Preserve runtime effects on send failure

API.md

@@ -58,7 +58,7 @@ pub trait Transport {
     fn send_frame(
         &mut self,
         connection: ConnectionId,
-        frame: FrameBytes,
+        frame: &FrameBytes,
     ) -> Result<(), Self::Error>;
 
     fn flush(&mut self) -> Result<(), Self::Error> {

unshell-runtime/src/node/runtime.rs

@@ -268,7 +268,9 @@ where
     fn flush_outbound(&mut self) -> Result<usize, NodeRuntimeError<T::Error>> {
         let mut retained = EffectQueue::new();
         let mut sent = 0usize;
-        for effect in self.effects.drain() {
+        let mut pending = core::mem::take(&mut self.effects);
+        let mut drained = pending.drain();
+        while let Some(effect) = drained.next() {
             match effect {
                 RuntimeEffect::SendFrame {
                     connection,
@@ -279,9 +281,18 @@ where
                     .registered(connection)
                     .is_some_and(|registered| registered.generation() == generation) =>
                 {
-                    self.transport
-                        .send_frame(connection, frame)
-                        .map_err(NodeRuntimeError::Transport)?;
+                    if let Err(error) = self.transport.send_frame(connection, &frame) {
+                        retained.push(RuntimeEffect::SendFrame {
+                            connection,
+                            generation,
+                            frame,
+                        });
+                        for remaining in drained {
+                            retained.push(remaining);
+                        }
+                        self.effects = retained;
+                        return Err(NodeRuntimeError::Transport(error));
+                    }
                     sent += 1;
                 }
                 RuntimeEffect::SendFrame { .. } => {}
@@ -316,16 +327,20 @@ mod tests {
     use unshell_protocol::tree::{ChildRoute, ProtocolEndpoint};
     use unshell_protocol::{CallMessage, FrameBytes, PacketHeader, PacketType, encode_packet};
 
-    use super::{EndpointState, NodeRuntime, TickBudget};
+    use super::{EndpointState, NodeRuntime, NodeRuntimeError, TickBudget};
 
     #[derive(Debug, Default)]
     struct RecordingTransport {
         inbound: Option<(ConnectionId, FrameBytes)>,
         sent: Vec<(ConnectionId, FrameBytes)>,
+        fail_send: bool,
     }
 
+    #[derive(Debug, Clone, Copy, Eq, PartialEq)]
+    struct SendError;
+
     impl Transport for RecordingTransport {
-        type Error = core::convert::Infallible;
+        type Error = SendError;
 
         fn poll_recv(&mut self) -> Result<Option<(ConnectionId, FrameBytes)>, Self::Error> {
             Ok(self.inbound.take())
@@ -334,9 +349,12 @@ mod tests {
         fn send_frame(
             &mut self,
             connection: ConnectionId,
-            frame: FrameBytes,
+            frame: &FrameBytes,
         ) -> Result<(), Self::Error> {
-            self.sent.push((connection, frame));
+            if self.fail_send {
+                return Err(SendError);
+            }
+            self.sent.push((connection, frame.clone()));
             Ok(())
         }
     }
@@ -388,6 +406,7 @@ mod tests {
         let transport = RecordingTransport {
             inbound: Some((parent, frame)),
             sent: Vec::new(),
+            fail_send: false,
         };
         let mut runtime = NodeRuntime::new(EndpointState::new(endpoint), connections, transport);
 
@@ -456,6 +475,7 @@ mod tests {
         let transport = RecordingTransport {
             inbound: Some((parent, frame)),
             sent: Vec::new(),
+            fail_send: false,
         };
         let mut runtime = NodeRuntime::new(EndpointState::new(endpoint), connections, transport);
 
@@ -509,6 +529,78 @@ mod tests {
         assert!(matches!(runtime.effects()[0], RuntimeEffect::Local(_)));
     }
 
+    #[test]
+    fn failed_send_preserves_failed_and_unprocessed_effects() {
+        let parent = ConnectionId::new(1);
+        let mut connections = Connections::new();
+        connections.push(Connection::registered(
+            parent,
+            ConnectionDirection::Parent,
+            vec![],
+            ConnectionGeneration::INITIAL,
+        ));
+
+        let mut endpoint =
+            ProtocolEndpoint::new(vec![String::from("agent")], Some(vec![]), vec![], vec![]);
+        endpoint
+            .add_endpoint_procedure("org.example.v1.echo.invoke")
+            .expect("procedure registers");
+        let frame = encode_packet(
+            &PacketHeader {
+                packet_type: PacketType::Call,
+                src_path: vec![],
+                dst_path: vec![String::from("agent")],
+                dst_leaf: None,
+                hook_id: None,
+            },
+            &CallMessage {
+                procedure_id: String::from("org.example.v1.echo.invoke"),
+                data: vec![],
+                response_hook: None,
+            },
+        )
+        .expect("frame encodes");
+
+        let mut runtime = NodeRuntime::new(
+            EndpointState::new(endpoint),
+            connections,
+            RecordingTransport {
+                inbound: None,
+                sent: Vec::new(),
+                fail_send: true,
+            },
+        );
+
+        runtime.effects.push(RuntimeEffect::SendFrame {
+            connection: parent,
+            generation: ConnectionGeneration::INITIAL,
+            frame: frame.clone(),
+        });
+        runtime
+            .receive_frame(parent, frame.clone())
+            .expect("local frame processes");
+        runtime.effects.push(RuntimeEffect::SendFrame {
+            connection: parent,
+            generation: ConnectionGeneration::INITIAL,
+            frame,
+        });
+
+        let error = runtime.flush_outbound().expect_err("send fails");
+
+        assert!(matches!(error, NodeRuntimeError::Transport(SendError)));
+        assert!(runtime.transport().sent.is_empty());
+        assert_eq!(runtime.effects().len(), 3);
+        assert!(matches!(
+            runtime.effects()[0],
+            RuntimeEffect::SendFrame { .. }
+        ));
+        assert!(matches!(runtime.effects()[1], RuntimeEffect::Local(_)));
+        assert!(matches!(
+            runtime.effects()[2],
+            RuntimeEffect::SendFrame { .. }
+        ));
+    }
+
     #[test]
     fn tick_counts_only_new_local_events() {
         let parent = ConnectionId::new(1);
@@ -544,6 +636,7 @@ mod tests {
         let transport = RecordingTransport {
             inbound: Some((parent, frame)),
             sent: Vec::new(),
+            fail_send: false,
         };
         let mut runtime = NodeRuntime::new(EndpointState::new(endpoint), connections, transport);
 
@@ -591,6 +684,7 @@ mod tests {
         let transport = RecordingTransport {
             inbound: Some((child, frame)),
             sent: Vec::new(),
+            fail_send: false,
         };
         let mut runtime = NodeRuntime::new(EndpointState::new(endpoint), connections, transport);
 

unshell-runtime/src/transport.rs

@@ -21,7 +21,7 @@ pub trait Transport {
     fn send_frame(
         &mut self,
         connection: ConnectionId,
-        frame: FrameBytes,
+        frame: &FrameBytes,
     ) -> Result<(), Self::Error>;
 
     /// Flushes buffered outbound transport data, if the transport has any.