tunnel: fix panic on send to closed writeCh during disconnect

When the server disconnects, Close() was closing both stopCh and
writeCh. The TAP→Server goroutine could race and send to the closed
writeCh, causing a panic. Fix: don't close writeCh in Close(), let
writeLoop exit via stopCh instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/client/tunnel.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"math/rand"
 	"sync"
+	"sync/atomic"
 	"time"
 )
 
@@ -29,8 +30,8 @@ const (
 // SoftEther VPN session. Each "block" is one Ethernet frame.
 type Tunnel struct {
 	sess    *Session
-	writeCh chan []byte // serialized messages queued for the single writer
-	writeErr error     // last write error
+	writeCh chan []byte       // serialized messages queued for the single writer
+	writeErr atomic.Value    // stores error from writeLoop
 	stopCh  chan struct{}
 	stopped sync.Once
 }
@@ -50,20 +51,23 @@ func NewTunnel(sess *Session) *Tunnel {
 // writeLoop is the single goroutine that writes to the connection.
 // All writes are serialized through writeCh — no mutex needed.
 func (t *Tunnel) writeLoop() {
-	for buf := range t.writeCh {
-		if _, err := t.sess.Conn.Write(buf); err != nil {
-			t.writeErr = err
+	for {
+		select {
+		case buf := <-t.writeCh:
+			if _, err := t.sess.Conn.Write(buf); err != nil {
+				t.writeErr.Store(err)
+				return
+			}
+		case <-t.stopCh:
 			return
 		}
 	}
 }
 
 // Close stops the keepalive goroutine and closes the underlying connection.
+// The writeCh is not closed here — writeLoop exits when the connection write fails.
 func (t *Tunnel) Close() error {
-	t.stopped.Do(func() {
-		close(t.stopCh)
-		close(t.writeCh)
-	})
+	t.stopped.Do(func() { close(t.stopCh) })
 	return t.sess.Conn.Close()
 }
 
@@ -124,7 +128,10 @@ func (t *Tunnel) WriteFrames(frames [][]byte) error {
 
 	select {
 	case t.writeCh <- buf:
-		return t.writeErr
+		if err, ok := t.writeErr.Load().(error); ok {
+			return err
+		}
+		return nil
 	case <-t.stopCh:
 		return fmt.Errorf("tunnel closed")
 	}