net: lazily look up the listenerBacklog value on first use

Don't open files or do sysctls in init.

Updates #26775

Change-Id: I017bed6c24ef1e4bc30040120349fb779f203225
Reviewed-on: https://go-review.googlesource.com/127655Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/net.go

@@ -357,7 +357,16 @@ type PacketConn interface {
 	SetWriteDeadline(t time.Time) error
 }
 
-var listenerBacklog = maxListenerBacklog()
+var listenerBacklogCache struct {
+	sync.Once
+	val int
+}
+
+// listenerBacklog is a caching wrapper around maxListenerBacklog.
+func listenerBacklog() int {
+	listenerBacklogCache.Do(func() { listenerBacklogCache.val = maxListenerBacklog() })
+	return listenerBacklogCache.val
+}
 
 // A Listener is a generic network listener for stream-oriented protocols.
 //

src/net/sock_posix.go

@@ -54,7 +54,7 @@ func socket(ctx context.Context, net string, family, sotype, proto int, ipv6only
 	if laddr != nil && raddr == nil {
 		switch sotype {
 		case syscall.SOCK_STREAM, syscall.SOCK_SEQPACKET:
-			if err := fd.listenStream(laddr, listenerBacklog, ctrlFn); err != nil {
+			if err := fd.listenStream(laddr, listenerBacklog(), ctrlFn); err != nil {
 				fd.Close()
 				return nil, err
 			}