database/sql: add NullTime

This matches NullBool, NullFloat64, and NullInt64.

Fixes #30305

Change-Id: I79bfcf04a3d43b965d2a3159b0ac22f3e8084a53
Reviewed-on: https://go-review.googlesource.com/c/go/+/170699
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/fakedb_test.go

@@ -1158,7 +1158,9 @@ func converterForType(typ string) driver.ValueConverter {
 		// TODO(coopernurse): add type-specific converter
 		return driver.Null{Converter: driver.DefaultParameterConverter}
 	case "datetime":
-		return driver.DefaultParameterConverter
+		return driver.NotNull{Converter: driver.DefaultParameterConverter}
+	case "nulldatetime":
+		return driver.Null{Converter: driver.DefaultParameterConverter}
 	case "any":
 		return anyTypeConverter{}
 	}

src/database/sql/sql.go

@@ -286,6 +286,32 @@ func (n NullBool) Value() (driver.Value, error) {
 	return n.Bool, nil
 }
 
+// NullTime represents a time.Time that may be null.
+// NullTime implements the Scanner interface so
+// it can be used as a scan destination, similar to NullString.
+type NullTime struct {
+	Time  time.Time
+	Valid bool // Valid is true if Time is not NULL
+}
+
+// Scan implements the Scanner interface.
+func (n *NullTime) Scan(value interface{}) error {
+	if value == nil {
+		n.Time, n.Valid = time.Time{}, false
+		return nil
+	}
+	n.Valid = true
+	return convertAssign(&n.Time, value)
+}
+
+// Value implements the driver Valuer interface.
+func (n NullTime) Value() (driver.Value, error) {
+	if !n.Valid {
+		return nil, nil
+	}
+	return n.Time, nil
+}
+
 // Scanner is an interface used by Scan.
 type Scanner interface {
 	// Scan assigns a value from a database driver.

src/database/sql/sql_test.go

@@ -1695,6 +1695,21 @@ func TestNullBoolParam(t *testing.T) {
 	nullTestRun(t, spec)
 }
 
+func TestNullTimeParam(t *testing.T) {
+	t0 := time.Time{}
+	t1 := time.Date(2000, 1, 1, 8, 9, 10, 11, time.UTC)
+	t2 := time.Date(2010, 1, 1, 8, 9, 10, 11, time.UTC)
+	spec := nullTestSpec{"nulldatetime", "datetime", [6]nullTestRow{
+		{NullTime{t1, true}, t2, NullTime{t1, true}},
+		{NullTime{t1, false}, t2, NullTime{t0, false}},
+		{t1, t2, NullTime{t1, true}},
+		{NullTime{t1, true}, t2, NullTime{t1, true}},
+		{NullTime{t1, false}, t2, NullTime{t0, false}},
+		{t2, NullTime{t1, false}, nil},
+	}}
+	nullTestRun(t, spec)
+}
+
 func nullTestRun(t *testing.T, spec nullTestSpec) {
 	db := newTestDB(t, "")
 	defer closeDB(t, db)