time: defer loading ZONEINFO until first time.LoadLocation call

A user application can now use os.Setenv("ZONEINFO", ..) becase the
value is no longer read on init of the time package.

Fixes #18619

Change-Id: Id8e303d67e6fb9c5d6ea9f969d8c94f6fff1bee3
Reviewed-on: https://go-review.googlesource.com/35639Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/time/export_test.go

@@ -18,6 +18,15 @@ func ForceUSPacificForTesting() {
 	localOnce.Do(initTestingZone)
 }
 
+func ZoneinfoForTesting() *string {
+	return zoneinfo
+}
+
+func ResetZoneinfoForTesting() {
+	zoneinfo = nil
+	zoneinfoOnce = sync.Once{}
+}
+
 var (
 	ForceZipFileForTesting = forceZipFileForTesting
 	ParseTimeZone          = parseTimeZone

src/time/zoneinfo.go

@@ -256,7 +256,8 @@ func (l *Location) lookupName(name string, unix int64) (offset int, isDST bool,
 // NOTE(rsc): Eventually we will need to accept the POSIX TZ environment
 // syntax too, but I don't feel like implementing it today.
 
-var zoneinfo, _ = syscall.Getenv("ZONEINFO")
+var zoneinfo *string
+var zoneinfoOnce sync.Once
 
 // LoadLocation returns the Location with the given name.
 //
@@ -279,8 +280,12 @@ func LoadLocation(name string) (*Location, error) {
 	if name == "Local" {
 		return Local, nil
 	}
-	if zoneinfo != "" {
-		if z, err := loadZoneFile(zoneinfo, name); err == nil {
+	zoneinfoOnce.Do(func() {
+		env, _ := syscall.Getenv("ZONEINFO")
+		zoneinfo = &env
+	})
+	if zoneinfo != nil && *zoneinfo != "" {
+		if z, err := loadZoneFile(*zoneinfo, name); err == nil {
 			z.name = name
 			return z, nil
 		}

src/time/zoneinfo_test.go

@@ -5,10 +5,36 @@
 package time_test
 
 import (
+	"fmt"
+	"os"
 	"testing"
 	"time"
 )
 
+func init() {
+	if time.ZoneinfoForTesting() != nil {
+		panic(fmt.Errorf("zoneinfo initialized before first LoadLocation"))
+	}
+}
+
+func TestEnvVarUsage(t *testing.T) {
+	time.ResetZoneinfoForTesting()
+
+	testZoneinfo := "foo.zip"
+	env := "ZONEINFO"
+
+	defer os.Setenv(env, os.Getenv(env))
+	os.Setenv(env, testZoneinfo)
+
+	// Result isn't important, we're testing the side effect of this command
+	time.LoadLocation("Asia/Jerusalem")
+	defer time.ResetZoneinfoForTesting()
+
+	if zoneinfo := time.ZoneinfoForTesting(); testZoneinfo != *zoneinfo {
+		t.Errorf("zoneinfo does not match env variable: got %q want %q", zoneinfo, testZoneinfo)
+	}
+}
+
 func TestVersion3(t *testing.T) {
 	time.ForceZipFileForTesting(true)
 	defer time.ForceZipFileForTesting(false)