feat: 移动端 TUN fd 复用，ReloadConfig 无需重新 SetTunFd

移动端（Android/iOS）开启 VPN 后更换订阅时，sing-box 销毁旧实例
会 close(fd) 导致 TUN fd 失效，新实例 OpenTun 报错。改为 dup()
复制 fd 给 sing-box 使用，保留原始 fd 不被消费，reload 时可重复
分配新副本。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: mapleafgo

core/dup_other.go

@@ -0,0 +1,7 @@
+//go:build !android && !ios
+
+package core
+
+func dupFd(fd int32) (int32, error) { return fd, nil }
+
+func closeFd(_ int32) error { return nil }

core/dup_unix.go

@@ -0,0 +1,15 @@
+//go:build android || ios
+
+package core
+
+import "syscall"
+
+func dupFd(fd int32) (int32, error) {
+	n, err := syscall.Dup(int(fd))
+	if err != nil {
+		return 0, err
+	}
+	return int32(n), nil
+}
+
+func closeFd(fd int32) error { return syscall.Close(int(fd)) }

core/platform.go

@@ -64,6 +64,9 @@ func NewPlatformIO() *PlatformIO {
 func (p *PlatformIO) SetTunFd(fd int32) {
 	p.mu.Lock()
 	defer p.mu.Unlock()
+	if p.tunFd != 0 && p.tunFd != fd {
+		closeFd(p.tunFd)
+	}
 	p.tunFd = fd
 	if fd != 0 {
 		p.externalTun = true
@@ -75,6 +78,9 @@ func (p *PlatformIO) SetTunFd(fd int32) {
 func (p *PlatformIO) ResetTunFd() {
 	p.mu.Lock()
 	defer p.mu.Unlock()
+	if p.tunFd != 0 {
+		closeFd(p.tunFd)
+	}
 	oldFd := p.tunFd
 	p.tunFd = 0
 	p.externalTun = false
@@ -164,15 +170,17 @@ func (p *PlatformIO) AutoDetectInterfaceControl(fd int32) error {
 func (p *PlatformIO) OpenTun(options libbox.TunOptions) (int32, error) {
 	p.mu.Lock()
 	fd := p.tunFd
-	if fd != 0 {
-		p.tunFd = 0
-	}
 	p.tunOptions = options
 	p.mu.Unlock()
 
 	if fd != 0 {
-		slog.Debug("[OpenTun] returning fd", "fd", fd)
-		return fd, nil
+		duped, err := dupFd(fd)
+		if err != nil {
+			slog.Error("[OpenTun] dup failed", "fd", fd, "error", err)
+			return 0, fmt.Errorf("dup tun fd: %w", err)
+		}
+		slog.Debug("[OpenTun] returning duped fd", "original", fd, "duped", duped)
+		return duped, nil
 	}
 	slog.Warn("[OpenTun] no TUN fd available; mobile platforms must call SetTunFd before starting/reloading")
 	return 0, os.ErrInvalid

core/platform_test.go

@@ -50,13 +50,16 @@ func TestOpenTun_ReturnsExternalFd(t *testing.T) {
 	}
 }
 
-func TestOpenTun_ConsumesFd(t *testing.T) {
+func TestOpenTun_ReusesFd(t *testing.T) {
 	p := newPlatform()
 	p.SetTunFd(42)
-	_, _ = p.OpenTun(nil)
 	_, err := p.OpenTun(nil)
-	if err == nil {
-		t.Fatal("second OpenTun should fail after fd was consumed")
+	if err != nil {
+		t.Fatalf("first OpenTun failed: %v", err)
+	}
+	_, err = p.OpenTun(nil)
+	if err != nil {
+		t.Fatalf("second OpenTun should succeed, fd is retained: %v", err)
 	}
 }