feat: report changes in supported protocols to ConnectionHandler

With this patch, implementations of `ConnectionHandler` (which are typically composed in a tree) can exchange information about the supported protocols of a remote with each other via `ConnectionHandlerEvent::ReportRemoteProtocols`. The provided `ProtocolSupport` enum can describe either additions or removals of the remote peer's protocols.

This information is aggregated in the connection and passed down to the `ConnectionHandler` via `ConnectionEvent::RemoteProtocolsChange`.

Similarly, if the listen protocols of a connection change, all `ConnectionHandler`s on the connection will be notified via `ConnectionEvent::LocalProtocolsChange`. This will allow us to eventually remove `PollParameters` from `NetworkBehaviour`.

This pattern allows protocols on a connection to communicate with each other. For example, protocols like identify can share the list of (supposedly) supported protocols by the remote with all other handlers. A protocol like kademlia can accurately add and remove a remote from its routing table as a result.

Resolves: #2680.
Related: #3124.

Pull-Request: #3651.

swarm/src/behaviour.rs

@@ -230,6 +230,9 @@ pub trait PollParameters {
     /// The iterator's elements are the ASCII names as reported on the wire.
     ///
     /// Note that the list is computed once at initialization and never refreshed.
+    #[deprecated(
+        note = "Use `libp2p_swarm::SupportedProtocols` in your `ConnectionHandler` instead."
+    )]
     fn supported_protocols(&self) -> Self::SupportedProtocolsIter;
 
     /// Returns the list of the addresses we're listening on.

swarm/src/behaviour/toggle.rs

@@ -362,6 +362,16 @@ where
             ConnectionEvent::ListenUpgradeError(listen_upgrade_error) => {
                 self.on_listen_upgrade_error(listen_upgrade_error)
             }
+            ConnectionEvent::LocalProtocolsChange(change) => {
+                if let Some(inner) = self.inner.as_mut() {
+                    inner.on_connection_event(ConnectionEvent::LocalProtocolsChange(change));
+                }
+            }
+            ConnectionEvent::RemoteProtocolsChange(change) => {
+                if let Some(inner) = self.inner.as_mut() {
+                    inner.on_connection_event(ConnectionEvent::RemoteProtocolsChange(change));
+                }
+            }
         }
     }
 }

swarm/src/connection.rs

@@ -21,18 +21,23 @@
 mod error;
 
 pub(crate) mod pool;
+mod supported_protocols;
 
 pub use error::ConnectionError;
 pub(crate) use error::{
     PendingConnectionError, PendingInboundConnectionError, PendingOutboundConnectionError,
 };
+pub use supported_protocols::SupportedProtocols;
 
 use crate::handler::{
     AddressChange, ConnectionEvent, ConnectionHandler, DialUpgradeError, FullyNegotiatedInbound,
-    FullyNegotiatedOutbound, ListenUpgradeError,
+    FullyNegotiatedOutbound, ListenUpgradeError, ProtocolSupport, ProtocolsAdded, ProtocolsChange,
+    UpgradeInfoSend,
 };
 use crate::upgrade::{InboundUpgradeSend, OutboundUpgradeSend, SendWrapper};
-use crate::{ConnectionHandlerEvent, KeepAlive, StreamUpgradeError, SubstreamProtocol};
+use crate::{
+    ConnectionHandlerEvent, KeepAlive, StreamProtocol, StreamUpgradeError, SubstreamProtocol,
+};
 use futures::stream::FuturesUnordered;
 use futures::FutureExt;
 use futures::StreamExt;
@@ -47,6 +52,7 @@ use libp2p_core::upgrade::{
 };
 use libp2p_core::Endpoint;
 use libp2p_identity::PeerId;
+use std::collections::HashSet;
 use std::future::Future;
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::task::Waker;
@@ -147,6 +153,9 @@ where
     requested_substreams: FuturesUnordered<
         SubstreamRequested<THandler::OutboundOpenInfo, THandler::OutboundProtocol>,
     >,
+
+    local_supported_protocols: HashSet<StreamProtocol>,
+    remote_supported_protocols: HashSet<StreamProtocol>,
 }
 
 impl<THandler> fmt::Debug for Connection<THandler>
@@ -171,10 +180,18 @@ where
     /// and connection handler.
     pub(crate) fn new(
         muxer: StreamMuxerBox,
-        handler: THandler,
+        mut handler: THandler,
         substream_upgrade_protocol_override: Option<upgrade::Version>,
         max_negotiating_inbound_streams: usize,
     ) -> Self {
+        let initial_protocols = gather_supported_protocols(&handler);
+
+        if !initial_protocols.is_empty() {
+            handler.on_connection_event(ConnectionEvent::LocalProtocolsChange(
+                ProtocolsChange::Added(ProtocolsAdded::from_set(&initial_protocols)),
+            ));
+        }
+
         Connection {
             muxing: muxer,
             handler,
@@ -184,6 +201,8 @@ where
             substream_upgrade_protocol_override,
             max_negotiating_inbound_streams,
             requested_substreams: Default::default(),
+            local_supported_protocols: initial_protocols,
+            remote_supported_protocols: Default::default(),
         }
     }
 
