[release-branch.go1.19] time: fix timezone lookup logic for non-DST zones

This change fixes time.LoadLocationFromTZData and time.Location.lookup logic if the given time is after the last transition and the extend string doesn't have the DST rule.

For #58682
Fixes #59074

Change-Id: Ie34a6d658d14c2b33098b29ab83c041ef0d34266
GitHub-Last-Rev: f6681eb44c0ea0772004e56eb68fcbd9023d971e
GitHub-Pull-Request: golang/go#58684
Reviewed-on: https://go-review.googlesource.com/c/go/+/471020
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
(cherry picked from commit 90dde5dec1126ddf2236730ec57511ced56a512d)
Reviewed-on: https://go-review.googlesource.com/c/go/+/478657
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Heschi Kreinick <heschi@google.com>

4 files changed, 24 insertions, 13 deletions

diff --git a/src/time/time_test.go b/src/time/time_test.go
index 6fde5f6470..1b7632830a 100644
--- a/src/time/time_test.go
+++ b/src/time/time_test.go
@@ -1748,16 +1748,26 @@ func TestZoneBounds(t *testing.T) {
 		5: {Date(1991, September, 14, 17, 0, 0, 0, loc), boundTwo, boundThree},
 		6: {Date(1991, September, 15, 0, 50, 0, 0, loc), boundTwo, boundThree},
 
+		// The ZoneBounds of a "Asia/Shanghai" after the last transition (Standard Time)
+		7:  {boundThree, boundThree, Time{}},
+		8:  {Date(1991, December, 15, 1, 50, 0, 0, loc), boundThree, Time{}},
+		9:  {Date(1992, April, 13, 17, 50, 0, 0, loc), boundThree, Time{}},
+		10: {Date(1992, April, 13, 18, 0, 0, 0, loc), boundThree, Time{}},
+		11: {Date(1992, April, 14, 1, 50, 0, 0, loc), boundThree, Time{}},
+		12: {Date(1992, September, 14, 16, 50, 0, 0, loc), boundThree, Time{}},
+		13: {Date(1992, September, 14, 17, 0, 0, 0, loc), boundThree, Time{}},
+		14: {Date(1992, September, 15, 0, 50, 0, 0, loc), boundThree, Time{}},
+
 		// The ZoneBounds of a local time would return two local Time.
 		// Note: We preloaded "America/Los_Angeles" as time.Local for testing
-		7:  {makeLocalTime(0), makeLocalTime(-5756400), makeLocalTime(9972000)},
-		8:  {makeLocalTime(1221681866), makeLocalTime(1205056800), makeLocalTime(1225616400)},
-		9:  {makeLocalTime(2152173599), makeLocalTime(2145916800), makeLocalTime(2152173600)},
-		10: {makeLocalTime(2152173600), makeLocalTime(2152173600), makeLocalTime(2172733200)},
-		11: {makeLocalTime(2152173601), makeLocalTime(2152173600), makeLocalTime(2172733200)},
-		12: {makeLocalTime(2159200800), makeLocalTime(2152173600), makeLocalTime(2172733200)},
-		13: {makeLocalTime(2172733199), makeLocalTime(2152173600), makeLocalTime(2172733200)},
-		14: {makeLocalTime(2172733200), makeLocalTime(2172733200), makeLocalTime(2177452800)},
+		15: {makeLocalTime(0), makeLocalTime(-5756400), makeLocalTime(9972000)},
+		16: {makeLocalTime(1221681866), makeLocalTime(1205056800), makeLocalTime(1225616400)},
+		17: {makeLocalTime(2152173599), makeLocalTime(2145916800), makeLocalTime(2152173600)},
+		18: {makeLocalTime(2152173600), makeLocalTime(2152173600), makeLocalTime(2172733200)},
+		19: {makeLocalTime(2152173601), makeLocalTime(2152173600), makeLocalTime(2172733200)},
+		20: {makeLocalTime(2159200800), makeLocalTime(2152173600), makeLocalTime(2172733200)},
+		21: {makeLocalTime(2172733199), makeLocalTime(2152173600), makeLocalTime(2172733200)},
+		22: {makeLocalTime(2172733200), makeLocalTime(2172733200), makeLocalTime(2177452800)},
 	}
 	for i, tt := range realTests {
 		start, end := tt.giveTime.ZoneBounds()
diff --git a/src/time/zoneinfo.go b/src/time/zoneinfo.go
index b3313583d8..eecee836b2 100644
--- a/src/time/zoneinfo.go
+++ b/src/time/zoneinfo.go
@@ -182,7 +182,7 @@ func (l *Location) lookup(sec int64) (name string, offset int, start, end int64,
 	// If we're at the end of the known zone transitions,
 	// try the extend string.
 	if lo == len(tx)-1 && l.extend != "" {
-		if ename, eoffset, estart, eend, eisDST, ok := tzset(l.extend, end, sec); ok {
+		if ename, eoffset, estart, eend, eisDST, ok := tzset(l.extend, start, sec); ok {
 			return ename, eoffset, estart, eend, eisDST
 		}
 	}
@@ -243,12 +243,12 @@ func (l *Location) firstZoneUsed() bool {
 }
 
