net: fix concurrent Accept on windows

Runtime netpoll supports at most one read waiter
and at most one write waiter. It's responsibility
of net package to ensure that. Currently windows
implementation allows more than one waiter in Accept.
It leads to "fatal error: netpollblock: double wait".

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12400045

src/pkg/net/fd_windows.go

@@ -579,6 +579,8 @@ func (fd *netFD) accept(toAddr func(syscall.Sockaddr) Addr) (*netFD, error) {
 	}
 
 	// Submit accept request.
+	fd.rio.Lock()
+	defer fd.rio.Unlock()
 	var o acceptOp
 	o.Init(fd, 'r')
 	o.newsock = s

src/pkg/net/tcp_test.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"reflect"
 	"runtime"
+	"sync"
 	"testing"
 	"time"
 )
@@ -294,3 +295,35 @@ func TestIPv6LinkLocalUnicastTCP(t *testing.T) {
 		<-done
 	}
 }
+
+func TestTCPConcurrentAccept(t *testing.T) {
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(4))
+	ln, err := Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("Listen failed: %v", err)
+	}
+	const N = 10
+	var wg sync.WaitGroup
+	wg.Add(N)
+	for i := 0; i < N; i++ {
+		go func() {
+			for {
+				c, err := ln.Accept()
+				if err != nil {
+					break
+				}
+				c.Close()
+			}
+			wg.Done()
+		}()
+	}
+	for i := 0; i < 10*N; i++ {
+		c, err := Dial("tcp", ln.Addr().String())
+		if err != nil {
+			t.Fatalf("Dial failed: %v", err)
+		}
+		c.Close()
+	}
+	ln.Close()
+	wg.Wait()
+}