Allow fallback names for protocols (#8682)

* Allow fallback names for protocols

* Apply suggestions from code review

Co-authored-by: Roman Proskuryakov <humbug@deeptown.org>

* Fix some issues

* Fix compilation after merging master

Co-authored-by: Roman Proskuryakov <humbug@deeptown.org>

substrate/client/network/src/protocol/event.rs

@@ -67,7 +67,16 @@ pub enum Event {
 		/// Node we opened the substream with.
 		remote: PeerId,
 		/// The concerned protocol. Each protocol uses a different substream.
+		/// This is always equal to the value of
+		/// [`crate::config::NonDefaultSetConfig::notifications_protocol`] of one of the
+		/// configured sets.
 		protocol: Cow<'static, str>,
+		/// If the negotiation didn't use the main name of the protocol (the one in
+		/// `notifications_protocol`), then this field contains which name has actually been
+		/// used.
+		/// Always contains a value equal to the value in
+		/// [`crate::config::NonDefaultSetConfig::fallback_names`].
+		negotiated_fallback: Option<Cow<'static, str>>,
 		/// Role of the remote.
 		role: ObservedRole,
 	},

substrate/client/network/src/protocol/notifications.rs

@@ -19,7 +19,7 @@
 //! Implementation of libp2p's `NetworkBehaviour` trait that establishes communications and opens
 //! notifications substreams.
 
-pub use self::behaviour::{Notifications, NotificationsOut};
+pub use self::behaviour::{Notifications, NotificationsOut, ProtocolConfig};
 pub use self::handler::{NotifsHandlerError, NotificationsSink, Ready};
 
 mod behaviour;

substrate/client/network/src/protocol/notifications/behaviour.rs

@@ -17,7 +17,7 @@
 // along with this program. If not, see <https://www.gnu.org/licenses/>.
 
 use crate::protocol::notifications::{
-	handler::{NotificationsSink, NotifsHandlerProto, NotifsHandlerOut, NotifsHandlerIn}
+	handler::{self, NotificationsSink, NotifsHandlerProto, NotifsHandlerOut, NotifsHandlerIn}
 };
 
 use bytes::BytesMut;
@@ -95,10 +95,8 @@ use wasm_timer::Instant;
 /// accommodates for any number of connections.
 ///
 pub struct Notifications {
-	/// Notification protocols. Entries are only ever added and not removed.
-	/// Contains, for each protocol, the protocol name and the message to send as part of the
-	/// initial handshake.
-	notif_protocols: Vec<(Cow<'static, str>, Arc<RwLock<Vec<u8>>>, u64)>,
+	/// Notification protocols. Entries never change after initialization.
+	notif_protocols: Vec<handler::ProtocolConfig>,
 
 	/// Receiver for instructions about who to connect to or disconnect from.
 	peerset: sc_peerset::Peerset,
@@ -130,6 +128,19 @@ pub struct Notifications {
 	events: VecDeque<NetworkBehaviourAction<NotifsHandlerIn, NotificationsOut>>,
 }
 
+/// Configuration for a notifications protocol.
+#[derive(Debug, Clone)]
+pub struct ProtocolConfig {
+	/// Name of the protocol.
+	pub name: Cow<'static, str>,
+	/// Names of the protocol to use if the main one isn't available.
+	pub fallback_names: Vec<Cow<'static, str>>,
+	/// Handshake of the protocol.
+	pub handshake: Vec<u8>,
+	/// Maximum allowed size for a notification.
+	pub max_notification_size: u64,
+}
+
 /// Identifier for a delay firing.
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 struct DelayId(u64);
@@ -311,6 +322,9 @@ pub enum NotificationsOut {
 		peer_id: PeerId,
 		/// Peerset set ID the substream is tied to.
 		set_id: sc_peerset::SetId,
+		/// If `Some`, a fallback protocol name has been used rather the main protocol name.
+		/// Always matches one of the fallback names passed at initialization.
+		negotiated_fallback: Option<Cow<'static, str>>,
 		/// Handshake that was sent to us.
 		/// This is normally a "Status" message, but this is out of the concern of this code.
 		received_handshake: Vec<u8>,
@@ -358,10 +372,15 @@ impl Notifications {
 	/// Creates a `CustomProtos`.
 	pub fn new(
 		peerset: sc_peerset::Peerset,
-		notif_protocols: impl Iterator<Item = (Cow<'static, str>, Vec<u8>, u64)>,
+		notif_protocols: impl Iterator<Item = ProtocolConfig>,
 	) -> Self {
 		let notif_protocols = notif_protocols
-			.map(|(n, hs, sz)| (n, Arc::new(RwLock::new(hs)), sz))
+			.map(|cfg| handler::ProtocolConfig {
+				name: cfg.name,
+				fallback_names: cfg.fallback_names,
+				handshake: Arc::new(RwLock::new(cfg.handshake)),
+				max_notification_size: cfg.max_notification_size,
+			})
 			.collect::<Vec<_>>();
 