 // tzset takes a timezone string like the one found in the TZ environment
-// variable, the end of the last time zone transition expressed as seconds
+// variable, the time of the last time zone transition expressed as seconds
 // since January 1, 1970 00:00:00 UTC, and a time expressed the same way.
 // We call this a tzset string since in C the function tzset reads TZ.
 // The return values are as for lookup, plus ok which reports whether the
 // parse succeeded.
-func tzset(s string, initEnd, sec int64) (name string, offset int, start, end int64, isDST, ok bool) {
+func tzset(s string, lastTxSec, sec int64) (name string, offset int, start, end int64, isDST, ok bool) {
 	var (
 		stdName, dstName     string
 		stdOffset, dstOffset int
@@ -269,7 +269,7 @@ func tzset(s string, initEnd, sec int64) (name string, offset int, start, end in
 
 	if len(s) == 0 || s[0] == ',' {
 		// No daylight savings time.
-		return stdName, stdOffset, initEnd, omega, false, true
+		return stdName, stdOffset, lastTxSec, omega, false, true
 	}
 
 	dstName, s, ok = tzsetName(s)
diff --git a/src/time/zoneinfo_read.go b/src/time/zoneinfo_read.go
index 90814ad36a..1c155dc59d 100644
--- a/src/time/zoneinfo_read.go
+++ b/src/time/zoneinfo_read.go
@@ -329,7 +329,7 @@ func LoadLocationFromTZData(name string, data []byte) (*Location, error) {
 			} else if l.extend != "" {
 				// If we're at the end of the known zone transitions,
 				// try the extend string.
-				if name, offset, estart, eend, isDST, ok := tzset(l.extend, l.cacheEnd, sec); ok {
+				if name, offset, estart, eend, isDST, ok := tzset(l.extend, l.cacheStart, sec); ok {
 					l.cacheStart = estart
 					l.cacheEnd = eend
 					// Find the zone that is returned by tzset to avoid allocation if possible.
diff --git a/src/time/zoneinfo_test.go b/src/time/zoneinfo_test.go
index 243ff8ebde..8cd37b5e27 100644
--- a/src/time/zoneinfo_test.go
+++ b/src/time/zoneinfo_test.go
@@ -271,6 +271,7 @@ func TestTzset(t *testing.T) {
 		{"PST8PDT,M3.2.0,M11.1.0", 0, 2172733199, "PDT", -7 * 60 * 60, 2152173600, 2172733200, true, true},
 		{"PST8PDT,M3.2.0,M11.1.0", 0, 2172733200, "PST", -8 * 60 * 60, 2172733200, 2177452800, false, true},
 		{"PST8PDT,M3.2.0,M11.1.0", 0, 2172733201, "PST", -8 * 60 * 60, 2172733200, 2177452800, false, true},
+		{"KST-9", 592333200, 1677246697, "KST", 9 * 60 * 60, 592333200, 1<<63 - 1, false, true},
 	} {
 		name, off, start, end, isDST, ok := time.Tzset(test.inStr, test.inEnd, test.inSec)
 		if name != test.name || off != test.off || start != test.start || end != test.end || isDST != test.isDST || ok != test.ok {