src/time: Support LMT in MarshalBinary, UnmarshalBinary

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

@@ -1138,16 +1138,31 @@ func (t Time) UnixNano() int64 {
 	return (t.unixSec())*1e9 + int64(t.nsec())
 }
 
-const timeBinaryVersion byte = 1
+const (
+	timeBinaryVersionV1 byte = iota + 1 // For general situation
+	timeBinaryVersionV2                 // For LMT only
+)
+
+var timeBinaryVersion byte
 
 // 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
 
 	if t.Location() == UTC {
 		offsetMin = -1
+	} else if name, offset := t.Zone(); name == "LMT" {
+		timeBinaryVersion = timeBinaryVersionV2
+		offsetSec = int16(offset % 60)
+
+		offset /= 60
+		if offset < -32768 || offset == -1 || offset > 32767 {
+			return nil, errors.New("Time.MarshalBinary: unexpected zone offset")
+		}
+		offsetMin = int16(offset)
 	} else {
-		_, offset := t.Zone()
+		timeBinaryVersion = timeBinaryVersionV1
 		if offset%60 != 0 {
 			return nil, errors.New("Time.MarshalBinary: zone offset has fractional minute")
 		}
@@ -1160,22 +1175,47 @@ func (t Time) MarshalBinary() ([]byte, error) {
 
 	sec := t.sec()
 	nsec := t.nsec()
-	enc := []byte{
-		timeBinaryVersion, // byte 0 : version
-		byte(sec >> 56),   // bytes 1-8: seconds
-		byte(sec >> 48),
-		byte(sec >> 40),
-		byte(sec >> 32),
-		byte(sec >> 24),
-		byte(sec >> 16),
-		byte(sec >> 8),
-		byte(sec),
-		byte(nsec >> 24), // bytes 9-12: nanoseconds
-		byte(nsec >> 16),
-		byte(nsec >> 8),
-		byte(nsec),
-		byte(offsetMin >> 8), // bytes 13-14: zone offset in minutes
-		byte(offsetMin),
+
+	var enc []byte
+	if timeBinaryVersion == timeBinaryVersionV1 {
+		enc = []byte{
+			timeBinaryVersion, // byte 0 : version
+			byte(sec >> 56),   // bytes 1-8: seconds
+			byte(sec >> 48),
+			byte(sec >> 40),
+			byte(sec >> 32),
+			byte(sec >> 24),
+			byte(sec >> 16),
+			byte(sec >> 8),
+			byte(sec),
+			byte(nsec >> 24), // bytes 9-12: nanoseconds
+			byte(nsec >> 16),
+			byte(nsec >> 8),
+			byte(nsec),
+			byte(offsetMin >> 8), // bytes 13-14: zone offset in minutes
+			byte(offsetMin),
+		}
+	} else {
+		// For timeBinaryVersionV2
+		enc = []byte{
+			timeBinaryVersion, // byte 0 : version
+			byte(sec >> 56),   // bytes 1-8: seconds
+			byte(sec >> 48),
+			byte(sec >> 40),
+			byte(sec >> 32),
+			byte(sec >> 24),
+			byte(sec >> 16),
+			byte(sec >> 8),
+			byte(sec),
+			byte(nsec >> 24), // bytes 9-12: nanoseconds
+			byte(nsec >> 16),
+			byte(nsec >> 8),
+			byte(nsec),
+			byte(offsetMin >> 8), // bytes 13-14: zone offset in minutes
+			byte(offsetMin),
+			byte(offsetSec >> 8), // bytes 15-16: zone offset in seconds which sum with offsetMin can get the total offset time
+			byte(offsetSec),
+		}
 	}
 
 	return enc, nil
@@ -1191,8 +1231,9 @@ func (t *Time) UnmarshalBinary(data []byte) error {
 	if buf[0] != timeBinaryVersion {
 		return errors.New("Time.UnmarshalBinary: unsupported version")
 	}
+	timeVersion := buf[0]
 
-	if len(buf) != /*version*/ 1+ /*sec*/ 8+ /*nsec*/ 4+ /*zone offset*/ 2 {
+	if (len(buf) != /*version*/ 1+ /*sec*/ 8+ /*nsec*/ 4+ /*zone offset*/ 2) && (len(buf) != /*version*/ 1+ /*sec*/ 8+ /*nsec*/ 4+ /*zone offset*/ 4) {
 		return errors.New("Time.UnmarshalBinary: invalid length")
 	}
 
@@ -1204,7 +1245,13 @@ 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

@@ -17,6 +17,7 @@ import (
 	"sync"
 	"testing"
 	"testing/quick"
+	"time"
 	. "time"
 )
 
@@ -1394,6 +1395,32 @@ func TestMarshalBinaryZeroTime(t *testing.T) {
 	}
 }
 
+func TestMarshalBinaryVersion2(t *testing.T) {
+	t0, err := time.Parse(time.RFC3339, "1880-01-01T00:00:00Z")
+	if err != nil {
+		t.Errorf("Failed to parse time, error = %v", err)
+	}
+	loc, err := time.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.Time{}
+	err = t2.UnmarshalBinary(b)
+	if err != nil {
+		t.Errorf("Failed to Unmarshal, error = %v", err)
+	}
+
+	if !(t0.Equal(t1) && t1.Equal(t2)) {
+		t.Errorf("The result after Unmarshal is not matched")
+	}
+}
+
 // 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 {