 		assert!(!notif_protocols.is_empty());
@@ -385,7 +404,7 @@ impl Notifications {
 		handshake_message: impl Into<Vec<u8>>
 	) {
 		if let Some(p) = self.notif_protocols.get_mut(usize::from(set_id)) {
-			*p.1.write() = handshake_message.into();
+			*p.handshake.write() = handshake_message.into();
 		} else {
 			log::error!(target: "sub-libp2p", "Unknown handshake change set: {:?}", set_id);
 			debug_assert!(false);
@@ -1728,7 +1747,9 @@ impl NetworkBehaviour for Notifications {
 				}
 			}
 
-			NotifsHandlerOut::OpenResultOk { protocol_index, received_handshake, notifications_sink, .. } => {
+			NotifsHandlerOut::OpenResultOk {
+				protocol_index, negotiated_fallback, received_handshake, notifications_sink, ..
+			} => {
 				let set_id = sc_peerset::SetId::from(protocol_index);
 				trace!(target: "sub-libp2p",
 					"Handler({}, {:?}) => OpenResultOk({:?})",
@@ -1748,6 +1769,7 @@ impl NetworkBehaviour for Notifications {
 								let event = NotificationsOut::CustomProtocolOpen {
 									peer_id: source,
 									set_id,
+									negotiated_fallback,
 									received_handshake,
 									notifications_sink: notifications_sink.clone(),
 								};

substrate/client/network/src/protocol/notifications/handler.rs

@@ -110,7 +110,7 @@ const INITIAL_KEEPALIVE_TIME: Duration = Duration::from_secs(5);
 pub struct NotifsHandlerProto {
 	/// Name of protocols, prototypes for upgrades for inbound substreams, and the message we
 	/// send or respond with in the handshake.
-	protocols: Vec<(Cow<'static, str>, NotificationsIn, Arc<RwLock<Vec<u8>>>, u64)>,
+	protocols: Vec<ProtocolConfig>,
 }
 
 /// The actual handler once the connection has been established.
@@ -135,20 +135,27 @@ pub struct NotifsHandler {
 	>,
 }
 
+/// Configuration for a notifications protocol.
+#[derive(Debug, Clone)]
+pub struct ProtocolConfig {
+	/// Name of the protocol.
+	pub name: Cow<'static, str>,
+	/// Names of the protocol to use if the main one isn't available.
+	pub fallback_names: Vec<Cow<'static, str>>,
+	/// Handshake of the protocol. The `RwLock` is locked every time a new substream is opened.
+	pub handshake: Arc<RwLock<Vec<u8>>>,
+	/// Maximum allowed size for a notification.
+	pub max_notification_size: u64,
+}
+
 /// Fields specific for each individual protocol.
 struct Protocol {
-	/// Name of the protocol.
-	name: Cow<'static, str>,
+	/// Other fields.
+	config: ProtocolConfig,
 
