database/sql: treat pointers as nullable types like encoding/json

- convert from nil pointers to the nil interface{}
- dereference non-nil pointers
- convert from nil interface{}s to nil pointers
- allocate pointers for non-nil interface{}s
- tests for all of the above

R=golang-dev, bradfitz, rsc, rogpeppe
CC=golang-dev
https://golang.org/cl/5630052

src/pkg/database/sql/convert.go

@@ -110,6 +110,14 @@ func convertAssign(dest, src interface{}) error {
 	}
 
 	switch dv.Kind() {
+	case reflect.Ptr:
+		if src == nil {
+			dv.Set(reflect.Zero(dv.Type()))
+			return nil
+		} else {
+			dv.Set(reflect.New(dv.Type().Elem()))
+			return convertAssign(dv.Interface(), src)
+		}
 	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
 		s := asString(src)
 		i64, err := strconv.ParseInt(s, 10, dv.Type().Bits())

src/pkg/database/sql/convert_test.go

@@ -13,6 +13,7 @@ import (
 )
 
 var someTime = time.Unix(123, 0)
+var answer int64 = 42
 
 type conversionTest struct {
 	s, d interface{} // source and destination
@@ -27,6 +28,8 @@ type conversionTest struct {
 	wantbool  bool // used if d is of type *bool
 	wanterr   string
 	wantiface interface{}
+	wantptr   *int64 // if non-nil, *d's pointed value must be equal to *wantptr
+	wantnil   bool   // if true, *d must be *int64(nil)
 }
 
 // Target variables for scanning into.
@@ -42,6 +45,7 @@ var (
 	scanf32    float32
 	scanf64    float64
 	scantime   time.Time
+	scanptr    *int64
 	scaniface  interface{}
 )
 
@@ -98,6 +102,10 @@ var conversionTests = []conversionTest{
 	{s: "1.5", d: &scanf32, wantf32: float32(1.5)},
 	{s: "1.5", d: &scanf64, wantf64: float64(1.5)},
 
+	// Pointers
+	{s: interface{}(nil), d: &scanptr, wantnil: true},
+	{s: int64(42), d: &scanptr, wantptr: &answer},
+
 	// To interface{}
 	{s: float64(1.5), d: &scaniface, wantiface: float64(1.5)},
 	{s: int64(1), d: &scaniface, wantiface: int64(1)},
@@ -107,6 +115,10 @@ var conversionTests = []conversionTest{
 	{s: nil, d: &scaniface},
 }
 
+func intPtrValue(intptr interface{}) interface{} {
+	return reflect.Indirect(reflect.Indirect(reflect.ValueOf(intptr))).Int()
+}
+
 func intValue(intptr interface{}) int64 {
 	return reflect.Indirect(reflect.ValueOf(intptr)).Int()
 }
@@ -162,6 +174,16 @@ func TestConversions(t *testing.T) {
 		if !ct.wanttime.IsZero() && !ct.wanttime.Equal(timeValue(ct.d)) {
 			errf("want time %v, got %v", ct.wanttime, timeValue(ct.d))
 		}
+		if ct.wantnil && *ct.d.(**int64) != nil {
+			errf("want nil, got %v", intPtrValue(ct.d))
+		}
+		if ct.wantptr != nil {
+			if *ct.d.(**int64) == nil {
+				errf("want pointer to %v, got nil", *ct.wantptr)
+			} else if *ct.wantptr != intPtrValue(ct.d) {
+				errf("want pointer to %v, got %v", *ct.wantptr, intPtrValue(ct.d))
+			}
+		}
 		if ifptr, ok := ct.d.(*interface{}); ok {
 			if !reflect.DeepEqual(ct.wantiface, scaniface) {
 				errf("want interface %#v, got %#v", ct.wantiface, scaniface)

src/pkg/database/sql/driver/types.go

@@ -248,6 +248,13 @@ func (defaultConverter) ConvertValue(v interface{}) (interface{}, error) {
 
 	rv := reflect.ValueOf(v)
 	switch rv.Kind() {
+	case reflect.Ptr:
+		// indirect pointers
+		if rv.IsNil() {
+			return nil, nil
+		} else {
+			return defaultConverter{}.ConvertValue(rv.Elem().Interface())
+		}
 	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
 		return rv.Int(), nil
 	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32:

src/pkg/database/sql/driver/types_test.go

@@ -18,6 +18,7 @@ type valueConverterTest struct {
 }
 
 var now = time.Now()
+var answer int64 = 42
 
 var valueConverterTests = []valueConverterTest{
 	{Bool, "true", true, ""},
@@ -37,6 +38,9 @@ var valueConverterTests = []valueConverterTest{
 	{c: Bool, in: "foo", err: "sql/driver: couldn't convert \"foo\" into type bool"},
 	{c: Bool, in: 2, err: "sql/driver: couldn't convert 2 into type bool"},
 	{DefaultParameterConverter, now, now, ""},
+	{DefaultParameterConverter, (*int64)(nil), nil, ""},
+	{DefaultParameterConverter, &answer, answer, ""},
+	{DefaultParameterConverter, &now, now, ""},
 }
 
 func TestValueConverters(t *testing.T) {