syscall: fix Gettimeofday on macOS Sierra

Fixes #16606

Change-Id: I57465061b90e901293cd8b6ef756d6aa84ebd4a3
Reviewed-on: https://go-review.googlesource.com/25495
Reviewed-by: Quentin Smith <quentin@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Quentin Smith <quentin@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/syscall_darwin_386.go

@@ -26,14 +26,21 @@ func NsecToTimeval(nsec int64) (tv Timeval) {
 }
 
 //sysnb	gettimeofday(tp *Timeval) (sec int32, usec int32, err error)
-func Gettimeofday(tv *Timeval) (err error) {
-	// The tv passed to gettimeofday must be non-nil
-	// but is otherwise unused. The answers come back
-	// in the two registers.
+func Gettimeofday(tv *Timeval) error {
+	// The tv passed to gettimeofday must be non-nil.
+	// Before macOS Sierra (10.12), tv was otherwise unused and
+	// the answers came back in the two registers.
+	// As of Sierra, gettimeofday return zeros and populates
+	// tv itself.
 	sec, usec, err := gettimeofday(tv)
-	tv.Sec = int32(sec)
-	tv.Usec = int32(usec)
-	return err
+	if err != nil {
+		return err
+	}
+	if sec != 0 || usec != 0 {
+		tv.Sec = int32(sec)
+		tv.Usec = int32(usec)
+	}
+	return nil
 }
 
 func SetKevent(k *Kevent_t, fd, mode, flags int) {

src/syscall/syscall_darwin_amd64.go

@@ -26,14 +26,21 @@ func NsecToTimeval(nsec int64) (tv Timeval) {
 }
 
 //sysnb	gettimeofday(tp *Timeval) (sec int64, usec int32, err error)
-func Gettimeofday(tv *Timeval) (err error) {
-	// The tv passed to gettimeofday must be non-nil
-	// but is otherwise unused. The answers come back
-	// in the two registers.
+func Gettimeofday(tv *Timeval) error {
+	// The tv passed to gettimeofday must be non-nil.
+	// Before macOS Sierra (10.12), tv was otherwise unused and
+	// the answers came back in the two registers.
+	// As of Sierra, gettimeofday return zeros and populates
+	// tv itself.
 	sec, usec, err := gettimeofday(tv)
-	tv.Sec = sec
-	tv.Usec = usec
-	return err
+	if err != nil {
+		return err
+	}
+	if sec != 0 || usec != 0 {
+		tv.Sec = sec
+		tv.Usec = usec
+	}
+	return nil
 }
 
 func SetKevent(k *Kevent_t, fd, mode, flags int) {

src/syscall/syscall_darwin_test.go

@@ -0,0 +1,23 @@
+// Copyright 2016 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.
+
+// +build darwin
+// +build amd64 386
+
+package syscall_test
+
+import (
+	"syscall"
+	"testing"
+)
+
+func TestDarwinGettimeofday(t *testing.T) {
+	tv := &syscall.Timeval{}
+	if err := syscall.Gettimeofday(tv); err != nil {
+		t.Fatal(err)
+	}
+	if tv.Sec == 0 && tv.Usec == 0 {
+		t.Fatal("Sec and Usec both zero")
+	}
+}