fix: TCP 双向转发半关闭时不再截断对端数据

问题：tcp_port_mapping::stream_copy 与 tcp_nat::stream_nat 用
tokio::select! 组合两个 io::copy，任一方向先结束就 drop 另一方向。
客户端半关闭（shutdown 写方向）后，服务端的响应数据会被静默截断。

修复：抽出 copy_bidirectional_split，用 tokio::io::join 合并分离的
读写半部后走 copy_bidirectional——任一方向 EOF 时对另一端 shutdown
并继续转发剩余方向，直到双向完成。

测试：test_half_close_no_truncation 用 duplex 流验证半关闭后响应
完整送达（旧实现此处读到空数据）。

vnt-core/src/nat/internal_nat/tcp_nat.rs

@@ -78,8 +78,8 @@ async fn stream_task(
 }
 
 pub(crate) async fn stream_nat<R, W, A: ToSocketAddrs + Debug>(
-    mut recv_stream: R,
-    mut send_stream: W,
+    recv_stream: R,
+    send_stream: W,
     addr: A,
 ) -> anyhow::Result<()>
 where
@@ -89,10 +89,11 @@ where
     let mut tokio_stream = TcpStream::connect(&addr)
         .await
         .with_context(|| format!("error connecting to {:?}", addr))?;
-    let (mut tcp_r, mut tcp_w) = tokio_stream.split();
-    tokio::select! {
-        _ = tokio::io::copy(&mut recv_stream, &mut tcp_w) => {},
-        _ = tokio::io::copy(&mut tcp_r, &mut send_stream) => {},
-    }
+    crate::port_mapping::tcp_port_mapping::copy_bidirectional_split(
+        &mut tokio_stream,
+        recv_stream,
+        send_stream,
+    )
+    .await?;
     Ok(())
 }

vnt-core/src/port_mapping/tcp_port_mapping.rs

@@ -7,8 +7,26 @@ use crate::utils::task_control::TaskGroup;
 use anyhow::Context;
 use pnet_packet::ip::IpNextHeaderProtocols;
 use std::net::{Ipv4Addr, SocketAddr};
+use tokio::io::{AsyncRead, AsyncWrite};
 use tokio::net::{TcpListener, TcpStream};
 
+/// 双向转发：任一方向 EOF 时对另一端执行 shutdown 并继续转发剩余方向，
+/// 直到两个方向都完成。避免 select! 下任一方向先结束就 drop 另一方向
+/// 造成的半关闭截断（如对端半关闭后响应数据丢失）。
+pub(crate) async fn copy_bidirectional_split<T, R, W>(
+    stream: &mut T,
+    reader: R,
+    writer: W,
+) -> anyhow::Result<(u64, u64)>
+where
+    T: AsyncRead + AsyncWrite + Unpin,
+    R: AsyncRead + Unpin,
+    W: AsyncWrite + Unpin,
+{
+    let mut other = tokio::io::join(reader, writer);
+    Ok(tokio::io::copy_bidirectional(stream, &mut other).await?)
+}
+
 pub async fn start(
     task_group: &TaskGroup,
     list: &Vec<PortMapping>,
@@ -64,7 +82,7 @@ async fn stream_copy(
     dst_port: u16,
     quic_tunnel_client: QuicTunnelClient,
 ) -> anyhow::Result<()> {
-    let (mut send_stream, mut recv_stream) = quic_tunnel_client.open_bi(target_ip).await?;
+    let (mut send_stream, recv_stream) = quic_tunnel_client.open_bi(target_ip).await?;
     let handshake = QuicProxyHandshake {
         handshake: Some(quic_proxy_handshake::Handshake::TcpPortMapping(
             PortProxyHandshake {
@@ -76,10 +94,47 @@ async fn stream_copy(
         )),
     };
     send_handshake(&mut send_stream, handshake).await?;
-    let (mut tcp_r, mut tcp_w) = tcp_stream.split();
-    tokio::select! {
-        _ = tokio::io::copy(&mut recv_stream, &mut tcp_w) => {},
-        _ = tokio::io::copy(&mut tcp_r, &mut send_stream) => {},
-    }
+    copy_bidirectional_split(&mut tcp_stream, recv_stream, send_stream).await?;
     Ok(())
 }
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use tokio::io::{AsyncReadExt, AsyncWriteExt};
+
+    /// 半关闭场景：客户端发完请求后 shutdown 写方向，
+    /// 服务端的响应必须完整送达，不能被截断。
+    #[tokio::test]
+    async fn test_half_close_no_truncation() {
+        let (mut client, mut relay_tcp) = tokio::io::duplex(64);
+        let (relay_tunnel, mut server) = tokio::io::duplex(64);
+        let (relay_tunnel_r, relay_tunnel_w) = tokio::io::split(relay_tunnel);
+
+        let relay = tokio::spawn(async move {
+            copy_bidirectional_split(&mut relay_tcp, relay_tunnel_r, relay_tunnel_w)
+                .await
+                .unwrap();
+        });
+
+        // 客户端发请求后立即半关闭写方向
+        client.write_all(b"ping").await.unwrap();
+        client.shutdown().await.unwrap();
+
+        // 服务端读到完整请求（读到 EOF 前数据不能丢）
+        let mut buf = [0u8; 4];
+        server.read_exact(&mut buf).await.unwrap();
+        assert_eq!(&buf, b"ping");
+        // 服务端回响应并关闭
+        server.write_all(b"pong").await.unwrap();
+        server.shutdown().await.unwrap();
+
+        // 客户端必须能读到完整响应（旧实现此处会被截断为空）
+        let mut out = Vec::new();
+        client.read_to_end(&mut out).await.unwrap();
+        assert_eq!(out, b"pong");
+
+        relay.await.unwrap();
+    }
+}