go.net: add netutil package with LimitListener

Update golang/go#6012

R=golang-dev, dsymonds, rsc
CC=golang-dev
https://golang.org/cl/12727043

netutil/listen.go

+// 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 "net"
+
+// LimitListener returns a Listener that accepts at most n simultaneous
+// connections from the provided Listener.
+func LimitListener(l net.Listener, n int) net.Listener {
+	ch := make(chan struct{}, n)
+	for i := 0; i < n; i++ {
+		ch <- struct{}{}
+	}
+	return &limitListener{l, ch}
+}
+
+type limitListener struct {
+	net.Listener
+	ch chan struct{}
+}
+
+func (l *limitListener) Accept() (net.Conn, error) {
+	<-l.ch
+	c, err := l.Listener.Accept()
+	if err != nil {
+		return nil, err
+	}
+	return &limitListenerConn{c, l.ch}, nil
+}
+
+type limitListenerConn struct {
+	net.Conn
+	ch chan<- struct{}
+}
+
+func (l *limitListenerConn) Close() error {
+	err := l.Conn.Close()
+	l.ch <- struct{}{}
+	return err
+}

netutil/listen_test.go

+// 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
+
+import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net"
+	"net/http"
+	"sync"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestLimitListener(t *testing.T) {
+	const (
+		max = 5
+		num = 200
+	)
+
+	l, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("Listen: %v", err)
+	}
+	defer l.Close()
+	l = LimitListener(l, max)
+
+	var open int32
+	go http.Serve(l, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if n := atomic.AddInt32(&open, 1); n > max {
+			t.Errorf("%d open connections, want <= %d", n, max)
+		}
+		defer atomic.AddInt32(&open, -1)
+		time.Sleep(10 * time.Millisecond)
+		fmt.Fprint(w, "some body")
+	}))
+
+	var wg sync.WaitGroup
+	var failed int32
+	for i := 0; i < num; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			r, err := http.Get("http://" + l.Addr().String())
+			if err != nil {
+				t.Logf("Get: %v", err)
+				atomic.AddInt32(&failed, 1)
+				return
+			}
+			defer r.Body.Close()
+			io.Copy(ioutil.Discard, r.Body)
+		}()
+	}
+	wg.Wait()
+
+	// We expect some Gets to fail as the kernel's accept queue is filled,
+	// but most should succeed.
+	if failed >= num/2 {
+		t.Errorf("too many Gets failed")
+	}
+}