// Copyright 2009 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

import "syscall"

func kernelVersion() (major int, minor int) {
	var uname syscall.Utsname
	if err := syscall.Uname(&uname); err != nil {
		return
	}

	rl := uname.Release
	var values [2]int
	vi := 0
	value := 0
	for _, c := range rl {
		if c >= '0' && c <= '9' {
			value = (value * 10) + int(c-'0')
		} else {
			// Note that we're assuming N.N.N here.  If we see anything else we are likely to
			// mis-parse it.
			values[vi] = value
			vi++
			if vi >= len(values) {
				break
			}
			value = 0
		}
	}
	switch vi {
	case 0:
		return 0, 0
	case 1:
		return values[0], 0
	case 2:
		return values[0], values[1]
	}
	return
}

// Linux stores the backlog as:
//
//  - uint16 in kernel version < 4.1,
//  - uint32 in kernel version >= 4.1
//
// Truncate number to avoid wrapping.
//
// See issue 5030 and 41470.
func maxAckBacklog(n int) int {
	major, minor := kernelVersion()
	size := 16
	if major > 4 || (major == 4 && minor >= 1) {
		size = 32
	}

	var max uint = 1<<size - 1
	if uint(n) > max {
		n = int(max)
	}
	return n
}

func maxListenerBacklog() int {
	fd, err := open("/proc/sys/net/core/somaxconn")
	if err != nil {
		return syscall.SOMAXCONN
	}
	defer fd.close()
	l, ok := fd.readLine()
	if !ok {
		return syscall.SOMAXCONN
	}
	f := getFields(l)
	n, _, ok := dtoi(f[0])
	if n == 0 || !ok {
		return syscall.SOMAXCONN
	}

	if n > 1<<16-1 {
		return maxAckBacklog(n)
	}
	return n
}