database/sql: check if src is nil before converting to string

A nil src (NULL database value) will result in a "nil" string,
which will never parse correctly in a ParseInt or similar
numeric conversion. The resulting error is confusing. Check
for a nil src prior to converting the value to string
if the resulting string will be parsed after that.

Closes #31274

Change-Id: I90f12cceff00fbbfdd3e343b04fa7e2596390e6d
Reviewed-on: https://go-review.googlesource.com/c/go/+/174177
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/convert.go

@@ -420,6 +420,9 @@ func convertAssignRows(dest, src interface{}, rows *Rows) error {
 		dv.Set(reflect.New(dv.Type().Elem()))
 		return convertAssignRows(dv.Interface(), src, rows)
 	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		if src == nil {
+			return fmt.Errorf("converting NULL to %s is unsupported", dv.Kind())
+		}
 		s := asString(src)
 		i64, err := strconv.ParseInt(s, 10, dv.Type().Bits())
 		if err != nil {
@@ -429,6 +432,9 @@ func convertAssignRows(dest, src interface{}, rows *Rows) error {
 		dv.SetInt(i64)
 		return nil
 	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64:
+		if src == nil {
+			return fmt.Errorf("converting NULL to %s is unsupported", dv.Kind())
+		}
 		s := asString(src)
 		u64, err := strconv.ParseUint(s, 10, dv.Type().Bits())
 		if err != nil {
@@ -438,6 +444,9 @@ func convertAssignRows(dest, src interface{}, rows *Rows) error {
 		dv.SetUint(u64)
 		return nil
 	case reflect.Float32, reflect.Float64:
+		if src == nil {
+			return fmt.Errorf("converting NULL to %s is unsupported", dv.Kind())
+		}
 		s := asString(src)
 		f64, err := strconv.ParseFloat(s, dv.Type().Bits())
 		if err != nil {
@@ -447,6 +456,9 @@ func convertAssignRows(dest, src interface{}, rows *Rows) error {
 		dv.SetFloat(f64)
 		return nil
 	case reflect.String:
+		if src == nil {
+			return fmt.Errorf("converting NULL to %s is unsupported", dv.Kind())
+		}
 		switch v := src.(type) {
 		case string:
 			dv.SetString(v)

src/database/sql/sql_test.go

@@ -1401,7 +1401,7 @@ func TestInvalidNilValues(t *testing.T) {
 		{
 			name:          "int",
 			input:         &date2,
-			expectedError: `sql: Scan error on column index 0, name "bdate": converting driver.Value type <nil> ("<nil>") to a int: invalid syntax`,
+			expectedError: `sql: Scan error on column index 0, name "bdate": converting NULL to int is unsupported`,
 		},
 	}