 	/// Prototype for the inbound upgrade.
 	in_upgrade: NotificationsIn,
 
-	/// Handshake to send when opening a substream or receiving an open request.
-	handshake: Arc<RwLock<Vec<u8>>>,
-
-	/// Maximum allowed size of individual notifications.
-	max_notification_size: u64,
-
 	/// Current state of the substreams for this protocol.
 	state: State,
 }
@@ -214,21 +221,25 @@ impl IntoProtocolsHandler for NotifsHandlerProto {
 
 	fn inbound_protocol(&self) -> UpgradeCollec<NotificationsIn> {
 		self.protocols.iter()
-			.map(|(_, p, _, _)| p.clone())
+			.map(|cfg| NotificationsIn::new(cfg.name.clone(), cfg.fallback_names.clone(), cfg.max_notification_size))
 			.collect::<UpgradeCollec<_>>()
 	}
 
 	fn into_handler(self, peer_id: &PeerId, connected_point: &ConnectedPoint) -> Self::Handler {
 		NotifsHandler {
-			protocols: self.protocols.into_iter().map(|(name, in_upgrade, handshake, max_size)| {
+			protocols: self.protocols.into_iter().map(|config| {
+				let in_upgrade = NotificationsIn::new(
+					config.name.clone(),
+					config.fallback_names.clone(),
+					config.max_notification_size
+				);
+
 				Protocol {
-					name,
+					config,
 					in_upgrade,
-					handshake,
 					state: State::Closed {
 						pending_opening: false,
 					},
-					max_notification_size: max_size,
 				}
 			}).collect(),
 			peer_id: peer_id.clone(),
@@ -271,6 +282,8 @@ pub enum NotifsHandlerOut {
 	OpenResultOk {
 		/// Index of the protocol in the list of protocols passed at initialization.
 		protocol_index: usize,
+		/// Name of the protocol that was actually negotiated, if the default one wasn't available.
+		negotiated_fallback: Option<Cow<'static, str>>,
 		/// The endpoint of the connection that is open for custom protocols.
 		endpoint: ConnectedPoint,
 		/// Handshake that was sent to us.
@@ -445,18 +458,10 @@ impl NotifsHandlerProto {
 	/// is always the same whether we open a substream ourselves or respond to handshake from
 	/// the remote.
 	pub fn new(
-		list: impl Into<Vec<(Cow<'static, str>, Arc<RwLock<Vec<u8>>>, u64)>>,
+		list: impl Into<Vec<ProtocolConfig>>,
 	) -> Self {
-		let protocols =	list
-			.into()
-			.into_iter()
-			.map(|(proto_name, msg, max_notif_size)| {
-				(proto_name.clone(), NotificationsIn::new(proto_name, max_notif_size), msg, max_notif_size)
-			})
-			.collect();
-
 		NotifsHandlerProto {
-			protocols,
+			protocols: list.into(),
 		}
 	}
 }
@@ -481,7 +486,7 @@ impl ProtocolsHandler for NotifsHandler {
 
 	fn inject_fully_negotiated_inbound(
 		&mut self,
-		((_remote_handshake, mut new_substream), protocol_index):
+		(mut in_substream_open, protocol_index):
 			<Self::InboundProtocol as InboundUpgrade<NegotiatedSubstream>>::Output,
 		(): ()
 	) {
@@ -495,7 +500,7 @@ impl ProtocolsHandler for NotifsHandler {
 				));
 
 				protocol_info.state = State::OpenDesiredByRemote {
-					in_substream: new_substream,
+					in_substream: in_substream_open.substream,
 					pending_opening,
 				};
 			},
@@ -518,16 +523,16 @@ impl ProtocolsHandler for NotifsHandler {
 
 				// Create `handshake_message` on a separate line to be sure that the
 				// lock is released as soon as possible.
-				let handshake_message = protocol_info.handshake.read().clone();
-				new_substream.send_handshake(handshake_message);
-				*in_substream = Some(new_substream);
+				let handshake_message = protocol_info.config.handshake.read().clone();
+				in_substream_open.substream.send_handshake(handshake_message);
+				*in_substream = Some(in_substream_open.substream);
 			},
 		}
 	}
 
 	fn inject_fully_negotiated_outbound(
 		&mut self,
-		(handshake, substream): <Self::OutboundProtocol as OutboundUpgrade<NegotiatedSubstream>>::Output,
+		new_open: <Self::OutboundProtocol as OutboundUpgrade<NegotiatedSubstream>>::Output,
 		protocol_index: Self::OutboundOpenInfo
 	) {
 		match self.protocols[protocol_index].state {
@@ -553,15 +558,16 @@ impl ProtocolsHandler for NotifsHandler {
 