@@ -213,6 +232,8 @@ where
             shutdown,
             max_negotiating_inbound_streams,
             substream_upgrade_protocol_override,
+            local_supported_protocols: supported_protocols,
+            remote_supported_protocols,
         } = self.get_mut();
 
         loop {
@@ -246,6 +267,31 @@ where
                 Poll::Ready(ConnectionHandlerEvent::Close(err)) => {
                     return Poll::Ready(Err(ConnectionError::Handler(err)));
                 }
+                Poll::Ready(ConnectionHandlerEvent::ReportRemoteProtocols(
+                    ProtocolSupport::Added(protocols),
+                )) => {
+                    if let Some(added) =
+                        ProtocolsChange::add(remote_supported_protocols, &protocols)
+                    {
+                        handler.on_connection_event(ConnectionEvent::RemoteProtocolsChange(added));
+                        remote_supported_protocols.extend(protocols);
+                    }
+
+                    continue;
+                }
+                Poll::Ready(ConnectionHandlerEvent::ReportRemoteProtocols(
+                    ProtocolSupport::Removed(protocols),
+                )) => {
+                    if let Some(removed) =
+                        ProtocolsChange::remove(remote_supported_protocols, &protocols)
+                    {
+                        handler
+                            .on_connection_event(ConnectionEvent::RemoteProtocolsChange(removed));
+                        remote_supported_protocols.retain(|p| !protocols.contains(p));
+                    }
+
+                    continue;
+                }
             }
 
             // In case the [`ConnectionHandler`] can not make any more progress, poll the negotiating outbound streams.
@@ -376,9 +422,33 @@ where
                 }
             }
 
+            let new_protocols = gather_supported_protocols(handler);
+
+            for change in ProtocolsChange::from_full_sets(supported_protocols, &new_protocols) {
+                handler.on_connection_event(ConnectionEvent::LocalProtocolsChange(change));
+            }
+
+            *supported_protocols = new_protocols;
+
             return Poll::Pending; // Nothing can make progress, return `Pending`.
         }
     }
+
+    #[cfg(test)]
+    fn poll_noop_waker(
+        &mut self,
+    ) -> Poll<Result<Event<THandler::OutEvent>, ConnectionError<THandler::Error>>> {
+        Pin::new(self).poll(&mut Context::from_waker(futures::task::noop_waker_ref()))
+    }
+}
+
+fn gather_supported_protocols(handler: &impl ConnectionHandler) -> HashSet<StreamProtocol> {
+    handler
+        .listen_protocol()
+        .upgrade()
+        .protocol_info()
+        .filter_map(|i| StreamProtocol::try_from_owned(i.as_ref().to_owned()).ok())
+        .collect()
 }
 
 /// Borrowed information about an incoming connection currently being negotiated.
@@ -605,9 +675,10 @@ enum Shutdown {
 mod tests {
     use super::*;
     use crate::keep_alive;
+    use futures::future;
     use futures::AsyncRead;
     use futures::AsyncWrite;
-    use libp2p_core::upgrade::DeniedUpgrade;
+    use libp2p_core::upgrade::{DeniedUpgrade, InboundUpgrade, OutboundUpgrade, UpgradeInfo};
     use libp2p_core::StreamMuxer;
     use quickcheck::*;
     use std::sync::{Arc, Weak};
@@ -629,8 +700,7 @@ mod tests {
                 max_negotiating_inbound_streams,
             );
 
-            let result = Pin::new(&mut connection)
-                .poll(&mut Context::from_waker(futures::task::noop_waker_ref()));
+            let result = connection.poll_noop_waker();
 
             assert!(result.is_pending());
             assert_eq!(
@@ -654,13 +724,11 @@ mod tests {
         );
 
         connection.handler.open_new_outbound();
-        let _ = Pin::new(&mut connection)
-            .poll(&mut Context::from_waker(futures::task::noop_waker_ref()));
+        let _ = connection.poll_noop_waker();
 
         std::thread::sleep(upgrade_timeout + Duration::from_secs(1));
 
-        let _ = Pin::new(&mut connection)
-            .poll(&mut Context::from_waker(futures::task::noop_waker_ref()));
+        let _ = connection.poll_noop_waker();
 
         assert!(matches!(
             connection.handler.error.unwrap(),
@@ -668,6 +736,94 @@ mod tests {
         ))
     }
 
