fix(websocket): release a pending Accept when the listener is closed (#5552)

the websocket listener kept its accept channel open on Close, so an Accept call that was already waiting blocked forever

every other listener in pkg/util/net closes the channel and returns an error, but this one never did, so the HandleListener goroutine stayed stuck after the server closed

i close the channel on Close, guard the hand off send so a late connection does not panic, and return ErrWebsocketListenerClosed which was declared but never reached

Signed-off-by: Abdel <62770500+hktitof@users.noreply.github.com>

pkg/util/net/websocket_test.go

@@ -0,0 +1,73 @@
+// Copyright 2026 The frp Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package net
+
+import (
+	"errors"
+	"net"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/require"
+)
+
+type closeStubListener struct {
+	closed chan struct{}
+}
+
+func (l *closeStubListener) Accept() (net.Conn, error) {
+	<-l.closed
+	return nil, net.ErrClosed
+}
+
+func (l *closeStubListener) Close() error {
+	select {
+	case <-l.closed:
+	default:
+		close(l.closed)
+	}
+	return nil
+}
+
+func (l *closeStubListener) Addr() net.Addr {
+	return &net.TCPAddr{IP: net.IPv4(127, 0, 0, 1)}
+}
+
+func TestWebsocketListenerAcceptUnblocksOnClose(t *testing.T) {
+	wl := NewWebsocketListener(&closeStubListener{closed: make(chan struct{})})
+
+	errCh := make(chan error, 1)
+	go func() {
+		_, err := wl.Accept()
+		errCh <- err
+	}()
+
+	select {
+	case err := <-errCh:
+		t.Fatalf("Accept returned before the listener was closed: %v", err)
+	case <-time.After(100 * time.Millisecond):
+	}
+
+	require.NoError(t, wl.Close())
+	// Closing twice must stay safe and keep reporting the closed error.
+	require.NoError(t, wl.Close())
+
+	select {
+	case err := <-errCh:
+		require.True(t, errors.Is(err, ErrWebsocketListenerClosed), "got %v", err)
+	case <-time.After(time.Second):
+		t.Fatal("Accept did not return after the listener was closed")
+	}
+}