wgengine/netstack: close forwarded TCP connections when incoming TCP dies

Updates #4522

Change-Id: I31a430da422b1e5fab834a2a670cddf448889ee6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/netstack/netstack.go

@@ -715,29 +715,46 @@ func (ns *Impl) forwardTCP(client *gonet.TCPConn, clientRemoteIP netaddr.IP, wq
 		cancel()
 	}()
 	var stdDialer net.Dialer
-	server, err := stdDialer.DialContext(ctx, "tcp", dialAddrStr)
+	s, err := stdDialer.DialContext(ctx, "tcp", dialAddrStr)
 	if err != nil {
 		ns.logf("netstack: could not connect to local server at %s: %v", dialAddrStr, err)
 		return
 	}
+	server := s.(*net.TCPConn)
 	defer server.Close()
 	backendLocalAddr := server.LocalAddr().(*net.TCPAddr)
 	backendLocalIPPort, _ := netaddr.FromStdAddr(backendLocalAddr.IP, backendLocalAddr.Port, backendLocalAddr.Zone)
 	ns.e.RegisterIPPortIdentity(backendLocalIPPort, clientRemoteIP)
 	defer ns.e.UnregisterIPPortIdentity(backendLocalIPPort)
-	connClosed := make(chan error, 2)
+
 	go func() {
-		_, err := io.Copy(server, client)
-		connClosed <- err
+		<-ctx.Done()
+		// Inform the server we won't read anymore, EOF will happen.
+		server.CloseRead()
+		// Shutdown the client, but don't close it, otherwise buffers would be dropped.
+		client.CloseRead()
+		client.CloseWrite()
 	}()
+
+	var wg sync.WaitGroup
+
+	wg.Add(1)
 	go func() {
-		_, err := io.Copy(client, server)
-		connClosed <- err
+		io.Copy(server, client)
+		server.CloseRead()
+		client.CloseWrite()
+		wg.Done()
 	}()
-	err = <-connClosed
-	if err != nil {
-		ns.logf("proxy connection closed with error: %v", err)
-	}
+
+	wg.Add(1)
+	go func() {
+		io.Copy(client, server)
+		server.CloseWrite()
+		client.CloseRead()
+		wg.Done()
+	}()
+
+	wg.Wait()
 	ns.logf("[v2] netstack: forwarder connection to %s closed", dialAddrStr)
 }