+    #[test]
+    fn propagates_changes_to_supported_inbound_protocols() {
+        let mut connection = Connection::new(
+            StreamMuxerBox::new(PendingStreamMuxer),
+            ConfigurableProtocolConnectionHandler::default(),
+            None,
+            0,
+        );
+
+        // First, start listening on a single protocol.
+        connection.handler.listen_on(&["/foo"]);
+        let _ = connection.poll_noop_waker();
+
+        assert_eq!(connection.handler.local_added, vec![vec!["/foo"]]);
+        assert!(connection.handler.local_removed.is_empty());
+
+        // Second, listen on two protocols.
+        connection.handler.listen_on(&["/foo", "/bar"]);
+        let _ = connection.poll_noop_waker();
+
+        assert_eq!(
+            connection.handler.local_added,
+            vec![vec!["/foo"], vec!["/bar"]],
+            "expect to only receive an event for the newly added protocols"
+        );
+        assert!(connection.handler.local_removed.is_empty());
+
+        // Third, stop listening on the first protocol.
+        connection.handler.listen_on(&["/bar"]);
+        let _ = connection.poll_noop_waker();
+
+        assert_eq!(
+            connection.handler.local_added,
+            vec![vec!["/foo"], vec!["/bar"]]
+        );
+        assert_eq!(connection.handler.local_removed, vec![vec!["/foo"]]);
+    }
+
+    #[test]
+    fn only_propagtes_actual_changes_to_remote_protocols_to_handler() {
+        let mut connection = Connection::new(
+            StreamMuxerBox::new(PendingStreamMuxer),
+            ConfigurableProtocolConnectionHandler::default(),
+            None,
+            0,
+        );
+
+        // First, remote supports a single protocol.
+        connection.handler.remote_adds_support_for(&["/foo"]);
+        let _ = connection.poll_noop_waker();
+
+        assert_eq!(connection.handler.remote_added, vec![vec!["/foo"]]);
+        assert!(connection.handler.remote_removed.is_empty());
+
+        // Second, it adds a protocol but also still includes the first one.
+        connection
+            .handler
+            .remote_adds_support_for(&["/foo", "/bar"]);
+        let _ = connection.poll_noop_waker();
+
+        assert_eq!(
+            connection.handler.remote_added,
+            vec![vec!["/foo"], vec!["/bar"]],
+            "expect to only receive an event for the newly added protocol"
+        );
+        assert!(connection.handler.remote_removed.is_empty());
+
+        // Third, stop listening on a protocol it never advertised (we can't control what handlers do so this needs to be handled gracefully).
+        connection.handler.remote_removes_support_for(&["/baz"]);
+        let _ = connection.poll_noop_waker();
+
+        assert_eq!(
+            connection.handler.remote_added,
+            vec![vec!["/foo"], vec!["/bar"]]
+        );
+        assert!(&connection.handler.remote_removed.is_empty());
+
+        // Fourth, stop listening on a protocol that was previously supported
+        connection.handler.remote_removes_support_for(&["/bar"]);
+        let _ = connection.poll_noop_waker();
+
+        assert_eq!(
+            connection.handler.remote_added,
+            vec![vec!["/foo"], vec!["/bar"]]
+        );
+        assert_eq!(connection.handler.remote_removed, vec![vec!["/bar"]]);
+    }
+
     struct DummyStreamMuxer {
         counter: Arc<()>,
     }
@@ -785,6 +941,40 @@ mod tests {
         }
     }
 
+    #[derive(Default)]
+    struct ConfigurableProtocolConnectionHandler {
+        events: Vec<ConnectionHandlerEvent<DeniedUpgrade, (), Void, Void>>,
+        active_protocols: HashSet<StreamProtocol>,
+        local_added: Vec<Vec<StreamProtocol>>,
+        local_removed: Vec<Vec<StreamProtocol>>,
+        remote_added: Vec<Vec<StreamProtocol>>,
+        remote_removed: Vec<Vec<StreamProtocol>>,
+    }
+
+    impl ConfigurableProtocolConnectionHandler {
+        fn listen_on(&mut self, protocols: &[&'static str]) {
+            self.active_protocols = protocols.iter().copied().map(StreamProtocol::new).collect();
+        }
+
+        fn remote_adds_support_for(&mut self, protocols: &[&'static str]) {
+            self.events
+                .push(ConnectionHandlerEvent::ReportRemoteProtocols(
+                    ProtocolSupport::Added(
+                        protocols.iter().copied().map(StreamProtocol::new).collect(),
+                    ),
+                ));
+        }
+
+        fn remote_removes_support_for(&mut self, protocols: &[&'static str]) {
+            self.events
+                .push(ConnectionHandlerEvent::ReportRemoteProtocols(
+                    ProtocolSupport::Removed(
+                        protocols.iter().copied().map(StreamProtocol::new).collect(),
+                    ),
+                ));
+        }
+    }
+
     impl ConnectionHandler for MockConnectionHandler {
         type InEvent = Void;
         type OutEvent = Void;
@@ -821,7 +1011,10 @@ mod tests {
                 ConnectionEvent::DialUpgradeError(DialUpgradeError { error, .. }) => {
                     self.error = Some(error)
                 }
-                ConnectionEvent::AddressChange(_) | ConnectionEvent::ListenUpgradeError(_) => {}
+                ConnectionEvent::AddressChange(_)
+                | ConnectionEvent::ListenUpgradeError(_)
+                | ConnectionEvent::LocalProtocolsChange(_)
+                | ConnectionEvent::RemoteProtocolsChange(_) => {}
             }
         }
 
