time: support fractional timezone minutes in MarshalBinary

If the time is in 'LMT' and has fractional minute, then
`MarshalBinary()` and `UnmarshalBinary()` will encode/decode the time
in `timeBinaryVersionV2` in which the fractional minute is at
bit 15 and 16, and presented in seconds.

Fixes #39616

Author: howjmay

src/time/time.go

@@ -1163,31 +1163,43 @@ func (t Time) UnixNano() int64 {
 	return (t.unixSec())*1e9 + int64(t.nsec())
 }
 
-const timeBinaryVersion byte = 1
+const (
+	timeBinaryVersionV1 byte = iota // For general situation
+	timeBinaryVersionV2             // For LMT only
+)
 
 // MarshalBinary implements the encoding.BinaryMarshaler interface.
 func (t Time) MarshalBinary() ([]byte, error) {
 	var offsetMin int16 // minutes east of UTC. -1 is UTC.
+	var offsetSec int16
+	var version byte = timeBinaryVersionV1
 
 	if t.Location() == UTC {
 		offsetMin = -1
 	} else {
 		_, offset := t.Zone()
 		if offset%60 != 0 {
-			return nil, errors.New("Time.MarshalBinary: zone offset has fractional minute")
+			version = timeBinaryVersionV2
+			offsetSec = int16(offset % 60)
+		} else {
+			if offset%60 != 0 {
+				return nil, errors.New("Time.MarshalBinary: zone offset has fractional minute")
+			}
 		}
+
 		offset /= 60
 		if offset < -32768 || offset == -1 || offset > 32767 {
 			return nil, errors.New("Time.MarshalBinary: unexpected zone offset")
 		}
+
 		offsetMin = int16(offset)
 	}
 
 	sec := t.sec()
 	nsec := t.nsec()
 	enc := []byte{
-		timeBinaryVersion, // byte 0 : version
-		byte(sec >> 56),   // bytes 1-8: seconds
+		version,         // byte 0 : version
+		byte(sec >> 56), // bytes 1-8: seconds
 		byte(sec >> 48),
 		byte(sec >> 40),
 		byte(sec >> 32),
@@ -1202,6 +1214,9 @@ func (t Time) MarshalBinary() ([]byte, error) {
 		byte(offsetMin >> 8), // bytes 13-14: zone offset in minutes
 		byte(offsetMin),
 	}
+	if version == timeBinaryVersionV2 {
+		enc = append(enc, byte(offsetSec>>8), byte(offsetSec))
+	}
 
 	return enc, nil
 }
@@ -1213,12 +1228,19 @@ func (t *Time) UnmarshalBinary(data []byte) error {
 		return errors.New("Time.UnmarshalBinary: no data")
 	}
 
-	if buf[0] != timeBinaryVersion {
+	if buf[0] != timeBinaryVersionV1 && buf[0] != timeBinaryVersionV2 {
 		return errors.New("Time.UnmarshalBinary: unsupported version")
 	}
 
-	if len(buf) != /*version*/ 1+ /*sec*/ 8+ /*nsec*/ 4+ /*zone offset*/ 2 {
-		return errors.New("Time.UnmarshalBinary: invalid length")
+	timeVersion := buf[0]
+	if timeVersion == timeBinaryVersionV1 {
+		if len(buf) != /*version*/ 1+ /*sec*/ 8+ /*nsec*/ 4+ /*zone offset*/ 2 {
+			return errors.New("Time.UnmarshalBinary: invalid length")
+		}
+	} else if timeVersion == timeBinaryVersionV2 {
+		if len(buf) != /*version*/ 1+ /*sec*/ 8+ /*nsec*/ 4+ /*zone offset*/ 4 {
+			return errors.New("Time.UnmarshalBinary: invalid length")
+		}
 	}
 
 	buf = buf[1:]
@@ -1229,7 +1251,13 @@ func (t *Time) UnmarshalBinary(data []byte) error {
 	nsec := int32(buf[3]) | int32(buf[2])<<8 | int32(buf[1])<<16 | int32(buf[0])<<24
 
 	buf = buf[4:]
-	offset := int(int16(buf[1])|int16(buf[0])<<8) * 60
+	var offset int
+	if timeVersion == timeBinaryVersionV2 {
+		offset = int(int16(buf[1])|int16(buf[0])<<8)*60 + int(int16(buf[3])|int16(buf[2])<<8)
+	} else {
+		// timeVersion is equal to timeBinaryVersionV1
+		offset = int(int16(buf[1])|int16(buf[0])<<8) * 60
+	}
 
 	*t = Time{}
 	t.wall = uint64(nsec)

src/time/time_test.go

@@ -745,7 +745,7 @@ var invalidEncodingTests = []struct {
 	want  string
 }{
 	{[]byte{}, "Time.UnmarshalBinary: no data"},
-	{[]byte{0, 2, 3}, "Time.UnmarshalBinary: unsupported version"},
+	{[]byte{3, 2, 3, 3, 2, 3, 3, 2, 3, 3, 2, 3, 3, 2, 3}, "Time.UnmarshalBinary: unsupported version"},
 	{[]byte{1, 2, 3}, "Time.UnmarshalBinary: invalid length"},
 }
 
@@ -767,7 +767,6 @@ var notEncodableTimes = []struct {
 	time Time
 	want string
 }{
-	{Date(0, 1, 2, 3, 4, 5, 6, FixedZone("", 1)), "Time.MarshalBinary: zone offset has fractional minute"},
 	{Date(0, 1, 2, 3, 4, 5, 6, FixedZone("", -1*60)), "Time.MarshalBinary: unexpected zone offset"},
 	{Date(0, 1, 2, 3, 4, 5, 6, FixedZone("", -32769*60)), "Time.MarshalBinary: unexpected zone offset"},
 	{Date(0, 1, 2, 3, 4, 5, 6, FixedZone("", 32768*60)), "Time.MarshalBinary: unexpected zone offset"},
@@ -1437,6 +1436,37 @@ func TestMarshalBinaryZeroTime(t *testing.T) {
 	}
 }
 
+func TestMarshalBinaryVersion2(t *testing.T) {
+	t0, err := Parse(RFC3339, "1880-01-01T00:00:00Z")
+	if err != nil {
+		t.Errorf("Failed to parse time, error = %v", err)
+	}
+	loc, err := LoadLocation("US/Eastern")
+	if err != nil {
+		t.Errorf("Failed to load location, error = %v", err)
+	}
+	t1 := t0.In(loc)
+	b, err := t1.MarshalBinary()
+	if err != nil {
+		t.Errorf("Failed to Marshal, error = %v", err)
+	}
+
+	t2 := Time{}
+	err = t2.UnmarshalBinary(b)
+	if err != nil {
+		t.Errorf("Failed to Unmarshal, error = %v", err)
+	}
+
+	if !(t0.Equal(t1) && t1.Equal(t2)) {
+		if !t0.Equal(t1) {
+			t.Errorf("The result t1: %+v after Marshal is not matched orignal t0: %+v", t1, t0)
+		}
+		if !t1.Equal(t2) {
+			t.Errorf("The result t2: %+v after Unmarshal is not matched orignal t1: %+v", t2, t1)
+		}
+	}
+}
+
 // Issue 17720: Zero value of time.Month fails to print
 func TestZeroMonthString(t *testing.T) {
 	if got, want := Month(0).String(), "%!Month(0)"; got != want {