net: consolidate listenerBacklog declaration

Also adds maxListenerBacklog placeholder for Plan 9.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/11674044

src/pkg/net/ipsock_plan9.go

@@ -12,9 +12,6 @@ import (
 	"syscall"
 )
 
-// /sys/include/ape/sys/socket.h:/SOMAXCONN
-var listenerBacklog = 5
-
 // probeIPv6Stack returns two boolean values.  If the first boolean
 // value is true, kernel supports basic IPv6 functionality.  If the
 // second boolean value is true, kernel supports IPv6 IPv4-mapping.

src/pkg/net/net.go

@@ -259,6 +259,8 @@ type PacketConn interface {
 	SetWriteDeadline(t time.Time) error
 }
 
+var listenerBacklog = maxListenerBacklog()
+
 // A Listener is a generic network listener for stream-oriented protocols.
 //
 // Multiple goroutines may invoke methods on a Listener simultaneously.

src/pkg/net/sock_plan9.go

@@ -0,0 +1,10 @@
+// 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 net
+
+func maxListenerBacklog() int {
+	// /sys/include/ape/sys/socket.h:/SOMAXCONN
+	return 5
+}

src/pkg/net/sock_posix.go

@@ -11,8 +11,6 @@ import (
 	"time"
 )
 
-var listenerBacklog = maxListenerBacklog()
-
 // Generic POSIX socket creation.
 func socket(net string, f, t, p int, ipv6only bool, ulsa, ursa syscall.Sockaddr, deadline time.Time, toAddr func(syscall.Sockaddr) Addr) (fd *netFD, err error) {
 	s, err := sysSocket(f, t, p)