@@ -855,6 +1048,112 @@ mod tests {
             Poll::Pending
         }
     }
+
+    impl ConnectionHandler for ConfigurableProtocolConnectionHandler {
+        type InEvent = Void;
+        type OutEvent = Void;
+        type Error = Void;
+        type InboundProtocol = ManyProtocolsUpgrade;
+        type OutboundProtocol = DeniedUpgrade;
+        type InboundOpenInfo = ();
+        type OutboundOpenInfo = ();
+
+        fn listen_protocol(
+            &self,
+        ) -> SubstreamProtocol<Self::InboundProtocol, Self::InboundOpenInfo> {
+            SubstreamProtocol::new(
+                ManyProtocolsUpgrade {
+                    protocols: Vec::from_iter(self.active_protocols.clone()),
+                },
+                (),
+            )
+        }
+
+        fn on_connection_event(
+            &mut self,
+            event: ConnectionEvent<
+                Self::InboundProtocol,
+                Self::OutboundProtocol,
+                Self::InboundOpenInfo,
+                Self::OutboundOpenInfo,
+            >,
+        ) {
+            match event {
+                ConnectionEvent::LocalProtocolsChange(ProtocolsChange::Added(added)) => {
+                    self.local_added.push(added.cloned().collect())
+                }
+                ConnectionEvent::LocalProtocolsChange(ProtocolsChange::Removed(removed)) => {
+                    self.local_removed.push(removed.cloned().collect())
+                }
+                ConnectionEvent::RemoteProtocolsChange(ProtocolsChange::Added(added)) => {
+                    self.remote_added.push(added.cloned().collect())
+                }
+                ConnectionEvent::RemoteProtocolsChange(ProtocolsChange::Removed(removed)) => {
+                    self.remote_removed.push(removed.cloned().collect())
+                }
+                _ => {}
+            }
+        }
+
+        fn on_behaviour_event(&mut self, event: Self::InEvent) {
+            void::unreachable(event)
+        }
+
+        fn connection_keep_alive(&self) -> KeepAlive {
+            KeepAlive::Yes
+        }
+
+        fn poll(
+            &mut self,
+            _: &mut Context<'_>,
+        ) -> Poll<
+            ConnectionHandlerEvent<
+                Self::OutboundProtocol,
+                Self::OutboundOpenInfo,
+                Self::OutEvent,
+                Self::Error,
+            >,
+        > {
+            if let Some(event) = self.events.pop() {
+                return Poll::Ready(event);
+            }
+
+            Poll::Pending
+        }
+    }
+
+    struct ManyProtocolsUpgrade {
+        protocols: Vec<StreamProtocol>,
+    }
+
+    impl UpgradeInfo for ManyProtocolsUpgrade {
+        type Info = StreamProtocol;
+        type InfoIter = std::vec::IntoIter<Self::Info>;
+
+        fn protocol_info(&self) -> Self::InfoIter {
+            self.protocols.clone().into_iter()
+        }
+    }
+
+    impl<C> InboundUpgrade<C> for ManyProtocolsUpgrade {
+        type Output = C;
+        type Error = Void;
+        type Future = future::Ready<Result<Self::Output, Self::Error>>;
+
+        fn upgrade_inbound(self, stream: C, _: Self::Info) -> Self::Future {
+            future::ready(Ok(stream))
+        }
+    }
+
+    impl<C> OutboundUpgrade<C> for ManyProtocolsUpgrade {
+        type Output = C;
+        type Error = Void;
+        type Future = future::Ready<Result<Self::Output, Self::Error>>;
+
+        fn upgrade_outbound(self, stream: C, _: Self::Info) -> Self::Future {
+            future::ready(Ok(stream))
+        }
+    }
 }
 
 /// The endpoint roles associated with a pending peer-to-peer connection.