 				self.protocols[protocol_index].state = State::Open {
 					notifications_sink_rx: stream::select(async_rx.fuse(), sync_rx.fuse()).peekable(),
-					out_substream: Some(substream),
+					out_substream: Some(new_open.substream),
 					in_substream: in_substream.take(),
 				};
 
 				self.events_queue.push_back(ProtocolsHandlerEvent::Custom(
 					NotifsHandlerOut::OpenResultOk {
 						protocol_index,
+						negotiated_fallback: new_open.negotiated_fallback,
 						endpoint: self.endpoint.clone(),
-						received_handshake: handshake,
+						received_handshake: new_open.handshake,
 						notifications_sink
 					}
 				));
@@ -577,9 +583,10 @@ impl ProtocolsHandler for NotifsHandler {
 					State::Closed { pending_opening } => {
 						if !*pending_opening {
 							let proto = NotificationsOut::new(
-								protocol_info.name.clone(),
-								protocol_info.handshake.read().clone(),
-								protocol_info.max_notification_size
+								protocol_info.config.name.clone(),
+								protocol_info.config.fallback_names.clone(),
+								protocol_info.config.handshake.read().clone(),
+								protocol_info.config.max_notification_size
 							);
 
 							self.events_queue.push_back(ProtocolsHandlerEvent::OutboundSubstreamRequest {
@@ -593,13 +600,14 @@ impl ProtocolsHandler for NotifsHandler {
 						};
 					},
 					State::OpenDesiredByRemote { pending_opening, in_substream } => {
-						let handshake_message = protocol_info.handshake.read().clone();
+						let handshake_message = protocol_info.config.handshake.read().clone();
 
 						if !*pending_opening {
 							let proto = NotificationsOut::new(
-								protocol_info.name.clone(),
+								protocol_info.config.name.clone(),
+								protocol_info.config.fallback_names.clone(),
 								handshake_message.clone(),
-								protocol_info.max_notification_size,
+								protocol_info.config.max_notification_size,
 							);
 
 							self.events_queue.push_back(ProtocolsHandlerEvent::OutboundSubstreamRequest {

substrate/client/network/src/protocol/notifications/tests.rs

@@ -18,7 +18,7 @@
 
 #![cfg(test)]
 
-use crate::protocol::notifications::{Notifications, NotificationsOut};
+use crate::protocol::notifications::{Notifications, NotificationsOut, ProtocolConfig};
 
 use futures::prelude::*;
 use libp2p::{PeerId, Multiaddr, Transport};
@@ -80,7 +80,12 @@ fn build_nodes() -> (Swarm<CustomProtoWithAddr>, Swarm<CustomProtoWithAddr>) {
 		});
 
 		let behaviour = CustomProtoWithAddr {
-			inner: Notifications::new(peerset, iter::once(("/foo".into(), Vec::new(), 1024 * 1024))),
+			inner: Notifications::new(peerset, iter::once(ProtocolConfig {
+				name: "/foo".into(),
+				fallback_names: Vec::new(),
+				handshake: Vec::new(),
+				max_notification_size: 1024 * 1024
+			})),
 			addrs: addrs
 				.iter()
 				.enumerate()

substrate/client/network/src/protocol/notifications/upgrade.rs

@@ -19,8 +19,10 @@
 pub use self::collec::UpgradeCollec;
 pub use self::notifications::{
 	NotificationsIn,
+	NotificationsInOpen,
 	NotificationsInSubstream,
 	NotificationsOut,
+	NotificationsOutOpen,
 	NotificationsOutSubstream,
 	NotificationsHandshakeError,
 	NotificationsOutError,

substrate/client/network/src/protocol/notifications/upgrade/notifications.rs

@@ -41,7 +41,7 @@ use futures::prelude::*;
 use asynchronous_codec::Framed;
 use libp2p::core::{UpgradeInfo, InboundUpgrade, OutboundUpgrade, upgrade};
 use log::error;
-use std::{borrow::Cow, convert::{Infallible, TryFrom as _}, io, iter, mem, pin::Pin, task::{Context, Poll}};
+use std::{borrow::Cow, convert::{Infallible, TryFrom as _}, io, mem, pin::Pin, task::{Context, Poll}, vec};
 use unsigned_varint::codec::UviBytes;
 
