go/src/net/rlimit_unix.go

// concurrentThreadsLimit returns the number of threads we permit to
// run concurrently doing DNS lookups via cgo. A DNS lookup may use a
// file descriptor so we limit this to less than the number of
// permitted open files. On some systems, notably Darwin, if
// getaddrinfo is unable to open a file descriptor it simply returns
// EAI_NONAME rather than a useful error. Limiting the number of
// concurrent getaddrinfo calls to less than the permitted number of
// file descriptors makes that error less likely. We don't bother to
// apply the same limit to DNS lookups run directly from Go, because
// there we will return a meaningful "too many open files" error.
func concurrentThreadsLimit() int { … }