Fix peer discovery delays. (#2982)

* Fix DiscoveryBehaviour::poll.

The previous implementation regularly returned `NotReady` from `poll`
despite the inner Kademlia behaviour having events ready, thus letting
the `poll`ing be largely driven by the task wakeups from the Delay for
the next random Kademlia query, inducing major delays in consuming
the ready Kademlia events and thus slowing progress.

The discovery test now passes quickly, as expected.

* Ensure the Delay is polled right after reset.

For task wakeup.

substrate/core/network/src/discovery.rs

@@ -181,65 +181,6 @@ where
 			Self::OutEvent,
 		>,
 	> {
-		// Poll Kademlia.
-		match self.kademlia.poll(params) {
-			Async::NotReady => (),
-			Async::Ready(NetworkBehaviourAction::GenerateEvent(ev)) => {
-				match ev {
-					KademliaOut::Discovered { .. } => {}
-					KademliaOut::KBucketAdded { peer_id, .. } => {
-						let ev = DiscoveryOut::Discovered(peer_id);
-						return Async::Ready(NetworkBehaviourAction::GenerateEvent(ev));
-					}
-					KademliaOut::FindNodeResult { key, closer_peers } => {
-						trace!(target: "sub-libp2p", "Libp2p => Query for {:?} yielded {:?} results",
-							key, closer_peers.len());
-						if closer_peers.is_empty() {
-							warn!(target: "sub-libp2p", "Libp2p => Random Kademlia query has yielded empty \
-								results");
-						}
-					}
-					KademliaOut::GetValueResult(res) => {
-						let ev = match res {
-							GetValueResult::Found { results } => {
-								let results = results
-										.into_iter()
-										.map(|r| (r.key, r.value))
-										.collect();
-
-								DiscoveryOut::ValueFound(results)
-							}
-							GetValueResult::NotFound { key, .. } => {
-								DiscoveryOut::ValueNotFound(key)
-							}
-						};
-						return Async::Ready(NetworkBehaviourAction::GenerateEvent(ev));
-					}
-					KademliaOut::PutValueResult(res) => {
-						let ev = match res {
-							PutValueResult::Ok{ key, .. } =>  {
-								DiscoveryOut::ValuePut(key)
-							}
-							PutValueResult::Err { key, .. } => {
-								DiscoveryOut::ValuePutFailed(key)
-							}
-						};
-						return Async::Ready(NetworkBehaviourAction::GenerateEvent(ev));
-					}
-					// We never start any other type of query.
-					KademliaOut::GetProvidersResult { .. } => {}
-				}
-			},
-			Async::Ready(NetworkBehaviourAction::DialAddress { address }) =>
-				return Async::Ready(NetworkBehaviourAction::DialAddress { address }),
-			Async::Ready(NetworkBehaviourAction::DialPeer { peer_id }) =>
-				return Async::Ready(NetworkBehaviourAction::DialPeer { peer_id }),
-			Async::Ready(NetworkBehaviourAction::SendEvent { peer_id, event }) =>
-				return Async::Ready(NetworkBehaviourAction::SendEvent { peer_id, event }),
-			Async::Ready(NetworkBehaviourAction::ReportObservedAddr { address }) =>
-				return Async::Ready(NetworkBehaviourAction::ReportObservedAddr { address }),
-		}
-
 		// Poll the stream that fires when we need to start a random Kademlia query.
 		loop {
 			match self.next_kad_random_query.poll() {
@@ -262,6 +203,67 @@ where
 			}
 		}
 
+		// Poll Kademlia.
+		loop {
+			match self.kademlia.poll(params) {
+				Async::NotReady => break,
+				Async::Ready(NetworkBehaviourAction::GenerateEvent(ev)) => {
+					match ev {
+						KademliaOut::Discovered { .. } => {}
+						KademliaOut::KBucketAdded { peer_id, .. } => {
+							let ev = DiscoveryOut::Discovered(peer_id);
+							return Async::Ready(NetworkBehaviourAction::GenerateEvent(ev));
+						}
+						KademliaOut::FindNodeResult { key, closer_peers } => {
+							trace!(target: "sub-libp2p", "Libp2p => Query for {:?} yielded {:?} results",
+								key, closer_peers.len());
+							if closer_peers.is_empty() {
+								warn!(target: "sub-libp2p", "Libp2p => Random Kademlia query has yielded empty \
+									results");
+							}
+						}
+						KademliaOut::GetValueResult(res) => {
+							let ev = match res {
+								GetValueResult::Found { results } => {
+									let results = results
+											.into_iter()
+											.map(|r| (r.key, r.value))
+											.collect();
+
+									DiscoveryOut::ValueFound(results)
+								}
+								GetValueResult::NotFound { key, .. } => {
+									DiscoveryOut::ValueNotFound(key)
+								}
+							};
+							return Async::Ready(NetworkBehaviourAction::GenerateEvent(ev));
+						}
+						KademliaOut::PutValueResult(res) => {
+							let ev = match res {
+								PutValueResult::Ok{ key, .. } =>  {
+									DiscoveryOut::ValuePut(key)
+								}
+								PutValueResult::Err { key, .. } => {
+									DiscoveryOut::ValuePutFailed(key)
+								}
+							};
+							return Async::Ready(NetworkBehaviourAction::GenerateEvent(ev));
+						}
+						// We never start any other type of query.
+						KademliaOut::GetProvidersResult { .. } => {}
+					}
+				},
+				Async::Ready(NetworkBehaviourAction::DialAddress { address }) =>
+					return Async::Ready(NetworkBehaviourAction::DialAddress { address }),
+				Async::Ready(NetworkBehaviourAction::DialPeer { peer_id }) =>
+					return Async::Ready(NetworkBehaviourAction::DialPeer { peer_id }),
+				Async::Ready(NetworkBehaviourAction::SendEvent { peer_id, event }) =>
+					return Async::Ready(NetworkBehaviourAction::SendEvent { peer_id, event }),
+				Async::Ready(NetworkBehaviourAction::ReportObservedAddr { address }) =>
+					return Async::Ready(NetworkBehaviourAction::ReportObservedAddr { address }),
+			}
+		}
+
 		Async::NotReady
 	}
 }