Immediately acknowledge GRPC subscriptions (#2712)

linera-core/src/notifier.rs

@@ -28,13 +28,27 @@ impl<N> Default for ChannelNotifier<N> {
 }
 
 impl<N> ChannelNotifier<N> {
-    /// Creates a subscription given a collection of ChainIds and a sender to the client.
-    pub fn subscribe(&self, chain_ids: Vec<ChainId>) -> UnboundedReceiver<N> {
-        let (tx, rx) = tokio::sync::mpsc::unbounded_channel();
+    fn add_sender(&self, chain_ids: Vec<ChainId>, sender: &UnboundedSender<N>) {
         for id in chain_ids {
             let mut senders = self.inner.entry(id).or_default();
-            senders.push(tx.clone());
+            senders.push(sender.clone());
         }
+    }
+
+    /// Creates a subscription given a collection of ChainIds and a sender to the client.
+    pub fn subscribe(&self, chain_ids: Vec<ChainId>) -> UnboundedReceiver<N> {
+        let (tx, rx) = tokio::sync::mpsc::unbounded_channel();
+        self.add_sender(chain_ids, &tx);
+        rx
+    }
+
+    /// Creates a subscription given a collection of ChainIds and a sender to the client.
+    /// Immediately posts a first notification as a ACK.
+    pub fn subscribe_with_ack(&self, chain_ids: Vec<ChainId>, ack: N) -> UnboundedReceiver<N> {
+        let (tx, rx) = tokio::sync::mpsc::unbounded_channel();
+        self.add_sender(chain_ids, &tx);
+        tx.send(ack)
+            .expect("pushing to a new channel should succeed");
         rx
     }
 }

linera-rpc/src/grpc/client.rs

@@ -17,7 +17,7 @@ use linera_core::{
 };
 use linera_version::VersionInfo;
 use tonic::{Code, IntoRequest, Request, Status};
-use tracing::{debug, error, info, instrument};
+use tracing::{debug, error, info, instrument, warn};
 #[cfg(not(web))]
 use {
     super::GrpcProtoConversionError,
@@ -259,7 +259,7 @@ impl ValidatorNode for GrpcClient {
         // terminates after unexpected or fatal errors.
         let notification_stream = endlessly_retrying_notification_stream
             .map(|result| {
-                Notification::try_from(result?).map_err(|err| {
+                Option::<Notification>::try_from(result?).map_err(|err| {
                     let message = format!("Could not deserialize notification: {}", err);
                     tonic::Status::new(Code::Internal, message)
                 })
@@ -279,7 +279,16 @@ impl ValidatorNode for GrpcClient {
                     true
                 })
             })
-            .filter_map(|result| future::ready(result.ok()));
+            .filter_map(|result| {
+                future::ready(match result {
+                    Ok(notification @ Some(_)) => notification,
+                    Ok(None) => None,
+                    Err(err) => {
+                        warn!("{}", err);
+                        None
+                    }
+                })
+            });
 
         Ok(Box::pin(notification_stream))
     }

linera-rpc/src/grpc/conversions.rs

@@ -143,14 +143,18 @@ impl TryFrom<Notification> for api::Notification {
     }
 }
 
-impl TryFrom<api::Notification> for Notification {
+impl TryFrom<api::Notification> for Option<Notification> {
     type Error = GrpcProtoConversionError;
 
     fn try_from(notification: api::Notification) -> Result<Self, Self::Error> {
-        Ok(Self {
-            chain_id: try_proto_convert(notification.chain_id)?,
-            reason: bincode::deserialize(&notification.reason)?,
-        })
+        if notification.chain_id.is_none() && notification.reason.is_empty() {
+            Ok(None)
+        } else {
+            Ok(Some(Notification {
+                chain_id: try_proto_convert(notification.chain_id)?,
+                reason: bincode::deserialize(&notification.reason)?,
+            }))
+        }
     }
 }
 
@@ -903,6 +907,13 @@ pub mod tests {
                 hash: CryptoHash::new(&Foo("".into())),
             },
         };
-        round_trip_check::<_, api::Notification>(notification);
+        let message = api::Notification::try_from(notification.clone()).unwrap();
+        assert_eq!(
+            Some(notification),
+            Option::<Notification>::try_from(message).unwrap()
+        );
+
+        let ack = api::Notification::default();
+        assert_eq!(None, Option::<Notification>::try_from(ack).unwrap());
     }
 }

linera-service/src/proxy/grpc.rs

@@ -406,7 +406,12 @@ where
             .into_iter()
             .map(ChainId::try_from)
             .collect::<Result<Vec<ChainId>, _>>()?;
-        let rx = self.0.notifier.subscribe(chain_ids);
+        // The empty notification seems to be needed in some cases to force
+        // completion of HTTP2 headers.
+        let rx = self
+            .0
+            .notifier
+            .subscribe_with_ack(chain_ids, Ok(Notification::default()));
         Ok(Response::new(UnboundedReceiverStream::new(rx)))
     }