fix(net): DpdkBackend::send_frame dropped every frame → zero TCP connections

dpdk-stdlib-net/src/backend_dpdk.rs

@@ -169,21 +169,26 @@ impl PacketBackend for DpdkBackend {
         let mut mbuf = self.mempool.alloc()
             .map_err(|e| io::Error::new(io::ErrorKind::Other, format!("mbuf alloc failed: {}", e)))?;
 
-        // Copy frame data into mbuf
-        let data = mbuf.data_mut()
-            .ok_or_else(|| io::Error::new(io::ErrorKind::Other, "Failed to get mbuf data"))?;
-
-        if data.len() < frame.len() {
+        // A freshly-allocated mbuf has data_len == 0, so data_mut() would return
+        // a zero-length slice. Compute capacity from the buffer and set the data
+        // length BEFORE data_mut() so it returns a correctly-sized slice. Getting
+        // this order wrong makes the capacity check read 0 and silently drops
+        // EVERY frame — this matches the proven UDP TX path in dpdk-udp.
+        let capacity = mbuf.buf_len() as usize - mbuf.data_offset() as usize;
+        if capacity < frame.len() {
             return Err(io::Error::new(
                 io::ErrorKind::InvalidInput,
-                format!("Frame too large: {} bytes, mbuf capacity: {}", frame.len(), data.len()),
+                format!("Frame too large: {} bytes, mbuf capacity: {}", frame.len(), capacity),
             ));
         }
 
-        data[..frame.len()].copy_from_slice(frame);
         mbuf.set_data_len(frame.len() as u16);
         mbuf.set_packet_len(frame.len() as u32);
 
+        let data = mbuf.data_mut()
+            .ok_or_else(|| io::Error::new(io::ErrorKind::Other, "Failed to get mbuf data"))?;
+        data[..frame.len()].copy_from_slice(frame);
+
         // Transmit
         let port = self.port.lock().unwrap();
         let mut packets = vec![mbuf];
@@ -298,10 +303,18 @@ mod tests {
         // EtherType (IPv4)
         frame[12..14].copy_from_slice(&[0x08, 0x00]);
 
-        // With stubs, tx_burst returns 0, so this will get WouldBlock
+        // The frame must pass the capacity check and reach tx_burst. With stubs
+        // tx_burst returns 0 → WouldBlock; on real hardware it returns Ok(len).
+        // It must NEVER fail the capacity check — that was the regression that
+        // silently dropped every TX frame (data_mut() before set_data_len()).
         let result = backend.send_frame(&frame);
-        // Stubs return 0 for tx_burst, so we expect WouldBlock
-        assert!(result.is_err() || result.unwrap() == 64);
+        if let Err(e) = &result {
+            assert_eq!(
+                e.kind(),
+                std::io::ErrorKind::WouldBlock,
+                "send_frame must reach tx_burst, not fail the capacity check: {e}"
+            );
+        }
     }
 
     #[test]

dpdk-stdlib-tcp/src/runtime.rs

@@ -121,6 +121,21 @@ pub fn init_dpdk_tcp_context(cfg: DpdkTcpRuntimeConfig) -> io::Result<()> {
 ///
 /// Split out from [`run_engine_driver`] so it can be unit-tested with a mock
 /// backend.
+/// Send a frame, warning ONCE if the backend rejects it for a non-transient
+/// reason. A persistent TX failure would otherwise be silent — silently
+/// dropping every SYN-ACK once cost a full EC2 perf run to diagnose. Transient
+/// WouldBlock (tx ring full) is normal backpressure and ignored.
+fn send_or_warn(backend: &Arc<dyn PacketBackend>, out: &[u8]) {
+    if let Err(e) = backend.send_frame(out) {
+        if e.kind() != std::io::ErrorKind::WouldBlock {
+            static TX_ERR_ONCE: std::sync::Once = std::sync::Once::new();
+            TX_ERR_ONCE.call_once(|| {
+                eprintln!("tcp-engine: backend.send_frame failed — frames are being dropped: {e}");
+            });
+        }
+    }
+}
+
 fn drive_once(
     backend: &Arc<dyn PacketBackend>,
     engine: &mut TcpEngine,
@@ -140,7 +155,7 @@ fn drive_once(
             src_mac.copy_from_slice(&frame[6..12]);
             if let Ok(seg) = parse_tcp_packet(frame) {
                 for out in engine.on_segment_with_macs(&seg, src_mac, dst_mac) {
-                    let _ = backend.send_frame(&out);
+                    send_or_warn(backend, &out);
                 }
             }
         }
@@ -149,14 +164,14 @@ fn drive_once(
     // Commands (Connect/Listen/Accept/Shutdown/SetOption/Close).
     while let Ok(cmd) = cmd_rx.try_recv() {
         for out in engine.on_command(cmd) {
-            let _ = backend.send_frame(&out);
+            send_or_warn(backend, &out);
         }
     }
 
     // Timers + tx-ring drain → segments.
     let now = engine.clock().now();
     for out in engine.on_tick(now) {
-        let _ = backend.send_frame(&out);
+        send_or_warn(backend, &out);
     }
 }