponse {chunk, proof}
+	}
+}
+
+impl ChunkResponse {
+	/// Re-build an `ErasureChunk` from response and request.
+	pub fn recombine_into_chunk(self, req: &AvailabilityFetchingRequest) -> ErasureChunk {
+		ErasureChunk {
+			chunk: self.chunk,
+			proof: self.proof,
+			index: req.index,
+		}
+	}
 }
 
 impl IsRequest for AvailabilityFetchingRequest {