net, syscall: use accept4 and SOCK_CLOEXEC on Linux

R=golang-dev, bradfitz, mikioh.mikioh, dave, minux.ma
CC=golang-dev
https://golang.org/cl/7227043

src/pkg/net/sys_cloexec.go

@@ -0,0 +1,54 @@
+// 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.
+
+// This file implements sysSocket and accept for platforms that do not
+// provide a fast path for setting SetNonblock and CloseOnExec.
+
+// +build darwin freebsd netbsd openbsd
+
+package net
+
+import "syscall"
+
+// Wrapper around the socket system call that marks the returned file
+// descriptor as nonblocking and close-on-exec.
+func sysSocket(f, t, p int) (int, error) {
+	// See ../syscall/exec_unix.go for description of ForkLock.
+	syscall.ForkLock.RLock()
+	s, err := syscall.Socket(f, t, p)
+	if err == nil {
+		syscall.CloseOnExec(s)
+	}
+	syscall.ForkLock.RUnlock()
+	if err != nil {
+		return -1, err
+	}
+	if err = syscall.SetNonblock(s, true); err != nil {
+		syscall.Close(s)
+		return -1, err
+	}
+	return s, nil
+}
+
+// Wrapper around the accept system call that marks the returned file
+// descriptor as nonblocking and close-on-exec.
+func accept(fd int) (int, syscall.Sockaddr, error) {
+	// See ../syscall/exec_unix.go for description of ForkLock.
+	// It is okay to hold the lock across syscall.Accept
+	// because we have put fd.sysfd into non-blocking mode.
+	syscall.ForkLock.RLock()
+	nfd, sa, err := syscall.Accept(fd)
+	if err == nil {
+		syscall.CloseOnExec(nfd)
+	}
+	syscall.ForkLock.RUnlock()
+	if err != nil {
+		return -1, nil, err
+	}
+	if err = syscall.SetNonblock(nfd, true); err != nil {
+		syscall.Close(nfd)
+		return -1, nil, err
+	}
+	return nfd, sa, nil
+}