swarm/src/connection/supported_protocols.rs

@@ -0,0 +1,88 @@
+use crate::handler::ProtocolsChange;
+use crate::StreamProtocol;
+use std::collections::HashSet;
+
+#[derive(Default, Clone, Debug)]
+pub struct SupportedProtocols {
+    protocols: HashSet<StreamProtocol>,
+}
+
+impl SupportedProtocols {
+    pub fn on_protocols_change(&mut self, change: ProtocolsChange) -> bool {
+        match change {
+            ProtocolsChange::Added(added) => {
+                let mut changed = false;
+
+                for p in added {
+                    changed |= self.protocols.insert(p.clone());
+                }
+
+                changed
+            }
+            ProtocolsChange::Removed(removed) => {
+                let mut changed = false;
+
+                for p in removed {
+                    changed |= self.protocols.remove(p);
+                }
+
+                changed
+            }
+        }
+    }
+
+    pub fn iter(&self) -> impl Iterator<Item = &StreamProtocol> {
+        self.protocols.iter()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::handler::{ProtocolsAdded, ProtocolsRemoved};
+    use once_cell::sync::Lazy;
+
+    #[test]
+    fn protocols_change_added_returns_correct_changed_value() {
+        let mut protocols = SupportedProtocols::default();
+
+        let changed = protocols.on_protocols_change(add_foo());
+        assert!(changed);
+
+        let changed = protocols.on_protocols_change(add_foo());
+        assert!(!changed);
+
+        let changed = protocols.on_protocols_change(add_foo_bar());
+        assert!(changed);
+    }
+
+    #[test]
+    fn protocols_change_removed_returns_correct_changed_value() {
+        let mut protocols = SupportedProtocols::default();
+
+        let changed = protocols.on_protocols_change(remove_foo());
+        assert!(!changed);
+
+        protocols.on_protocols_change(add_foo());
+
+        let changed = protocols.on_protocols_change(remove_foo());
+        assert!(changed);
+    }
+
+    fn add_foo() -> ProtocolsChange<'static> {
+        ProtocolsChange::Added(ProtocolsAdded::from_set(&FOO_PROTOCOLS))
+    }
+
+    fn add_foo_bar() -> ProtocolsChange<'static> {
+        ProtocolsChange::Added(ProtocolsAdded::from_set(&FOO_BAR_PROTOCOLS))
+    }
+
+    fn remove_foo() -> ProtocolsChange<'static> {
+        ProtocolsChange::Removed(ProtocolsRemoved::from_set(&FOO_PROTOCOLS))
+    }
+
+    static FOO_PROTOCOLS: Lazy<HashSet<StreamProtocol>> =
+        Lazy::new(|| HashSet::from([StreamProtocol::new("/foo")]));
+    static FOO_BAR_PROTOCOLS: Lazy<HashSet<StreamProtocol>> =
+        Lazy::new(|| HashSet::from([StreamProtocol::new("/foo"), StreamProtocol::new("/bar")]));
+}

swarm/src/dummy.rs

@@ -138,7 +138,10 @@ impl crate::handler::ConnectionHandler for ConnectionHandler {
                     unreachable!("Denied upgrade does not support any protocols")
                 }
             },
-            ConnectionEvent::AddressChange(_) | ConnectionEvent::ListenUpgradeError(_) => {}
+            ConnectionEvent::AddressChange(_)
+            | ConnectionEvent::ListenUpgradeError(_)
+            | ConnectionEvent::LocalProtocolsChange(_)
+            | ConnectionEvent::RemoteProtocolsChange(_) => {}
         }
     }
 }

swarm/src/handler.rs

@@ -47,17 +47,24 @@ mod pending;
 mod select;
 
 pub use crate::upgrade::{InboundUpgradeSend, OutboundUpgradeSend, SendWrapper, UpgradeInfoSend};
-
-use instant::Instant;
-use libp2p_core::Multiaddr;
-use std::{cmp::Ordering, error, fmt, io, task::Context, task::Poll, time::Duration};
-
 pub use map_in::MapInEvent;
 pub use map_out::MapOutEvent;
 pub use one_shot::{OneShotHandler, OneShotHandlerConfig};
 pub use pending::PendingConnectionHandler;
 pub use select::ConnectionHandlerSelect;
 
+use crate::StreamProtocol;
+use ::either::Either;
+use instant::Instant;
+use libp2p_core::Multiaddr;
+use once_cell::sync::Lazy;
+use smallvec::SmallVec;
+use std::collections::hash_map::RandomState;
+use std::collections::hash_set::{Difference, Intersection};
+use std::collections::HashSet;
+use std::iter::Peekable;
+use std::{cmp::Ordering, error, fmt, io, task::Context, task::Poll, time::Duration};
+
 /// A handler for a set of protocols used on a connection with a remote.
 ///
 /// This trait should be implemented for a type that maintains the state for
@@ -209,6 +216,10 @@ pub enum ConnectionEvent<'a, IP: InboundUpgradeSend, OP: OutboundUpgradeSend, IO
     DialUpgradeError(DialUpgradeError<OOI, OP>),
     /// Informs the handler that upgrading an inbound substream to the given protocol has failed.
     ListenUpgradeError(ListenUpgradeError<IOI, IP>),
+    /// The local [`ConnectionHandler`] added or removed support for one or more protocols.
+    LocalProtocolsChange(ProtocolsChange<'a>),
+    /// The remote [`ConnectionHandler`] now supports a different set of protocols.
+    RemoteProtocolsChange(ProtocolsChange<'a>),
 }
 
 impl<'a, IP: InboundUpgradeSend, OP: OutboundUpgradeSend, IOI, OOI>
@@ -222,6 +233,8 @@ impl<'a, IP: InboundUpgradeSend, OP: OutboundUpgradeSend, IOI, OOI>
             }
             ConnectionEvent::FullyNegotiatedInbound(_)
             | ConnectionEvent::AddressChange(_)
+            | ConnectionEvent::LocalProtocolsChange(_)
+            | ConnectionEvent::RemoteProtocolsChange(_)
             | ConnectionEvent::ListenUpgradeError(_) => false,
         }
     }
@@ -234,6 +247,8 @@ impl<'a, IP: InboundUpgradeSend, OP: OutboundUpgradeSend, IOI, OOI>
             }
             ConnectionEvent::FullyNegotiatedOutbound(_)
             | ConnectionEvent::AddressChange(_)
+            | ConnectionEvent::LocalProtocolsChange(_)
+            | ConnectionEvent::RemoteProtocolsChange(_)
             | ConnectionEvent::DialUpgradeError(_) => false,
         }
     }
@@ -266,6 +281,122 @@ pub struct AddressChange<'a> {
     pub new_address: &'a Multiaddr,
 }
 
