database/sql: accept nil pointers to Valuers implemented on value receivers

The driver.Valuer interface lets types map their Go representation to
a suitable database/sql/driver.Value.

If a user defines the Value method with a value receiver, such as:

    type MyStr string

    func (s MyStr) Value() (driver.Value, error) {
        return strings.ToUpper(string(s)), nil
    }

Then they can't use (*MyStr)(nil) as an argument to an SQL call via
database/sql, because *MyStr also implements driver.Value, but via a
compiler-generated wrapper which checks whether the pointer is nil and
panics if so.

We now accept (*MyStr)(nil) and map it to "nil" (an SQL "NULL")
if the Valuer method is implemented on MyStr instead of *MyStr.

If a user implements the driver.Value interface with a pointer
receiver, they retain full control of what nil means:

    type MyStr string

    func (s *MyStr) Value() (driver.Value, error) {
        if s == nil {
            return "missing MyStr", nil
        }
        return strings.ToUpper(string(*s)), nil
    }

Adds tests for both cases.

Fixes #8415

Change-Id: I897d609d80d46e2354d2669a8a3e090688eee3ad
Reviewed-on: https://go-review.googlesource.com/31259
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

1 files changed, 24 insertions, 2 deletions

diff --git a/src/database/sql/convert.go b/src/database/sql/convert.go
index cee96319da..594a816966 100644
--- a/src/database/sql/convert.go
+++ b/src/database/sql/convert.go
@@ -57,8 +57,8 @@ func driverArgs(ds *driverStmt, args []interface{}) ([]driver.NamedValue, error)
 		// First, see if the value itself knows how to convert
 		// itself to a driver type. For example, a NullString
 		// struct changing into a string or nil.
-		if svi, ok := arg.(driver.Valuer); ok {
-			sv, err := svi.Value()
+		if vr, ok := arg.(driver.Valuer); ok {
+			sv, err := callValuerValue(vr)
 			if err != nil {
 				return nil, fmt.Errorf("sql: argument index %d from Value: %v", n, err)
 			}
@@ -341,3 +341,25 @@ func asBytes(buf []byte, rv reflect.Value) (b []byte, ok bool) {
 	}
 	return
 }
+
+var valuerReflectType = reflect.TypeOf((*driver.Valuer)(nil)).Elem()
+
+// callValuerValue returns vr.Value(), with one exception:
+// If vr.Value is an auto-generated method on a pointer type and the
+// pointer is nil, it would panic at runtime in the panicwrap
+// method. Treat it like nil instead.
+// Issue 8415.
+//
+// This is so people can implement driver.Value on value types and
+// still use nil pointers to those types to mean nil/NULL, just like
+// string/*string.
+//
+// This function is mirrored in the database/sql/driver package.
+func callValuerValue(vr driver.Valuer) (v driver.Value, err error) {
+	if rv := reflect.ValueOf(vr); rv.Kind() == reflect.Ptr &&
+		rv.IsNil() &&
+		rv.Type().Elem().Implements(valuerReflectType) {
+		return nil, nil
+	}
+	return vr.Value()
+}