fix(s2n-quic-core): always wake application on available datagram capacity

quic/s2n-quic-core/src/datagram/default.rs

@@ -253,6 +253,7 @@ pub struct Sender {
     capacity: usize,
     min_packet_space: usize,
     max_packet_space: usize,
+    dropped_datagrams: u64,
     smoothed_packet_size: f64,
     waker: Option<Waker>,
     max_datagram_payload: u64,
@@ -449,43 +450,64 @@ impl Sender {
     pub fn smoothed_packet_space(&self) -> usize {
         self.smoothed_packet_size as usize
     }
+
+    /// Returns the number of datagrams that have been dropped by the sender
+    ///
+    /// The cause of drops is due to the datagrams being larger than the current path MTU. If this
+    /// number is non-zero, applications should try to send smaller datagrams.
+    #[inline]
+    pub fn dropped_datagrams(&self) -> u64 {
+        self.dropped_datagrams
+    }
 }
 
 impl super::Sender for Sender {
+    #[inline]
     fn on_transmit<P: Packet>(&mut self, packet: &mut P) {
         // Cede space to stream data when datagrams are not prioritized
         if packet.has_pending_streams() && !packet.datagrams_prioritized() {
             return;
         }
+
         self.record_capacity_stats(packet.remaining_capacity());
+
         let mut has_written = false;
+
         while packet.remaining_capacity() > 0 {
-            if let Some(datagram) = self.queue.pop_front() {
-                // Ensure there is enough space in the packet to send a datagram
-                if packet.remaining_capacity() >= datagram.data.len() {
-                    match packet.write_datagram(&datagram.data) {
-                        Ok(()) => has_written = true,
-                        Err(_error) => {
-                            continue;
-                        }
-                    }
-                    // Since a datagram was popped off the queue, wake the
-                    // stored waker if we have one to let the application know
-                    // that there is space on the queue for more datagrams.
-                    if let Some(w) = self.waker.take() {
-                        w.wake();
-                    }
-                } else {
-                    // This check keeps us from popping all the datagrams off the
-                    // queue when packet space remaining is smaller than the datagram.
-                    if has_written {
-                        self.queue.push_front(datagram);
-                        return;
-                    }
+            let Some(datagram) = self.queue.pop_front() else {
+                break;
+            };
+
+            // Ensure there is enough space in the packet to send a datagram
+            if packet.remaining_capacity() < datagram.data.len() {
+                // This check keeps us from popping all the datagrams off the
+                // queue when packet space remaining is smaller than the datagram.
+                if has_written {
+                    self.queue.push_front(datagram);
+                    break;
+                }
+
+                // the datagram is too large for the current packet and unlikely to ever fit so
+                // record a metric and try the next datagram in the queue
+                self.dropped_datagrams += 1;
+                continue;
+            }
+
+            match packet.write_datagram(&datagram.data) {
+                Ok(()) => has_written = true,
+                Err(_error) => {
+                    // TODO log this
+                    self.dropped_datagrams += 1;
+                    continue;
                 }
-            } else {
-                // If there are no datagrams on the queue we return
-                return;
+            }
+        }
+
+        // If we now have additional capacity wake the stored waker if we have one to
+        // let the application know that there is space on the queue for more datagrams.
+        if self.capacity > self.queue.len() {
+            if let Some(w) = self.waker.take() {
+                w.wake();
             }
         }
     }
@@ -540,6 +562,7 @@ impl SenderBuilder {
             queue: VecDeque::with_capacity(self.queue_capacity),
             capacity: self.queue_capacity,
             max_datagram_payload: self.max_datagram_payload,
+            dropped_datagrams: 0,
             max_packet_space: 0,
             min_packet_space: 0,
             smoothed_packet_size: 0.0,
@@ -780,6 +803,50 @@ mod tests {
         assert!(!default_sender.queue.is_empty());
     }
 
+    /// Ensures the application waker is called when capacity becomes available
+    #[test]
+    fn wake_with_capacity() {
+        let (waker, wake_count) = new_count_waker();
+        let mut cx = Context::from_waker(&waker);
+
+        let conn_info = ConnectionInfo::new(100, waker.clone());
+
+        let mut default_sender = Sender::builder()
+            .with_capacity(1)
+            .with_connection_info(&conn_info)
+            .build()
+            .unwrap();
+
+        let datagram = bytes::Bytes::from_static(&[1, 2, 3]);
+
+        assert!(default_sender
+            .poll_send_datagram(&mut datagram.clone(), &mut cx)
+            .is_ready());
+        assert!(default_sender
+            .poll_send_datagram(&mut datagram.clone(), &mut cx)
+            .is_pending());
+
+        assert_eq!(wake_count.get(), 0);
+
+        // Packet size is just enough to write the first datagram with some
+        // room left over, but not enough to write the second.
+        let mut packet = MockPacket {
+            remaining_capacity: 2,
+            has_pending_streams: false,
+            datagrams_prioritized: false,
+        };
+        crate::datagram::Sender::on_transmit(&mut default_sender, &mut packet);
+
+        // Packet capacity has not changed
+        assert_eq!(packet.remaining_capacity, 2);
+        // Send queue is completely depleted
+        assert!(default_sender.queue.is_empty());
+        // The waker was called since we now have capacity
+        assert_eq!(wake_count.get(), 1);
+        // The sender should record the number of dropped datagrams
+        assert_eq!(default_sender.dropped_datagrams(), 1);
+    }
+
     fn fake_receive_context() -> crate::datagram::ReceiveContext<'static> {
         crate::datagram::ReceiveContext {
             path: crate::event::api::Path {