+/// [`ConnectionEvent`] variant that informs the handler about a change in the protocols supported on the connection.
+#[derive(Clone)]
+pub enum ProtocolsChange<'a> {
+    Added(ProtocolsAdded<'a>),
+    Removed(ProtocolsRemoved<'a>),
+}
+
+impl<'a> ProtocolsChange<'a> {
+    /// Compute the [`ProtocolsChange`] that results from adding `to_add` to `existing_protocols`.
+    ///
+    /// Returns `None` if the change is a no-op, i.e. `to_add` is a subset of `existing_protocols`.
+    pub(crate) fn add(
+        existing_protocols: &'a HashSet<StreamProtocol>,
+        to_add: &'a HashSet<StreamProtocol>,
+    ) -> Option<Self> {
+        let mut actually_added_protocols = to_add.difference(existing_protocols).peekable();
+
+        actually_added_protocols.peek()?;
+
+        Some(ProtocolsChange::Added(ProtocolsAdded {
+            protocols: actually_added_protocols,
+        }))
+    }
+
+    /// Compute the [`ProtocolsChange`] that results from removing `to_remove` from `existing_protocols`.
+    ///
+    /// Returns `None` if the change is a no-op, i.e. none of the protocols in `to_remove` are in `existing_protocols`.
+    pub(crate) fn remove(
+        existing_protocols: &'a HashSet<StreamProtocol>,
+        to_remove: &'a HashSet<StreamProtocol>,
+    ) -> Option<Self> {
+        let mut actually_removed_protocols = existing_protocols.intersection(to_remove).peekable();
+
+        actually_removed_protocols.peek()?;
+
+        Some(ProtocolsChange::Removed(ProtocolsRemoved {
+            protocols: Either::Right(actually_removed_protocols),
+        }))
+    }
+
+    /// Compute the [`ProtocolsChange`]s required to go from `existing_protocols` to `new_protocols`.
+    pub(crate) fn from_full_sets(
+        existing_protocols: &'a HashSet<StreamProtocol>,
+        new_protocols: &'a HashSet<StreamProtocol>,
+    ) -> SmallVec<[Self; 2]> {
+        if existing_protocols == new_protocols {
+            return SmallVec::new();
+        }
+
+        let mut changes = SmallVec::new();
+
+        let mut added_protocols = new_protocols.difference(existing_protocols).peekable();
+        let mut removed_protocols = existing_protocols.difference(new_protocols).peekable();
+
+        if added_protocols.peek().is_some() {
+            changes.push(ProtocolsChange::Added(ProtocolsAdded {
+                protocols: added_protocols,
+            }));
+        }
+
+        if removed_protocols.peek().is_some() {
+            changes.push(ProtocolsChange::Removed(ProtocolsRemoved {
+                protocols: Either::Left(removed_protocols),
+            }));
+        }
+
+        changes
+    }
+}
+
+/// An [`Iterator`] over all protocols that have been added.
+#[derive(Clone)]
+pub struct ProtocolsAdded<'a> {
+    protocols: Peekable<Difference<'a, StreamProtocol, RandomState>>,
+}
+
+impl<'a> ProtocolsAdded<'a> {
+    pub(crate) fn from_set(protocols: &'a HashSet<StreamProtocol, RandomState>) -> Self {
+        ProtocolsAdded {
+            protocols: protocols.difference(&EMPTY_HASHSET).peekable(),
+        }
+    }
+}
+
+/// An [`Iterator`] over all protocols that have been removed.
+#[derive(Clone)]
+pub struct ProtocolsRemoved<'a> {
+    protocols: Either<
+        Peekable<Difference<'a, StreamProtocol, RandomState>>,
+        Peekable<Intersection<'a, StreamProtocol, RandomState>>,
+    >,
+}
+
+impl<'a> ProtocolsRemoved<'a> {
+    #[cfg(test)]
+    pub(crate) fn from_set(protocols: &'a HashSet<StreamProtocol, RandomState>) -> Self {
+        ProtocolsRemoved {
+            protocols: Either::Left(protocols.difference(&EMPTY_HASHSET).peekable()),
+        }
+    }
+}
+
+impl<'a> Iterator for ProtocolsAdded<'a> {
+    type Item = &'a StreamProtocol;
+    fn next(&mut self) -> Option<Self::Item> {
+        self.protocols.next()
+    }
+}
+
+impl<'a> Iterator for ProtocolsRemoved<'a> {
+    type Item = &'a StreamProtocol;
+    fn next(&mut self) -> Option<Self::Item> {
+        self.protocols.next()
+    }
+}
+
 /// [`ConnectionEvent`] variant that informs the handler
 /// that upgrading an outbound substream to the given protocol has failed.
 pub struct DialUpgradeError<OOI, OP: OutboundUpgradeSend> {
@@ -357,7 +488,7 @@ impl<TUpgrade, TInfo> SubstreamProtocol<TUpgrade, TInfo> {
 }
 