 /// Maximum allowed size of the two handshake messages, in bytes.
@@ -52,7 +52,8 @@ const MAX_HANDSHAKE_SIZE: usize = 1024;
 #[derive(Debug, Clone)]
 pub struct NotificationsIn {
 	/// Protocol name to use when negotiating the substream.
-	protocol_name: Cow<'static, str>,
+	/// The first one is the main name, while the other ones are fall backs.
+	protocol_names: Vec<Cow<'static, str>>,
 	/// Maximum allowed size for a single notification.
 	max_notification_size: u64,
 }
@@ -62,7 +63,8 @@ pub struct NotificationsIn {
 #[derive(Debug, Clone)]
 pub struct NotificationsOut {
 	/// Protocol name to use when negotiating the substream.
-	protocol_name: Cow<'static, str>,
+	/// The first one is the main name, while the other ones are fall backs.
+	protocol_names: Vec<Cow<'static, str>>,
 	/// Message to send when we start the handshake.
 	initial_message: Vec<u8>,
 	/// Maximum allowed size for a single notification.
@@ -106,51 +108,54 @@ pub struct NotificationsOutSubstream<TSubstream> {
 
 impl NotificationsIn {
 	/// Builds a new potential upgrade.
-	pub fn new(protocol_name: impl Into<Cow<'static, str>>, max_notification_size: u64) -> Self {
+	pub fn new(
+		main_protocol_name: impl Into<Cow<'static, str>>,
+		fallback_names: Vec<Cow<'static, str>>,
+		max_notification_size: u64
+	) -> Self {
+		let mut protocol_names = fallback_names;
+		protocol_names.insert(0, main_protocol_name.into());
+
 		NotificationsIn {
-			protocol_name: protocol_name.into(),
+			protocol_names,
 			max_notification_size,
 		}
 	}
 }
 
 impl UpgradeInfo for NotificationsIn {
-	type Info = Cow<'static, [u8]>;
-	type InfoIter = iter::Once<Self::Info>;
+	type Info = StringProtocolName;
+	type InfoIter = vec::IntoIter<Self::Info>;
 
 	fn protocol_info(&self) -> Self::InfoIter {
-		let bytes: Cow<'static, [u8]> = match &self.protocol_name {
-			Cow::Borrowed(s) => Cow::Borrowed(s.as_bytes()),
-			Cow::Owned(s) => Cow::Owned(s.as_bytes().to_vec())
-		};
-		iter::once(bytes)
+		self.protocol_names.iter().cloned().map(StringProtocolName).collect::<Vec<_>>().into_iter()
 	}
 }
 
 impl<TSubstream> InboundUpgrade<TSubstream> for NotificationsIn
 where TSubstream: AsyncRead + AsyncWrite + Unpin + Send + 'static,
 {
-	type Output = (Vec<u8>, NotificationsInSubstream<TSubstream>);
+	type Output = NotificationsInOpen<TSubstream>;
 	type Future = Pin<Box<dyn Future<Output = Result<Self::Output, Self::Error>> + Send>>;
 	type Error = NotificationsHandshakeError;
 
 	fn upgrade_inbound(
 		self,
 		mut socket: TSubstream,
-		_: Self::Info,
+		negotiated_name: Self::Info,
 	) -> Self::Future {
 		Box::pin(async move {
-			let initial_message_len = unsigned_varint::aio::read_usize(&mut socket).await?;
-			if initial_message_len > MAX_HANDSHAKE_SIZE {
+			let handshake_len = unsigned_varint::aio::read_usize(&mut socket).await?;
+			if handshake_len > MAX_HANDSHAKE_SIZE {
 				return Err(NotificationsHandshakeError::TooLarge {
-					requested: initial_message_len,
+					requested: handshake_len,
 					max: MAX_HANDSHAKE_SIZE,
 				});
 			}
 
-			let mut initial_message = vec![0u8; initial_message_len];
-			if !initial_message.is_empty() {
-				socket.read_exact(&mut initial_message).await?;
+			let mut handshake = vec![0u8; handshake_len];
+			if !handshake.is_empty() {
+				socket.read_exact(&mut handshake).await?;
 			}
 
 			let mut codec = UviBytes::default();
@@ -161,11 +166,30 @@ where TSubstream: AsyncRead + AsyncWrite + Unpin + Send + 'static,
 				handshake: NotificationsInSubstreamHandshake::NotSent,
 			};
 
