fix(dialer): split netns into platform file to support android cross-compile

internal/net/dialer/dialer.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"fmt"
 	"net"
-	"runtime"
 	"strings"
 	"syscall"
 	"time"
@@ -12,7 +11,6 @@ import (
 	"github.com/go-gost/core/logger"
 	ctxvalue "github.com/go-gost/x/ctx"
 	xnet "github.com/go-gost/x/internal/net"
-	"github.com/vishvananda/netns"
 )
 
 const (
@@ -54,29 +52,11 @@ func (d *Dialer) Dial(ctx context.Context, network, addr string) (conn net.Conn,
 	})
 
 	if d.Netns != "" {
-		runtime.LockOSThread()
-		defer runtime.UnlockOSThread()
-
-		originNs, err := netns.Get()
+		restore, err := switchNetns(d.Netns)
 		if err != nil {
-			return nil, fmt.Errorf("netns.Get(): %v", err)
-		}
-		defer netns.Set(originNs)
-
-		var ns netns.NsHandle
-		if strings.HasPrefix(d.Netns, "/") {
-			ns, err = netns.GetFromPath(d.Netns)
-		} else {
-			ns, err = netns.GetFromName(d.Netns)
-		}
-		if err != nil {
-			return nil, fmt.Errorf("netns.Get(%s): %v", d.Netns, err)
-		}
-		defer ns.Close()
-
-		if err := netns.Set(ns); err != nil {
-			return nil, fmt.Errorf("netns.Set(%s): %v", d.Netns, err)
+			return nil, err
 		}
+		defer restore()
 	}
 
 	if d.DialFunc != nil {

internal/net/dialer/dialer_android.go

@@ -0,0 +1,11 @@
+//go:build android
+
+package dialer
+
+import "fmt"
+
+// switchNetns is a no-op stub — network namespace switching is not supported
+// on Android (requires privileged kernel syscalls unavailable to app sandboxes).
+func switchNetns(name string) (restore func(), err error) {
+	return nil, fmt.Errorf("netns not supported on android")
+}

internal/net/dialer/dialer_netns.go

@@ -0,0 +1,49 @@
+//go:build linux && !android
+
+package dialer
+
+import (
+	"fmt"
+	"runtime"
+	"strings"
+
+	"github.com/vishvananda/netns"
+)
+
+// switchNetns enters the named network namespace and returns a cleanup
+// function that restores the original namespace. The caller must defer
+// the returned function.
+func switchNetns(name string) (restore func(), err error) {
+	runtime.LockOSThread()
+
+	originNs, err := netns.Get()
+	if err != nil {
+		runtime.UnlockOSThread()
+		return nil, fmt.Errorf("netns.Get(): %v", err)
+	}
+
+	var ns netns.NsHandle
+	if strings.HasPrefix(name, "/") {
+		ns, err = netns.GetFromPath(name)
+	} else {
+		ns, err = netns.GetFromName(name)
+	}
+	if err != nil {
+		netns.Set(originNs)
+		runtime.UnlockOSThread()
+		return nil, fmt.Errorf("netns.Get(%s): %v", name, err)
+	}
+
+	if err := netns.Set(ns); err != nil {
+		ns.Close()
+		netns.Set(originNs)
+		runtime.UnlockOSThread()
+		return nil, fmt.Errorf("netns.Set(%s): %v", name, err)
+	}
+
+	return func() {
+		netns.Set(originNs)
+		ns.Close()
+		runtime.UnlockOSThread()
+	}, nil
+}