 /// Event produced by a handler.
-#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq, Eq)]
 pub enum ConnectionHandlerEvent<TConnectionUpgrade, TOutboundOpenInfo, TCustom, TErr> {
     /// Request a new outbound substream to be opened with the remote.
     OutboundSubstreamRequest {
@@ -374,11 +505,21 @@ pub enum ConnectionHandlerEvent<TConnectionUpgrade, TOutboundOpenInfo, TCustom,
     /// the connection, return [`KeepAlive::No`] in
     /// [`ConnectionHandler::connection_keep_alive`].
     Close(TErr),
+    /// We learned something about the protocols supported by the remote.
+    ReportRemoteProtocols(ProtocolSupport),
 
     /// Other event.
     Custom(TCustom),
 }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum ProtocolSupport {
+    /// The remote now supports these additional protocols.
+    Added(HashSet<StreamProtocol>),
+    /// The remote no longer supports these protocols.
+    Removed(HashSet<StreamProtocol>),
+}
+
 /// Event produced by a handler.
 impl<TConnectionUpgrade, TOutboundOpenInfo, TCustom, TErr>
     ConnectionHandlerEvent<TConnectionUpgrade, TOutboundOpenInfo, TCustom, TErr>
@@ -400,6 +541,9 @@ impl<TConnectionUpgrade, TOutboundOpenInfo, TCustom, TErr>
             }
             ConnectionHandlerEvent::Custom(val) => ConnectionHandlerEvent::Custom(val),
             ConnectionHandlerEvent::Close(val) => ConnectionHandlerEvent::Close(val),
+            ConnectionHandlerEvent::ReportRemoteProtocols(support) => {
+                ConnectionHandlerEvent::ReportRemoteProtocols(support)
+            }
         }
     }
 
@@ -420,6 +564,9 @@ impl<TConnectionUpgrade, TOutboundOpenInfo, TCustom, TErr>
             }
             ConnectionHandlerEvent::Custom(val) => ConnectionHandlerEvent::Custom(val),
             ConnectionHandlerEvent::Close(val) => ConnectionHandlerEvent::Close(val),
+            ConnectionHandlerEvent::ReportRemoteProtocols(support) => {
+                ConnectionHandlerEvent::ReportRemoteProtocols(support)
+            }
         }
     }
 
@@ -437,6 +584,9 @@ impl<TConnectionUpgrade, TOutboundOpenInfo, TCustom, TErr>
             }
             ConnectionHandlerEvent::Custom(val) => ConnectionHandlerEvent::Custom(map(val)),
             ConnectionHandlerEvent::Close(val) => ConnectionHandlerEvent::Close(val),
+            ConnectionHandlerEvent::ReportRemoteProtocols(support) => {
+                ConnectionHandlerEvent::ReportRemoteProtocols(support)
+            }
         }
     }
 
@@ -454,6 +604,9 @@ impl<TConnectionUpgrade, TOutboundOpenInfo, TCustom, TErr>
             }
             ConnectionHandlerEvent::Custom(val) => ConnectionHandlerEvent::Custom(val),
             ConnectionHandlerEvent::Close(val) => ConnectionHandlerEvent::Close(map(val)),
+            ConnectionHandlerEvent::ReportRemoteProtocols(support) => {
+                ConnectionHandlerEvent::ReportRemoteProtocols(support)
+            }
         }
     }
 }
@@ -558,3 +711,7 @@ impl Ord for KeepAlive {
         }
     }
 }
+
+/// A statically declared, empty [`HashSet`] allows us to work around borrow-checker rules for
+/// [`ProtocolsAdded::from_set`]. The lifetimes don't work unless we have a [`HashSet`] with a `'static' lifetime.
+static EMPTY_HASHSET: Lazy<HashSet<StreamProtocol>> = Lazy::new(HashSet::new);

swarm/src/handler/either.rs

@@ -208,6 +208,22 @@ where
                     handler.on_connection_event(ConnectionEvent::AddressChange(address_change))
                 }
             },
+            ConnectionEvent::LocalProtocolsChange(supported_protocols) => match self {
+                Either::Left(handler) => handler.on_connection_event(
+                    ConnectionEvent::LocalProtocolsChange(supported_protocols),
+                ),
+                Either::Right(handler) => handler.on_connection_event(
+                    ConnectionEvent::LocalProtocolsChange(supported_protocols),
+                ),
+            },
+            ConnectionEvent::RemoteProtocolsChange(supported_protocols) => match self {
+                Either::Left(handler) => handler.on_connection_event(
+                    ConnectionEvent::RemoteProtocolsChange(supported_protocols),
+                ),
+                Either::Right(handler) => handler.on_connection_event(
+                    ConnectionEvent::RemoteProtocolsChange(supported_protocols),
+                ),
+            },
         }
     }
 }