-			Ok((initial_message, substream))
+			Ok(NotificationsInOpen {
+				handshake,
+				negotiated_fallback: if negotiated_name.0 == self.protocol_names[0] {
+					None
+				} else {
+					Some(negotiated_name.0)
+				},
+				substream,
+			})
 		})
 	}
 }
 
+/// Yielded by the [`NotificationsIn`] after a successfuly upgrade.
+pub struct NotificationsInOpen<TSubstream> {
+	/// Handshake sent by the remote.
+	pub handshake: Vec<u8>,
+	/// If the negotiated name is not the "main" protocol name but a fallback, contains the
+	/// name of the negotiated fallback.
+	pub negotiated_fallback: Option<Cow<'static, str>>,
+	/// Implementation of `Stream` that allows receives messages from the substream.
+	pub substream: NotificationsInSubstream<TSubstream>,
+}
+
 impl<TSubstream> NotificationsInSubstream<TSubstream>
 where TSubstream: AsyncRead + AsyncWrite + Unpin,
 {
@@ -296,7 +320,8 @@ where TSubstream: AsyncRead + AsyncWrite + Unpin,
 impl NotificationsOut {
 	/// Builds a new potential upgrade.
 	pub fn new(
-		protocol_name: impl Into<Cow<'static, str>>,
+		main_protocol_name: impl Into<Cow<'static, str>>,
+		fallback_names: Vec<Cow<'static, str>>,
 		initial_message: impl Into<Vec<u8>>,
 		max_notification_size: u64,
 	) -> Self {
@@ -305,38 +330,47 @@ impl NotificationsOut {
 			error!(target: "sub-libp2p", "Outbound networking handshake is above allowed protocol limit");
 		}
 
+		let mut protocol_names = fallback_names;
+		protocol_names.insert(0, main_protocol_name.into());
+
 		NotificationsOut {
-			protocol_name: protocol_name.into(),
+			protocol_names,
 			initial_message,
 			max_notification_size,
 		}
 	}
 }
 
+/// Implementation of the `ProtocolName` trait, where the protocol name is a string.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct StringProtocolName(Cow<'static, str>);
+
+impl upgrade::ProtocolName for StringProtocolName {
+	fn protocol_name(&self) -> &[u8] {
+		self.0.as_bytes()
+	}
+}
+
 impl UpgradeInfo for NotificationsOut {
-	type Info = Cow<'static, [u8]>;
-	type InfoIter = iter::Once<Self::Info>;
+	type Info = StringProtocolName;
+	type InfoIter = vec::IntoIter<Self::Info>;
 
 	fn protocol_info(&self) -> Self::InfoIter {
-		let bytes: Cow<'static, [u8]> = match &self.protocol_name {
-			Cow::Borrowed(s) => Cow::Borrowed(s.as_bytes()),
-			Cow::Owned(s) => Cow::Owned(s.as_bytes().to_vec())
-		};
-		iter::once(bytes)
+		self.protocol_names.iter().cloned().map(StringProtocolName).collect::<Vec<_>>().into_iter()
 	}
 }
 
 impl<TSubstream> OutboundUpgrade<TSubstream> for NotificationsOut
 where TSubstream: AsyncRead + AsyncWrite + Unpin + Send + 'static,
 {
-	type Output = (Vec<u8>, NotificationsOutSubstream<TSubstream>);
+	type Output = NotificationsOutOpen<TSubstream>;
 	type Future = Pin<Box<dyn Future<Output = Result<Self::Output, Self::Error>> + Send>>;
 	type Error = NotificationsHandshakeError;
 
 	fn upgrade_outbound(
 		self,
 		mut socket: TSubstream,
-		_: Self::Info,
+		negotiated_name: Self::Info,
 	) -> Self::Future {
 		Box::pin(async move {
 			upgrade::write_with_len_prefix(&mut socket, &self.initial_message).await?;
@@ -358,13 +392,32 @@ where TSubstream: AsyncRead + AsyncWrite + Unpin + Send + 'static,
 			let mut codec = UviBytes::default();
 			codec.set_max_len(usize::try_from(self.max_notification_size).unwrap_or(usize::max_value()));
 
