Report RPC Errors to the application on peer disconnections (#5680)

* Report RPC Errors to the application on peer disconnections

Co-authored-by: Age Manning <Age@AgeManning.com>

* Expect RPCError::Disconnect to fail ongoing requests

* Drop lookups after peer disconnect and not awaiting events

* Allow RPCError disconnect through network service

* Update beacon_node/lighthouse_network/src/service/mod.rs

Co-authored-by: Age Manning <Age@AgeManning.com>

* Merge branch 'unstable' into rpc-error-on-disconnect

beacon_node/lighthouse_network/src/rpc/handler.rs

@@ -352,6 +352,31 @@ where
         !matches!(self.state, HandlerState::Deactivated)
     }
 
+    // NOTE: This function gets polled to completion upon a connection close.
+    fn poll_close(&mut self, _: &mut Context<'_>) -> Poll<Option<Self::ToBehaviour>> {
+        // Inform the network behaviour of any failed requests
+
+        while let Some(substream_id) = self.outbound_substreams.keys().next().cloned() {
+            let outbound_info = self
+                .outbound_substreams
+                .remove(&substream_id)
+                .expect("The value must exist for a key");
+            // If the state of the connection is closing, we do not need to report this case to
+            // the behaviour, as the connection has just closed non-gracefully
+            if matches!(outbound_info.state, OutboundSubstreamState::Closing(_)) {
+                continue;
+            }
+
+            // Register this request as an RPC Error
+            return Poll::Ready(Some(HandlerEvent::Err(HandlerErr::Outbound {
+                error: RPCError::Disconnected,
+                proto: outbound_info.proto,
+                id: outbound_info.req_id,
+            })));
+        }
+        Poll::Ready(None)
+    }
+
     fn poll(
         &mut self,
         cx: &mut Context<'_>,