client/authority-discovery: Limit the amount of sentries per authority (#4302)

When receiving more addresses for a given authority than a defined
threshold (5), the authority discovery drops the remaining in order to
prevent a single authority to fill all priority group slots.

substrate/Cargo.lock

@@ -4814,6 +4814,7 @@ version = "2.0.0"
 dependencies = [
  "bytes 0.4.12 (registry+https://github.com/rust-lang/crates.io-index)",
  "derive_more 0.99.2 (registry+https://github.com/rust-lang/crates.io-index)",
+ "env_logger 0.7.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "futures 0.3.1 (registry+https://github.com/rust-lang/crates.io-index)",
  "futures-timer 2.0.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "libp2p 0.13.0 (registry+https://github.com/rust-lang/crates.io-index)",

substrate/client/authority-discovery/Cargo.toml

@@ -27,6 +27,7 @@ serde_json = "1.0.41"
 sp-runtime = { path = "../../primitives/sr-primitives" }
 
 [dev-dependencies]
+env_logger = "0.7.0"
 parking_lot = "0.9.0"
 peerset = { package = "sc-peerset", path = "../peerset" }
 test-client = { package = "substrate-test-runtime-client", path = "../../test/utils/runtime/client" }

substrate/client/authority-discovery/src/lib.rs

@@ -86,6 +86,12 @@ const LIBP2P_KADEMLIA_BOOTSTRAP_TIME: Duration = Duration::from_secs(30);
 /// discovery module.
 const AUTHORITIES_PRIORITY_GROUP_NAME: &'static str = "authorities";
 
+/// The maximum number of sentry node public addresses that we accept per authority.
+///
+/// Everything above this threshold should be dropped to prevent a single authority from filling up
+/// our peer set priority group.
+const MAX_NUM_SENTRY_ADDRESSES_PER_AUTHORITY: usize = 5;
+
 /// An `AuthorityDiscovery` makes a given authority discoverable and discovers other authorities.
 pub struct AuthorityDiscovery<Client, Network, Block>
 where
@@ -316,7 +322,7 @@ where
 				return Err(Error::VerifyingDhtPayload);
 			}
 
-			let addresses: Vec<libp2p::Multiaddr> = schema::AuthorityAddresses::decode(addresses)
+			let mut addresses: Vec<libp2p::Multiaddr> = schema::AuthorityAddresses::decode(addresses)
 				.map(|a| a.addresses)
 				.map_err(Error::DecodingProto)?
 				.into_iter()
@@ -324,6 +330,18 @@ where
 				.collect::<std::result::Result<_, _>>()
 				.map_err(Error::ParsingMultiaddress)?;
 
+			if addresses.len() > MAX_NUM_SENTRY_ADDRESSES_PER_AUTHORITY {
+				warn!(
+					target: "sub-authority-discovery",
+					"Got more than MAX_NUM_SENTRY_ADDRESSES_PER_AUTHORITY ({:?}) for Authority
+					'{:?}' from DHT, dropping the remainder.",
+					MAX_NUM_SENTRY_ADDRESSES_PER_AUTHORITY,	authority_id,
+				);
+				addresses = addresses.into_iter()
+					.take(MAX_NUM_SENTRY_ADDRESSES_PER_AUTHORITY)
+					.collect();
+			}
+
 			self.address_cache.insert(authority_id.clone(), addresses);
 		}
 
@@ -800,6 +818,7 @@ mod tests {
 
 	#[test]
 	fn handle_dht_events_with_value_found_should_call_set_priority_group() {
+		let _ = ::env_logger::try_init();
 		// Create authority discovery.
 
 		let (mut dht_event_tx, dht_event_rx) = channel(1000);