-			Ok((handshake, NotificationsOutSubstream {
-				socket: Framed::new(socket, codec),
-			}))
+			Ok(NotificationsOutOpen {
+				handshake,
+				negotiated_fallback: if negotiated_name.0 == self.protocol_names[0] {
+					None
+				} else {
+					Some(negotiated_name.0)
+				},
+				substream: NotificationsOutSubstream {
+					socket: Framed::new(socket, codec),
+				}
+			})
 		})
 	}
 }
 
+/// Yielded by the [`NotificationsOut`] after a successfuly upgrade.
+pub struct NotificationsOutOpen<TSubstream> {
+	/// Handshake returned by the remote.
+	pub handshake: Vec<u8>,
+	/// If the negotiated name is not the "main" protocol name but a fallback, contains the
+	/// name of the negotiated fallback.
+	pub negotiated_fallback: Option<Cow<'static, str>>,
+	/// Implementation of `Sink` that allows sending messages on the substream.
+	pub substream: NotificationsOutSubstream<TSubstream>,
+}
+
 impl<TSubstream> Sink<Vec<u8>> for NotificationsOutSubstream<TSubstream>
 	where TSubstream: AsyncRead + AsyncWrite + Unpin,
 {
@@ -436,7 +489,7 @@ pub enum NotificationsOutError {
 
 #[cfg(test)]
 mod tests {
-	use super::{NotificationsIn, NotificationsOut};
+	use super::{NotificationsIn, NotificationsInOpen, NotificationsOut, NotificationsOutOpen};
 
 	use async_std::net::{TcpListener, TcpStream};
 	use futures::{prelude::*, channel::oneshot};
@@ -450,9 +503,9 @@ mod tests {
 
 		let client = async_std::task::spawn(async move {
 			let socket = TcpStream::connect(listener_addr_rx.await.unwrap()).await.unwrap();
-			let (handshake, mut substream) = upgrade::apply_outbound(
+			let NotificationsOutOpen { handshake, mut substream, .. } = upgrade::apply_outbound(
 				socket,
-				NotificationsOut::new(PROTO_NAME, &b"initial message"[..], 1024 * 1024),
+				NotificationsOut::new(PROTO_NAME, Vec::new(), &b"initial message"[..], 1024 * 1024),
 				upgrade::Version::V1
 			).await.unwrap();
 
@@ -465,12 +518,12 @@ mod tests {
 			listener_addr_tx.send(listener.local_addr().unwrap()).unwrap();
 
 			let (socket, _) = listener.accept().await.unwrap();
-			let (initial_message, mut substream) = upgrade::apply_inbound(
+			let NotificationsInOpen { handshake, mut substream, .. } = upgrade::apply_inbound(
 				socket,
-				NotificationsIn::new(PROTO_NAME, 1024 * 1024)
+				NotificationsIn::new(PROTO_NAME, Vec::new(), 1024 * 1024)
 			).await.unwrap();
 
-			assert_eq!(initial_message, b"initial message");
+			assert_eq!(handshake, b"initial message");
 			substream.send_handshake(&b"hello world"[..]);
 
 			let msg = substream.next().await.unwrap().unwrap();
@@ -489,9 +542,9 @@ mod tests {
 
 		let client = async_std::task::spawn(async move {
 			let socket = TcpStream::connect(listener_addr_rx.await.unwrap()).await.unwrap();
-			let (handshake, mut substream) = upgrade::apply_outbound(
+			let NotificationsOutOpen { handshake, mut substream, .. } = upgrade::apply_outbound(
 				socket,
-				NotificationsOut::new(PROTO_NAME, vec![], 1024 * 1024),
+				NotificationsOut::new(PROTO_NAME, Vec::new(), vec![], 1024 * 1024),
 				upgrade::Version::V1
 			).await.unwrap();
 
