Fix future-polling loop in availability and add a better early-exit (#2779)

* onto the front

* fix early exit for waiting for requests

* add logging back

polkadot/node/network/availability-recovery/src/lib.rs

@@ -18,7 +18,7 @@
 
 #![warn(missing_docs)]
 
-use std::collections::HashMap;
+use std::collections::{HashMap, VecDeque};
 use std::pin::Pin;
 
 use futures::{channel::oneshot, prelude::*, stream::FuturesUnordered};
@@ -78,7 +78,7 @@ struct RequestFromBackersPhase {
 struct RequestChunksPhase {
 	// a random shuffling of the validators which indicates the order in which we connect to the validators and
 	// request the chunk from them.
-	shuffling: Vec<ValidatorIndex>,
+	shuffling: VecDeque<ValidatorIndex>,
 	received_chunks: HashMap<ValidatorIndex, ErasureChunk>,
 	requesting_chunks: FuturesUnordered<BoxFuture<
 		'static,
@@ -198,12 +198,25 @@ impl RequestChunksPhase {
 		shuffling.shuffle(&mut rand::thread_rng());
 
 		RequestChunksPhase {
-			shuffling,
+			shuffling: shuffling.into(),
 			received_chunks: HashMap::new(),
 			requesting_chunks: FuturesUnordered::new(),
 		}
 	}
 
+	fn is_unavailable(&self, params: &InteractionParams) -> bool {
+		is_unavailable(
+			self.received_chunks.len(),
+			self.requesting_chunks.len(),
+			self.shuffling.len(),
+			params.threshold,
+		)
+	}
+
+	fn can_conclude(&self, params: &InteractionParams) -> bool {
+		self.received_chunks.len() >= params.threshold || self.is_unavailable(params)
+	}
+
 	async fn launch_parallel_requests(
 		&mut self,
 		params: &InteractionParams,
@@ -211,7 +224,7 @@ impl RequestChunksPhase {
 	) {
 		let max_requests = std::cmp::min(N_PARALLEL, params.threshold);
 		while self.requesting_chunks.len() < max_requests {
-			if let Some(validator_index) = self.shuffling.pop() {
+			if let Some(validator_index) = self.shuffling.pop_back() {
 				let validator = params.validator_authority_keys[validator_index.0 as usize].clone();
 				tracing::trace!(
 					target: LOG_TARGET,
@@ -255,10 +268,8 @@ impl RequestChunksPhase {
 		&mut self,
 		params: &InteractionParams,
 	) {
-		// Poll for new updates from requesting_chunks.
-		while let Poll::Ready(Some(request_result))
-			= futures::poll!(self.requesting_chunks.next())
-		{
+		// Wait for all current requests to conclude or time-out, or until we reach enough chunks.
+		while let Some(request_result) = self.requesting_chunks.next().await {
 			match request_result {
 				Ok(Some(chunk)) => {
 					// Check merkle proofs of any received chunks.
@@ -306,11 +317,15 @@ impl RequestChunksPhase {
 					match e {
 						RequestError::InvalidResponse(_) => {}
 						RequestError::NetworkError(_) | RequestError::Canceled(_) => {
-							self.shuffling.push(validator_index);
+							self.shuffling.push_front(validator_index);
 						}
 					}
 				}
 			}
+
+			// Stop waiting for requests when we either can already recover the data
+			// or have gotten firm 'No' responses from enough validators.
+			if self.can_conclude(params) { break }
 		}
 	}
 
@@ -320,12 +335,7 @@ impl RequestChunksPhase {
 		sender: &mut impl SubsystemSender,
 	) -> Result<AvailableData, RecoveryError> {
 		loop {
-			if is_unavailable(
-				self.received_chunks.len(),
-				self.requesting_chunks.len(),
-				self.shuffling.len(),
-				params.threshold,
-			) {
+			if self.is_unavailable(&params) {
 				tracing::debug!(
 					target: LOG_TARGET,
 					candidate_hash = ?params.candidate_hash,

polkadot/roadmap/implementers-guide/src/node/availability/availability-recovery.md

@@ -133,7 +133,7 @@ Loop:
 
   * If the phase is `InteractionPhase::RequestChunks`:
     * If `received_chunks + requesting_chunks + shuffling` lengths are less than the threshold, break and return `Err(Unavailable)`.
-    * Poll for new updates from `requesting_chunks`. Check merkle proofs of any received chunks. If the request simply fails due to network issues, push onto the back of `shuffling` to be retried.
+    * Poll for new updates from `requesting_chunks`. Check merkle proofs of any received chunks. If the request simply fails due to network issues, insert into the front of `shuffling` to be retried.
     * If `received_chunks` has more than `threshold` entries, attempt to recover the data. If that fails, or a re-encoding produces an incorrect erasure-root, break and issue a `Err(RecoveryError::Invalid)`. If correct, break and issue `Ok(available_data)`.
     * While there are fewer than `N_PARALLEL` entries in `requesting_chunks`,
         * Pop the next item from `shuffling`. If it's empty and `requesting_chunks` is empty, return `Err(RecoveryError::Unavailable)`.