1// Copyright 2023 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5//go:build unix || wasip1
6
7package net
8
9import "syscall"
10
11// concurrentThreadsLimit returns the number of threads we permit to
12// run concurrently doing DNS lookups via cgo. A DNS lookup may use a
13// file descriptor so we limit this to less than the number of
14// permitted open files. On some systems, notably Darwin, if
15// getaddrinfo is unable to open a file descriptor it simply returns
16// EAI_NONAME rather than a useful error. Limiting the number of
17// concurrent getaddrinfo calls to less than the permitted number of
18// file descriptors makes that error less likely. We don't bother to
19// apply the same limit to DNS lookups run directly from Go, because
20// there we will return a meaningful "too many open files" error.
21func concurrentThreadsLimit() int {
22	var rlim syscall.Rlimit
23	if err := syscall.Getrlimit(syscall.RLIMIT_NOFILE, &rlim); err != nil {
24		return 500
25	}
26	r := rlim.Cur
27	if r > 500 {
28		r = 500
29	} else if r > 30 {
30		r -= 30
31	}
32	return int(r)
33}
34