time: fix race

Fixes #4622

R=golang-dev, dave, dvyukov
CC=golang-dev
https://golang.org/cl/7103046

src/pkg/time/export_test.go

+// Copyright 2013 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 time
+
+import (
+	"sync"
+)
+
+func ResetLocalOnceForTest() {
+	localOnce = sync.Once{}
+	localLoc = Location{}
+}
+
+func ForceUSPacificForTesting() {
+	ResetLocalOnceForTest()
+	localOnce.Do(initTestingZone)
+}

src/pkg/time/internal_test.go

@@ -6,7 +6,7 @@ package time
 
 func init() {
 	// force US/Pacific for time zone tests
-	localOnce.Do(initTestingZone)
+	ForceUSPacificForTesting()
 }
 
 var Interrupt = interrupt

src/pkg/time/time.go

@@ -261,8 +261,8 @@ func (t Time) abs() uint64 {
 // extracting both return values from a single zone lookup.
 func (t Time) locabs() (name string, offset int, abs uint64) {
 	l := t.loc
-	if l == nil {
-		l = &utcLoc
+	if l == nil || l == &localLoc {
+		l = l.get()
 	}
 	// Avoid function call if we hit the local time cache.
 	sec := t.sec + internalToUnix

src/pkg/time/time_test.go

@@ -1227,6 +1227,22 @@ func TestParseDurationRoundTrip(t *testing.T) {
 	}
 }
 
+// golang.org/issue/4622
+func TestLocationRace(t *testing.T) {
+	ResetLocalOnceForTest() // reset the Once to trigger the race
+
+	c := make(chan string, 1)
+	go func() {
+		c <- Now().String()
+	}()
+	Now().String()
+	<-c
+	Sleep(100 * Millisecond)
+
+	// Back to Los Angeles for subsequent tests:
+	ForceUSPacificForTesting()
+}
+
 var (
 	t Time
 	u int64