Allow StreamMuxer to notify changes in the address (#1621)

* Allow StreamMuxer to notify changes in the address

* Fix doc link

* Revert accidental rename

* Other accidental rename

Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>

core/src/connection.rs

@@ -132,6 +132,16 @@ impl ConnectedPoint {
             ConnectedPoint::Listener { .. } => true
         }
     }
+
+    /// Modifies the address of the remote stored in this struct.
+    ///
+    /// For `Dialer`, this modifies `address`. For `Listener`, this modifies `send_back_addr`.
+    pub fn set_remote_address(&mut self, new_address: Multiaddr) {
+        match self {
+            ConnectedPoint::Dialer { address } => *address = new_address,
+            ConnectedPoint::Listener { send_back_addr, .. } => *send_back_addr = new_address,
+        }
+    }
 }
 
 /// Information about a successfully established connection.
@@ -169,6 +179,15 @@ impl ConnectionInfo for PeerId {
     }
 }
 
+/// Event generated by a [`Connection`].
+#[derive(Debug, Clone)]
+pub enum Event<T> {
+    /// Event generated by the [`ConnectionHandler`].
+    Handler(T),
+    /// Address of the remote has changed.
+    AddressChange(Multiaddr),
+}
+
 /// A multiplexed connection to a peer with an associated `ConnectionHandler`.
 pub struct Connection<TMuxer, THandler>
 where
@@ -239,7 +258,7 @@ where
     /// Polls the connection for events produced by the associated handler
     /// as a result of I/O activity on the substream multiplexer.
     pub fn poll(mut self: Pin<&mut Self>, cx: &mut Context)
-        -> Poll<Result<THandler::OutEvent, ConnectionError<THandler::Error>>>
+        -> Poll<Result<Event<THandler::OutEvent>, ConnectionError<THandler::Error>>>
     {
         loop {
             let mut io_pending = false;
@@ -255,6 +274,10 @@ where
                     let endpoint = SubstreamEndpoint::Dialer(user_data);
                     self.handler.inject_substream(substream, endpoint)
                 }
+                Poll::Ready(Ok(SubstreamEvent::AddressChange(address))) => {
+                    self.handler.inject_address_change(&address);
+                    return Poll::Ready(Ok(Event::AddressChange(address)));
+                }
                 Poll::Ready(Err(err)) => return Poll::Ready(Err(ConnectionError::IO(err))),
             }
 
@@ -269,7 +292,7 @@ where
                     self.muxing.open_substream(user_data);
                 }
                 Poll::Ready(Ok(ConnectionHandlerEvent::Custom(event))) => {
-                    return Poll::Ready(Ok(event));
+                    return Poll::Ready(Ok(Event::Handler(event)));
                 }
                 Poll::Ready(Err(err)) => return Poll::Ready(Err(ConnectionError::Handler(err))),
             }