CVPN-956: handling of Full error on tun io-uring send

Treating tun io-uring Full error as OK scenario.
It is effectively the same scenario as a buffer in a network switch/router filling up so dropping the traffic is appropriate, higher level protocols (e.g. TCP) running over the tunnel will use their congestion control algorithms to adjust their send rate.

lightway-app-utils/Cargo.toml

@@ -32,6 +32,7 @@ tokio-eventfd = "0.2.1"
 dashmap = "5.5.3"
 thiserror = "1.0.57"
 tokio-tun = "0.11.2"
+metrics = "0.23.0"
 
 [[example]]
 name = "udprelay"

lightway-app-utils/src/iouring.rs

@@ -18,6 +18,7 @@ use std::{
 };
 use tokio::{io::AsyncReadExt, sync::Semaphore};
 use tokio_eventfd::EventFd;
+use crate::metrics::tun_iouring_data_dropped;
 
 const REGISTERED_FD_INDEX: u32 = 0;
 const IOURING_SQPOLL_IDLE_TIME: u32 = 100;
@@ -190,11 +191,17 @@ impl<T: AsRawFd> IOUring<T> {
 
     /// Try Send packet to Tun device
     pub fn try_send(&self, buf: BytesMut) -> IOUringResult<()> {
-        self.inner
-            .send_q
-            .tx
-            .try_send(buf)
-            .map_err(IOUringError::SendError)
+        let try_send_res = self.inner.send_q.tx.try_send(buf);
+        match try_send_res {
+            Ok(()) => Ok(()),
+            Err(e) if e.is_full() => {
+                // it is effectively the same scenario as a buffer in a network
+                // switch/router filling up so dropping the traffic is appropriate
+                tun_iouring_data_dropped();
+                Ok(())
+            }
+            Err(e) => Err(IOUringError::SendError(e)),
+        }
     }
 }
 

lightway-app-utils/src/lib.rs

@@ -26,5 +26,6 @@ pub use iouring::IOUring;
 
 pub use tun::Tun;
 
+mod metrics;
 mod utils;
 pub use utils::is_file_path_valid;

lightway-app-utils/src/metrics.rs

@@ -0,0 +1,8 @@
+use metrics::counter;
+
+const METRIC_TUN_IOURING_DATA_DROPPED: &str = "tun_iouring_data_dropped";
+
+/// Counter for "sending into a full channel" type of error ([`async_channel::TrySendError::Full`])
+pub(crate) fn tun_iouring_data_dropped() {
+    counter!(METRIC_TUN_IOURING_DATA_DROPPED).increment(1);
+}