fix(rumqttc): fix possible starvation with pending requests

Store the pending throttle interval within the EventLoop.

Fixes: bytebeamio#814

rumqttc/src/v5/eventloop.rs

@@ -5,12 +5,14 @@ use crate::eventloop::socket_connect;
 use crate::framed::AsyncReadWrite;
 
 use flume::Receiver;
+use futures_util::{Stream, StreamExt};
 use tokio::select;
 use tokio::time::{self, error::Elapsed, Instant, Sleep};
 
 use std::collections::VecDeque;
 use std::io;
 use std::pin::Pin;
+use std::task::{Context, Poll};
 use std::time::Duration;
 
 use super::mqttbytes::v5::ConnectReturnCode;
@@ -77,8 +79,8 @@ pub struct EventLoop {
     pub state: MqttState,
     /// Request stream
     request_rx: Receiver<Request>,
-    /// Pending packets from last session
-    pub pending: VecDeque<Request>,
+    /// Pending requests from the last session
+    pending: PendingRequests,
     /// Network connection to the broker
     network: Option<Network>,
     /// Keep alive time
@@ -98,9 +100,9 @@ impl EventLoop {
     /// When connection encounters critical errors (like auth failure), user has a choice to
     /// access and update `options`, `state` and `requests`.
     pub fn new(options: MqttOptions, request_rx: Receiver<Request>) -> EventLoop {
-        let pending = VecDeque::new();
         let inflight_limit = options.outgoing_inflight_upper_limit.unwrap_or(u16::MAX);
         let manual_acks = options.manual_acks;
+        let pending = PendingRequests::new(options.pending_throttle);
 
         EventLoop {
             options,
@@ -161,8 +163,6 @@ 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 await_acks = self.state.await_acks;
-
         let inflight_full = self.state.inflight >= self.state.max_outgoing_inflight;
         let collision = self.state.collision.is_some();
 
@@ -202,17 +202,16 @@ impl EventLoop {
             // After collision with pkid 1        -> [1b ,2, x, 4, 5].
             // 1a is saved to state and event loop is set to collision mode stopping new
             // outgoing requests (along with 1b).
-            o = Self::next_request(
-                &mut self.pending,
-                &self.request_rx,
-                self.options.pending_throttle
-            ), if !self.pending.is_empty() || (!inflight_full && !collision) => match o {
-                Ok(request) => {
-                    self.state.handle_outgoing_packet(request)?;
-                    network.flush().await?;
-                    Ok(self.state.events.pop_front().unwrap())
-                }
-                Err(_) => Err(ConnectionError::RequestsDone),
+            Some(request) = self.pending.next(), if !inflight_full && !collision => {
+                self.state.handle_outgoing_packet(request)?;
+                network.flush().await?;
+                Ok(self.state.events.pop_front().unwrap())
+            },
+            request = self.request_rx.recv_async(), if self.pending.is_empty() && !inflight_full && !collision => {
+                let request = request.map_err(|_| ConnectionError::RequestsDone)?;
+                self.state.handle_outgoing_packet(request)?;
+                network.flush().await?;
+                Ok(self.state.events.pop_front().unwrap())
             },
             // Pull a bunch of packets from network, reply in bunch and yield the first item
             o = network.readb(&mut self.state, BATCH_SIZE) => {
@@ -231,26 +230,57 @@ impl EventLoop {
                 network.flush().await?;
                 Ok(self.state.events.pop_front().unwrap())
             }
+            else => unreachable!("Eventloop select is exhaustive"),
         }
     }
+}
 
-    async fn next_request(
-        pending: &mut VecDeque<Request>,
-        rx: &Receiver<Request>,
-        pending_throttle: Duration,
-    ) -> Result<Request, ConnectionError> {
-        if !pending.is_empty() {
-            time::sleep(pending_throttle).await;
-            // We must call .next() AFTER sleep() otherwise .next() would
-            // advance the iterator but the future might be canceled before return
-            Ok(pending.pop_front().unwrap())
+/// Pending requets yielded with a configured rate. If the queue is empty the stream will yield pending.
+struct PendingRequests {
+    /// Interval
+    interval: Option<time::Interval>,
+    /// Pending requests
+    requests: VecDeque<Request>,
+}
+
+impl PendingRequests {
+    pub fn new(interval: Duration) -> Self {
+        let interval = (!interval.is_zero()).then(|| time::interval(interval));
+        PendingRequests {
+            interval,
+            requests: VecDeque::new(),
+        }
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.requests.is_empty()
+    }
+
+    pub fn extend(&mut self, requests: impl IntoIterator<Item = Request>) {
+        self.requests.extend(requests);
+    }
+}
+
+impl Stream for PendingRequests {
+    type Item = Request;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Request>> {
+        if self.is_empty() {
+            Poll::Pending
         } else {
-            match rx.recv_async().await {
-                Ok(r) => Ok(r),
-                Err(_) => Err(ConnectionError::RequestsDone),
+            match self.interval.as_mut() {
+                Some(interval) => match interval.poll_tick(cx) {
+                    Poll::Ready(_) => Poll::Ready(self.requests.pop_front()),
+                    Poll::Pending => Poll::Pending,
+                },
+                None => Poll::Ready(self.requests.pop_front()),
             }
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.requests.len(), Some(self.requests.len()))
+    }
 }
 
 /// This stream internally processes requests from the request stream provided to the eventloop