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// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Package netutil provides network utility functions, complementing the more
// common ones in the net package.
package netutil // import ""
import (
// LimitListener returns a Listener that accepts at most n simultaneous
// connections from the provided Listener.
func LimitListener(l net.Listener, n int) net.Listener {
return &limitListener{
Listener: l,
sem: make(chan struct{}, n),
done: make(chan struct{}),
type limitListener struct {
sem chan struct{}
closeOnce sync.Once // ensures the done chan is only closed once
done chan struct{} // no values sent; closed when Close is called
// acquire acquires the limiting semaphore. Returns true if successfully
// accquired, false if the listener is closed and the semaphore is not
// acquired.
func (l *limitListener) acquire() bool {
select {
case <-l.done:
return false
case l.sem <- struct{}{}:
return true
func (l *limitListener) release() { <-l.sem }
func (l *limitListener) Accept() (net.Conn, error) {
acquired := l.acquire()
// If the semaphore isn't acquired because the listener was closed, expect
// that this call to accept won't block, but immediately return an error.
c, err := l.Listener.Accept()
if err != nil {
if acquired {
return nil, err
return &limitListenerConn{Conn: c, release: l.release}, nil
func (l *limitListener) Close() error {
err := l.Listener.Close()
l.closeOnce.Do(func() { close(l.done) })
return err
type limitListenerConn struct {
releaseOnce sync.Once
release func()
func (l *limitListenerConn) Close() error {
err := l.Conn.Close()
return err