feat: Add data plane support to FFI (#2287)

1. Overview

This PR adds data plane APIs to easytier-ffi:

TCP Outbound:

- data_plane_tcp_connect
- data_plane_tcp_read
- data_plane_tcp_write
- data_plane_tcp_close

TCP Listener:

- data_plane_tcp_bind
- data_plane_tcp_accept
- data_plane_tcp_listener_close

UDP:

- data_plane_udp_bind
- data_plane_udp_send_to
- data_plane_udp_recv_from
- data_plane_udp_close

2. Key Changes

The main changes are focused on:

- easytier-contrib/easytier-ffi/src/lib.rs: Added FFI interfaces;
  made ERROR_MSG thread-safe.

- easytier/src/gateway/socks5.rs: Bridges the data plane to the
  existing Socks5 server logic.
  - Added EasyTierUdpSocket, mainly wrapping ref-counting and
    critical object (e.g., Socks5EntrySet) hold & drop logic,
    and exposing common fields (e.g., local_addr).
  - Extended Socks5Server functionality to expose TCP and UDP
    socket creation interfaces for FFI calls.

- Other files: Mostly pass-through logic.

- Added a relatively large Go usage example.

easytier-contrib/easytier-ffi/examples/go/easytier_test.go

@@ -0,0 +1,140 @@
+package easytierffi
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"net"
+	"os"
+	"strconv"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestSSHIntegration(t *testing.T) {
+	config := os.Getenv("EASYTIER_FFI_CONFIG")
+	instance := os.Getenv("EASYTIER_FFI_INSTANCE")
+	target := os.Getenv("EASYTIER_FFI_TARGET")
+	if config == "" || instance == "" || target == "" {
+		t.Skip("set EASYTIER_FFI_CONFIG, EASYTIER_FFI_INSTANCE and EASYTIER_FFI_TARGET to run integration test")
+	}
+
+	n, err := Open(defaultLibraryPath())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer n.Close()
+
+	if err := n.RunNetworkInstance(config); err != nil {
+		t.Fatal(err)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 120*time.Second)
+	defer cancel()
+
+	var lastErr error
+	for attempt := 1; ctx.Err() == nil; attempt++ {
+		conn, err := n.DialContext(ctx, instance, "tcp", target)
+		if err != nil {
+			lastErr = err
+			t.Logf("attempt %d: dial failed: %v", attempt, err)
+			time.Sleep(3 * time.Second)
+			continue
+		}
+
+		_ = conn.SetReadDeadline(time.Now().Add(10 * time.Second))
+		buf := make([]byte, 128)
+		nn, err := conn.Read(buf)
+		_ = conn.Close()
+		if err != nil {
+			lastErr = err
+			t.Logf("attempt %d: read failed: %v", attempt, err)
+			time.Sleep(3 * time.Second)
+			continue
+		}
+		banner := string(buf[:nn])
+		if !strings.HasPrefix(banner, "SSH-") {
+			t.Fatalf("attempt %d: expected SSH banner, got %q", attempt, banner)
+		}
+		t.Logf("attempt %d: got banner %q", attempt, strings.TrimRight(banner, "\r\n"))
+		return
+	}
+	t.Fatalf("never got SSH banner, last err: %v", lastErr)
+}
+
+func TestTCPListenIntegration(t *testing.T) {
+	config := os.Getenv("EASYTIER_FFI_LISTEN_CONFIG")
+	instance := os.Getenv("EASYTIER_FFI_LISTEN_INSTANCE")
+	listenPort := os.Getenv("EASYTIER_FFI_LISTEN_PORT")
+	if config == "" || instance == "" || listenPort == "" {
+		t.Skip("set EASYTIER_FFI_LISTEN_CONFIG, EASYTIER_FFI_LISTEN_INSTANCE and EASYTIER_FFI_LISTEN_PORT to run integration test")
+	}
+	port, err := strconv.ParseUint(listenPort, 10, 16)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	n, err := Open(defaultLibraryPath())
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer n.Close()
+
+	if err := n.RunNetworkInstance(config); err != nil {
+		t.Fatal(err)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 120*time.Second)
+	defer cancel()
+
+	// Data-plane readiness is asynchronous: the instance must finish starting
+	// before the data plane accepts binds. Retry until ready or ctx expires.
+	var listener net.Listener
+	for attempt := 1; ; attempt++ {
+		listener, err = n.ListenContext(ctx, instance, "tcp", net.JoinHostPort("0.0.0.0", strconv.Itoa(int(port))))
+		if err == nil {
+			break
+		}
+		if ctx.Err() != nil {
+			t.Fatalf("bind never succeeded, last err: %v", err)
+		}
+		t.Logf("attempt %d: bind failed: %v", attempt, err)
+		time.Sleep(3 * time.Second)
+	}
+	t.Logf("listening on %s; connect from another EasyTier peer and send ping", listener.Addr())
+
+	accepted := make(chan error, 1)
+	go func() {
+		conn, err := listener.Accept()
+		if err != nil {
+			accepted <- err
+			return
+		}
+		defer conn.Close()
+
+		_ = conn.SetDeadline(time.Now().Add(10 * time.Second))
+		buf := make([]byte, 4)
+		if _, err := io.ReadFull(conn, buf); err != nil {
+			accepted <- err
+			return
+		}
+		if string(buf) != "ping" {
+			accepted <- fmt.Errorf("expected %q, got %q", "ping", string(buf))
+			return
+		}
+		_, err = conn.Write([]byte("pong"))
+		accepted <- err
+	}()
+
+	select {
+	case err := <-accepted:
+		_ = listener.Close()
+		if err != nil {
+			t.Fatal(err)
+		}
+	case <-ctx.Done():
+		_ = listener.Close()
+		t.Fatal(ctx.Err())
+	}
+}