From 77f21eea597f374e025497c167caccfd72864e13 Mon Sep 17 00:00:00 2001
From: Dmitriy Vyukov <dvyukov@google.com>
Date: Sun, 4 Aug 2013 23:31:23 +0400
Subject: [PATCH] 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 |  2 ++
 src/pkg/net/tcp_test.go   | 33 +++++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/src/pkg/net/fd_windows.go b/src/pkg/net/fd_windows.go
index 5d584621a7..f51d1616e0 100644
--- a/src/pkg/net/fd_windows.go
+++ b/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
diff --git a/src/pkg/net/tcp_test.go b/src/pkg/net/tcp_test.go
index a71b02b477..f356f92f09 100644
--- a/src/pkg/net/tcp_test.go
+++ b/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()
+}
-- 
2.48.1