@@ -504,12 +557,12 @@ mod tests {
 			listener_addr_tx.send(listener.local_addr().unwrap()).unwrap();
 
 			let (socket, _) = listener.accept().await.unwrap();
-			let (initial_message, mut substream) = upgrade::apply_inbound(
+			let NotificationsInOpen { handshake, mut substream, .. } = upgrade::apply_inbound(
 				socket,
-				NotificationsIn::new(PROTO_NAME, 1024 * 1024)
+				NotificationsIn::new(PROTO_NAME, Vec::new(), 1024 * 1024)
 			).await.unwrap();
 
-			assert!(initial_message.is_empty());
+			assert!(handshake.is_empty());
 			substream.send_handshake(vec![]);
 
 			let msg = substream.next().await.unwrap().unwrap();
@@ -528,7 +581,7 @@ mod tests {
 			let socket = TcpStream::connect(listener_addr_rx.await.unwrap()).await.unwrap();
 			let outcome = upgrade::apply_outbound(
 				socket,
-				NotificationsOut::new(PROTO_NAME, &b"hello"[..], 1024 * 1024),
+				NotificationsOut::new(PROTO_NAME, Vec::new(), &b"hello"[..], 1024 * 1024),
 				upgrade::Version::V1
 			).await;
 
@@ -543,12 +596,12 @@ mod tests {
 			listener_addr_tx.send(listener.local_addr().unwrap()).unwrap();
 
 			let (socket, _) = listener.accept().await.unwrap();
-			let (initial_msg, substream) = upgrade::apply_inbound(
+			let NotificationsInOpen { handshake, substream, .. } = upgrade::apply_inbound(
 				socket,
-				NotificationsIn::new(PROTO_NAME, 1024 * 1024)
+				NotificationsIn::new(PROTO_NAME, Vec::new(), 1024 * 1024)
 			).await.unwrap();
 
-			assert_eq!(initial_msg, b"hello");
+			assert_eq!(handshake, b"hello");
 
 			// We successfully upgrade to the protocol, but then close the substream.
 			drop(substream);
@@ -567,7 +620,7 @@ mod tests {
 			let ret = upgrade::apply_outbound(
 				socket,
 				// We check that an initial message that is too large gets refused.
-				NotificationsOut::new(PROTO_NAME, (0..32768).map(|_| 0).collect::<Vec<_>>(), 1024 * 1024),
+				NotificationsOut::new(PROTO_NAME, Vec::new(), (0..32768).map(|_| 0).collect::<Vec<_>>(), 1024 * 1024),
 				upgrade::Version::V1
 			).await;
 			assert!(ret.is_err());
@@ -580,7 +633,7 @@ mod tests {
 			let (socket, _) = listener.accept().await.unwrap();
 			let ret = upgrade::apply_inbound(
 				socket,
-				NotificationsIn::new(PROTO_NAME, 1024 * 1024)
+				NotificationsIn::new(PROTO_NAME, Vec::new(), 1024 * 1024)
 			).await;
 			assert!(ret.is_err());
 		});
@@ -597,7 +650,7 @@ mod tests {
 			let socket = TcpStream::connect(listener_addr_rx.await.unwrap()).await.unwrap();
 			let ret = upgrade::apply_outbound(
 				socket,
-				NotificationsOut::new(PROTO_NAME, &b"initial message"[..], 1024 * 1024),
+				NotificationsOut::new(PROTO_NAME, Vec::new(), &b"initial message"[..], 1024 * 1024),
 				upgrade::Version::V1
 			).await;
 			assert!(ret.is_err());
@@ -608,11 +661,11 @@ mod tests {
 			listener_addr_tx.send(listener.local_addr().unwrap()).unwrap();
 
 			let (socket, _) = listener.accept().await.unwrap();
-			let (initial_message, mut substream) = upgrade::apply_inbound(
+			let NotificationsInOpen { handshake, mut substream, .. } = upgrade::apply_inbound(
 				socket,
-				NotificationsIn::new(PROTO_NAME, 1024 * 1024)
+				NotificationsIn::new(PROTO_NAME, Vec::new(), 1024 * 1024)
 			).await.unwrap();
-			assert_eq!(initial_message, b"initial message");
+			assert_eq!(handshake, b"initial message");
 
 			// We check that a handshake that is too large gets refused.
 			substream.send_handshake((0..32768).map(|_| 0).collect::<Vec<_>>());