feat(transport/quic_datagrams): expose send queue capacity

neqo-http3/src/connection_client.rs

@@ -820,6 +820,8 @@
 
     /// Send `WebTransport` datagram.
     ///
+    /// Consider calling [`Http3Client::datagram_send_queue_capacity`] before.
+    ///
     /// # Errors
     ///
     /// It may return `InvalidStreamId` if a stream does not exist anymore.
@@ -838,6 +840,8 @@
 
     /// Send `ConnectUdp` datagram.
     ///
+    /// Consider calling [`Http3Client::datagram_send_queue_capacity`] before.
+    ///
     /// # Errors
     ///
     /// It may return `InvalidStreamId` if a stream does not exist anymore.
@@ -854,6 +858,13 @@
             .connect_udp_send_datagram(session_id, &mut self.conn, buf, id)
     }
 
+    /// Returns the number of QUIC datagrams that can still be queued for
+    /// sending before the queue is full.
+    #[must_use]
+    pub fn datagram_send_queue_capacity(&self) -> usize {
+        self.conn.datagram_send_queue_capacity()
+    }
+
     /// Returns the current max size of a datagram that can fit into a packet.
     /// The value will change over time depending on the encoded size of the
     /// packet number, ack frames, etc.

neqo-transport/src/connection/mod.rs

@@ -3868,6 +3868,13 @@
             .add_datagram(buf, id.into(), &mut self.stats.borrow_mut())
     }
 
+    /// Returns the number of QUIC datagrams that can still be queued for
+    /// sending before the queue is full.
+    #[must_use]
+    pub fn datagram_send_queue_capacity(&self) -> usize {
+        self.quic_datagrams.send_queue_capacity()
+    }
+
     /// Return the PLMTU of the primary path.
     ///
     /// # Panics

neqo-transport/src/quic_datagrams.rs

@@ -186,6 +186,12 @@
         Ok(())
     }
 
+    /// Returns the number of QUIC datagrams that can still be queued for
+    /// sending before the queue is full.
+    pub fn send_queue_capacity(&self) -> usize {
+        self.max_queued_outgoing_datagrams - self.datagrams.len()
+    }
+
     pub fn handle_datagram(&self, data: &[u8], stats: &mut Stats) -> Res<()> {
         if self.local_datagram_size < u64::try_from(data.len())? {
             return Err(Error::ProtocolViolation);