feat(rumqttc): batch request processing before flushing the network

Collect up to BATCH_SIZE request from the client channel before flushing the network.

Fixes bytebeamio#810

rumqttc/CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- Process multiple outgoing client requests before flushing the network buffer (reduces number of system calls)
 
 * `size()` method on `Packet` calculates size once serialized.
 * `read()` and `write()` methods on `Packet`.

rumqttc/src/v5/eventloop.rs

@@ -163,8 +163,8 @@ impl EventLoop {
     /// Select on network and requests and generate keepalive pings when necessary
     async fn select(&mut self) -> Result<Event, ConnectionError> {
         let network = self.network.as_mut().unwrap();
-        let inflight_full = self.state.inflight >= self.state.max_outgoing_inflight;
-        let collision = self.state.collision.is_some();
+        let inflight_full = self.state.is_inflight_full();
+        let collision = self.state.has_collision();
 
         // Read buffered events from previous polls before calling a new poll
         if let Some(event) = self.state.events.pop_front() {
@@ -208,8 +208,38 @@ impl EventLoop {
                 Ok(self.state.events.pop_front().unwrap())
             },
             request = self.request_rx.recv_async(), if self.pending.is_empty() && !inflight_full && !collision => {
+                // Process first request
                 let request = request.map_err(|_| ConnectionError::RequestsDone)?;
                 self.state.handle_outgoing_packet(request)?;
+
+                // Take up to BATCH_SIZE - 1 requests from the channel until
+                // - the channel is empty
+                // - the inflight queue is full
+                // - there is a collision
+                // If the channel is closed this is reported in the next iteration from the async recv above.
+                for _ in 0..(BATCH_SIZE - 1) {
+                    if self.request_rx.is_empty() || self.state.is_inflight_full() || self.state.has_collision()
+                    {
+                        break;
+                    }
+
+                    // Safe to call the blocking `recv` in here since we know the channel is not empty.
+                    // Ensure a flush in case of any error.
+                    if let Err(e) = self
+                        .request_rx
+                        .recv()
+                        .map_err(|_| ConnectionError::RequestsDone)
+                        .and_then(|request| {
+                            self.state
+                                .handle_outgoing_packet(request)
+                                .map_err(Into::into)
+                        })
+                    {
+                        network.flush().await?;
+                        return Err(e);
+                    }
+                }
+
                 network.flush().await?;
                 Ok(self.state.events.pop_front().unwrap())
             },

rumqttc/src/v5/state.rs

@@ -185,6 +185,16 @@ impl MqttState {
         self.inflight
     }
 
+    /// Returns true if the inflight limit is reached
+    pub fn is_inflight_full(&self) -> bool {
+        self.inflight >= self.max_outgoing_inflight
+    }
+
+    /// Returns true if the state has a unresolved collision
+    pub fn has_collision(&self) -> bool {
+        self.collision.is_some()
+    }
+
     /// Consolidates handling of all outgoing mqtt packet logic. Returns a packet which should
     /// be put on to the network by the eventloop
     pub fn handle_outgoing_packet(