swarm/src/handler/map_out.rs

@@ -81,6 +81,9 @@ where
             ConnectionHandlerEvent::OutboundSubstreamRequest { protocol } => {
                 ConnectionHandlerEvent::OutboundSubstreamRequest { protocol }
             }
+            ConnectionHandlerEvent::ReportRemoteProtocols(support) => {
+                ConnectionHandlerEvent::ReportRemoteProtocols(support)
+            }
         })
     }
 

swarm/src/handler/multi.rs

@@ -205,6 +205,20 @@ where
             ConnectionEvent::ListenUpgradeError(listen_upgrade_error) => {
                 self.on_listen_upgrade_error(listen_upgrade_error)
             }
+            ConnectionEvent::LocalProtocolsChange(supported_protocols) => {
+                for h in self.handlers.values_mut() {
+                    h.on_connection_event(ConnectionEvent::LocalProtocolsChange(
+                        supported_protocols.clone(),
+                    ));
+                }
+            }
+            ConnectionEvent::RemoteProtocolsChange(supported_protocols) => {
+                for h in self.handlers.values_mut() {
+                    h.on_connection_event(ConnectionEvent::RemoteProtocolsChange(
+                        supported_protocols.clone(),
+                    ));
+                }
+            }
         }
     }
 

swarm/src/handler/one_shot.rs

@@ -217,7 +217,10 @@ where
                     self.keep_alive = KeepAlive::No;
                 }
             }
-            ConnectionEvent::AddressChange(_) | ConnectionEvent::ListenUpgradeError(_) => {}
+            ConnectionEvent::AddressChange(_)
+            | ConnectionEvent::ListenUpgradeError(_)
+            | ConnectionEvent::LocalProtocolsChange(_)
+            | ConnectionEvent::RemoteProtocolsChange(_) => {}
         }
     }
 }

swarm/src/handler/pending.rs

@@ -99,7 +99,9 @@ impl ConnectionHandler for PendingConnectionHandler {
             }
             ConnectionEvent::AddressChange(_)
             | ConnectionEvent::DialUpgradeError(_)
-            | ConnectionEvent::ListenUpgradeError(_) => {}
+            | ConnectionEvent::ListenUpgradeError(_)
+            | ConnectionEvent::LocalProtocolsChange(_)
+            | ConnectionEvent::RemoteProtocolsChange(_) => {}
         }
     }
 }

swarm/src/handler/select.rs

@@ -240,6 +240,9 @@ where
                         .map_info(Either::Left),
                 });
             }
+            Poll::Ready(ConnectionHandlerEvent::ReportRemoteProtocols(support)) => {
+                return Poll::Ready(ConnectionHandlerEvent::ReportRemoteProtocols(support));
+            }
             Poll::Pending => (),
         };
 
@@ -257,6 +260,9 @@ where
                         .map_info(Either::Right),
                 });
             }
+            Poll::Ready(ConnectionHandlerEvent::ReportRemoteProtocols(support)) => {
+                return Poll::Ready(ConnectionHandlerEvent::ReportRemoteProtocols(support));
+            }
             Poll::Pending => (),
         };
 
@@ -317,6 +323,26 @@ where
             ConnectionEvent::ListenUpgradeError(listen_upgrade_error) => {
                 self.on_listen_upgrade_error(listen_upgrade_error)
             }
+            ConnectionEvent::LocalProtocolsChange(supported_protocols) => {
+                self.proto1
+                    .on_connection_event(ConnectionEvent::LocalProtocolsChange(
+                        supported_protocols.clone(),
+                    ));
+                self.proto2
+                    .on_connection_event(ConnectionEvent::LocalProtocolsChange(
+                        supported_protocols,
+                    ));
+            }
+            ConnectionEvent::RemoteProtocolsChange(supported_protocols) => {
+                self.proto1
+                    .on_connection_event(ConnectionEvent::RemoteProtocolsChange(
+                        supported_protocols.clone(),
+                    ));
+                self.proto2
+                    .on_connection_event(ConnectionEvent::RemoteProtocolsChange(
+                        supported_protocols,
+                    ));
+            }
         }
     }
 }

swarm/src/keep_alive.rs

@@ -136,7 +136,9 @@ impl crate::handler::ConnectionHandler for ConnectionHandler {
             }) => void::unreachable(protocol),
             ConnectionEvent::DialUpgradeError(_)
             | ConnectionEvent::ListenUpgradeError(_)
-            | ConnectionEvent::AddressChange(_) => {}
+            | ConnectionEvent::AddressChange(_)
+            | ConnectionEvent::LocalProtocolsChange(_)
+            | ConnectionEvent::RemoteProtocolsChange(_) => {}
         }
     }
 }

swarm/src/lib.rs

@@ -116,7 +116,7 @@ pub use behaviour::{
     PollParameters, ToSwarm,
 };
 pub use connection::pool::ConnectionCounters;
-pub use connection::{ConnectionError, ConnectionId};
+pub use connection::{ConnectionError, ConnectionId, SupportedProtocols};
 pub use executor::Executor;
 pub use handler::{
     ConnectionHandler, ConnectionHandlerEvent, ConnectionHandlerSelect, KeepAlive, OneShotHandler,