Fix TCP send: server sends 12-byte status messages when receiving

pcap of MikroTik-as-server showed it sends periodic 12-byte status
messages back to the client even in RX-only mode. The client needs
these to display speed. Added tcp_status_sender that writes status
messages containing rx_bytes on the TCP write half every 1 second.

Reverted the "always bidirectional" change — TCP direction is
conditional, but RX mode now uses the writer for status instead
of keeping it idle.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/server.rs

@@ -293,53 +293,83 @@ async fn handle_client(
 async fn run_tcp_test_server(stream: TcpStream, cmd: Command) -> Result<()> {
     let state = BandwidthState::new();
     let tx_size = cmd.tx_size as usize;
+    let server_should_tx = cmd.server_tx();
+    let server_should_rx = cmd.server_rx();
     let tx_speed = cmd.remote_tx_speed;
 
     let (reader, writer) = stream.into_split();
 
-    // TCP mode: ALWAYS start both TX and RX threads.
-    // MikroTik expects bidirectional data flow on TCP regardless of direction.
-    // The direction only controls what gets measured/reported.
+    let mut _writer_keepalive = None;
+    let mut _reader_keepalive = None;
+
     let state_tx = state.clone();
-    let tx_handle = tokio::spawn(async move {
-        tcp_tx_loop(writer, tx_size, tx_speed, state_tx).await
-    });
+    let tx_handle = if server_should_tx {
+        Some(tokio::spawn(async move {
+            tcp_tx_loop(writer, tx_size, tx_speed, state_tx).await
+        }))
+    } else if server_should_rx {
+        // Server receives: use the writer to send periodic status messages
+        // back to the client (MikroTik needs this to show speed)
+        let st = state.clone();
+        Some(tokio::spawn(async move {
+            tcp_status_sender(writer, st).await
+        }))
+    } else {
+        _writer_keepalive = Some(writer);
+        None
+    };
 
     let state_rx = state.clone();
-    let rx_handle = tokio::spawn(async move {
-        tcp_rx_loop(reader, state_rx).await
-    });
+    let rx_handle = if server_should_rx {
+        Some(tokio::spawn(async move {
+            tcp_rx_loop(reader, state_rx).await
+        }))
+    } else {
+        _reader_keepalive = Some(reader);
+        None
+    };
 
     status_report_loop(&cmd, &state).await;
 
     state.running.store(false, Ordering::SeqCst);
-    let _ = tx_handle.await;
-    let _ = rx_handle.await;
+    if let Some(h) = tx_handle { let _ = h.await; }
+    if let Some(h) = rx_handle { let _ = h.await; }
     Ok(())
 }
 
-/// TCP multi-connection: always TX+RX on all streams.
+/// TCP multi-connection.
 async fn run_tcp_multiconn_server(streams: Vec<TcpStream>, cmd: Command) -> Result<()> {
     let state = BandwidthState::new();
     let tx_size = cmd.tx_size as usize;
+    let server_should_tx = cmd.server_tx();
+    let server_should_rx = cmd.server_rx();
     let tx_speed = cmd.remote_tx_speed;
 
     let mut tx_handles = Vec::new();
     let mut rx_handles = Vec::new();
+    let mut _writer_keepalives: Vec<tokio::net::tcp::OwnedWriteHalf> = Vec::new();
+    let mut _reader_keepalives: Vec<tokio::net::tcp::OwnedReadHalf> = Vec::new();
 
-    // TCP: always start both TX and RX on every stream
     for tcp_stream in streams {
         let (reader, writer) = tcp_stream.into_split();
 
-        let st = state.clone();
-        tx_handles.push(tokio::spawn(async move {
-            tcp_tx_loop(writer, tx_size, tx_speed, st).await
-        }));
+        if server_should_tx {
+            let st = state.clone();
+            tx_handles.push(tokio::spawn(async move {
+                tcp_tx_loop(writer, tx_size, tx_speed, st).await
+            }));
+        } else {
+            _writer_keepalives.push(writer);
+        }
 
-        let st = state.clone();
-        rx_handles.push(tokio::spawn(async move {
-            tcp_rx_loop(reader, st).await
-        }));
+        if server_should_rx {
+            let st = state.clone();
+            rx_handles.push(tokio::spawn(async move {
+                tcp_rx_loop(reader, st).await
+            }));
+        } else {
+            _reader_keepalives.push(reader);
+        }
     }
 
     tracing::info!(
@@ -413,6 +443,38 @@ async fn tcp_rx_loop(mut reader: tokio::net::tcp::OwnedReadHalf, state: Arc<Band
     }
 }
 
+/// Send periodic 12-byte status messages on the TCP connection.
+/// Used when server is in RX mode — tells the client how many bytes we received.
+async fn tcp_status_sender(
+    mut writer: tokio::net::tcp::OwnedWriteHalf,
+    state: Arc<BandwidthState>,
+) {
+    let mut seq: u32 = 0;
+    let mut interval = tokio::time::interval(Duration::from_secs(1));
+    interval.tick().await; // consume first immediate tick
+
+    while state.running.load(Ordering::Relaxed) {
+        interval.tick().await;
+        if !state.running.load(Ordering::Relaxed) {
+            break;
+        }
+
+        seq += 1;
+        let rx_bytes = state.rx_bytes.load(Ordering::Relaxed);
+
+        let status = StatusMessage {
+            seq,
+            bytes_received: rx_bytes as u32,
+        };
+
+        if writer.write_all(&status.serialize()).await.is_err() {
+            state.running.store(false, Ordering::SeqCst);
+            break;
+        }
+        let _ = writer.flush().await;
+    }
+}
+
 // --- UDP Test Server ---
 
 async fn run_udp_test_server(