time: support colon at start of TZ value

According to POSIX, there are three formats for TZ variable. When
it refers to timezone file, it should starts with a colon. This commit
removes the colon if it exists, so that it keeps compatible with both
the spec and the old behavior.

Change-Id: I30cfeaea530d24e174de309952338cb1146694a5
GitHub-Last-Rev: 11d83d11ca
GitHub-Pull-Request: golang/go#27570
Reviewed-on: https://go-review.googlesource.com/c/go/+/134217
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Dmitri Shuralyov <dmitshur@golang.org>

src/time/zoneinfo_unix.go

@@ -29,7 +29,9 @@ func initLocal() {
 	// consult $TZ to find the time zone to use.
 	// no $TZ means use the system default /etc/localtime.
 	// $TZ="" means use UTC.
-	// $TZ="foo" means use /usr/share/zoneinfo/foo.
+	// $TZ="foo" or $TZ=":foo" if foo is an absolute path, then the file pointed
+	// by foo will be used to initialize timezone; otherwise, file
+	// /usr/share/zoneinfo/foo will be used.
 
 	tz, ok := syscall.Getenv("TZ")
 	switch {
@@ -40,10 +42,25 @@ func initLocal() {
 			localLoc.name = "Local"
 			return
 		}
-	case tz != "" && tz != "UTC":
-		if z, err := loadLocation(tz, zoneSources); err == nil {
-			localLoc = *z
-			return
+	case tz != "":
+		if tz[0] == ':' {
+			tz = tz[1:]
+		}
+		if tz != "" && tz[0] == '/' {
+			if z, err := loadLocation(tz, []string{""}); err == nil {
+				localLoc = *z
+				if tz == "/etc/localtime" {
+					localLoc.name = "Local"
+				} else {
+					localLoc.name = tz
+				}
+				return
+			}
+		} else if tz != "" && tz != "UTC" {
+			if z, err := loadLocation(tz, zoneSources); err == nil {
+				localLoc = *z
+				return
+			}
 		}
 	}
 

src/time/zoneinfo_unix_test.go

@@ -0,0 +1,90 @@
+// Copyright 2020 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 aix darwin,amd64 dragonfly freebsd linux,!android netbsd openbsd solaris
+
+package time_test
+
+import (
+	"os"
+	"testing"
+	"time"
+)
+
+func TestEnvTZUsage(t *testing.T) {
+	const env = "TZ"
+	tz, ok := os.LookupEnv(env)
+	if !ok {
+		defer os.Unsetenv(env)
+	} else {
+		defer os.Setenv(env, tz)
+	}
+	defer time.ForceUSPacificForTesting()
+
+	localZoneName := "Local"
+	// The file may not exist.
+	if _, err := os.Stat("/etc/localtime"); os.IsNotExist(err) {
+		localZoneName = "UTC"
+	}
+
+	cases := []struct {
+		nilFlag bool
+		tz      string
+		local   string
+	}{
+		// no $TZ means use the system default /etc/localtime.
+		{true, "", localZoneName},
+		// $TZ="" means use UTC.
+		{false, "", "UTC"},
+		{false, ":", "UTC"},
+		{false, "Asia/Shanghai", "Asia/Shanghai"},
+		{false, ":Asia/Shanghai", "Asia/Shanghai"},
+		{false, "/etc/localtime", localZoneName},
+		{false, ":/etc/localtime", localZoneName},
+	}
+
+	for _, c := range cases {
+		time.ResetLocalOnceForTest()
+		if c.nilFlag {
+			os.Unsetenv(env)
+		} else {
+			os.Setenv(env, c.tz)
+		}
+		if time.Local.String() != c.local {
+			t.Errorf("invalid Local location name for %q: got %q want %q", c.tz, time.Local, c.local)
+		}
+	}
+
+	time.ResetLocalOnceForTest()
+	// The file may not exist on Solaris 2 and IRIX 6.
+	path := "/usr/share/zoneinfo/Asia/Shanghai"
+	os.Setenv(env, path)
+	if _, err := os.Stat(path); os.IsNotExist(err) {
+		if time.Local.String() != "UTC" {
+			t.Errorf(`invalid path should fallback to UTC: got %q want "UTC"`, time.Local)
+		}
+		return
+	}
+	if time.Local.String() != path {
+		t.Errorf(`custom path should lead to path itself: got %q want %q`, time.Local, path)
+	}
+
+	timeInUTC := time.Date(2009, 1, 1, 12, 0, 0, 0, time.UTC)
+	sameTimeInShanghai := time.Date(2009, 1, 1, 20, 0, 0, 0, time.Local)
+	if !timeInUTC.Equal(sameTimeInShanghai) {
+		t.Errorf("invalid timezone: got %q want %q", timeInUTC, sameTimeInShanghai)
+	}
+
+	time.ResetLocalOnceForTest()
+	os.Setenv(env, ":"+path)
+	if time.Local.String() != path {
+		t.Errorf(`custom path should lead to path itself: got %q want %q`, time.Local, path)
+	}
+
+	time.ResetLocalOnceForTest()
+	os.Setenv(env, path[:len(path)-1])
+	if time.Local.String() != "UTC" {
+		t.Errorf(`invalid path should fallback to UTC: got %q want "UTC"`, time.Local)
+	}
+}