support *sql.Scanner for null handling

Fixes jackc/pgx#1211

pgtype.go

@@ -533,8 +533,22 @@ type scanPlanDataTypeSQLScanner DataType
 func (plan *scanPlanDataTypeSQLScanner) Scan(ci *ConnInfo, oid uint32, formatCode int16, src []byte, dst interface{}) error {
 	scanner, ok := dst.(sql.Scanner)
 	if !ok {
-		newPlan := ci.PlanScan(oid, formatCode, dst)
+		dv := reflect.ValueOf(dst)
-		return newPlan.Scan(ci, oid, formatCode, src, dst)
+		if dv.Kind() != reflect.Ptr || !dv.Type().Elem().Implements(scannerType) {
+			newPlan := ci.PlanScan(oid, formatCode, dst)
+			return newPlan.Scan(ci, oid, formatCode, src, dst)
+		}
+		if src == nil {
+			// Ensure the pointer points to a zero version of the value
+			dv.Elem().Set(reflect.Zero(dv.Type().Elem()))
+			return nil
+		}
+		dv = dv.Elem()
+		// If the pointer is to a nil pointer then set that before scanning
+		if dv.Kind() == reflect.Ptr && dv.IsNil() {
+			dv.Set(reflect.New(dv.Type().Elem()))
+		}
+		scanner = dv.Interface().(sql.Scanner)
 	}
 
 	dt := (*DataType)(plan)
@@ -593,7 +607,25 @@ func (plan *scanPlanDataTypeAssignTo) Scan(ci *ConnInfo, oid uint32, formatCode
 type scanPlanSQLScanner struct{}
 
 func (scanPlanSQLScanner) Scan(ci *ConnInfo, oid uint32, formatCode int16, src []byte, dst interface{}) error {
-	scanner := dst.(sql.Scanner)
+	scanner, ok := dst.(sql.Scanner)
+	if !ok {
+		dv := reflect.ValueOf(dst)
+		if dv.Kind() != reflect.Ptr || !dv.Type().Elem().Implements(scannerType) {
+			newPlan := ci.PlanScan(oid, formatCode, dst)
+			return newPlan.Scan(ci, oid, formatCode, src, dst)
+		}
+		if src == nil {
+			// Ensure the pointer points to a zero version of the value
+			dv.Elem().Set(reflect.Zero(dv.Type()))
+			return nil
+		}
+		dv = dv.Elem()
+		// If the pointer is to a nil pointer then set that before scanning
+		if dv.Kind() == reflect.Ptr && dv.IsNil() {
+			dv.Set(reflect.New(dv.Type().Elem()))
+		}
+		scanner = dv.Interface().(sql.Scanner)
+	}
 	if src == nil {
 		// This is necessary because interface value []byte:nil does not equal nil:nil for the binary format path and the
 		// text format path would be converted to empty string.
@@ -761,6 +793,18 @@ func (scanPlanString) Scan(ci *ConnInfo, oid uint32, formatCode int16, src []byt
 	return newPlan.Scan(ci, oid, formatCode, src, dst)
 }
 
+var scannerType = reflect.TypeOf((*sql.Scanner)(nil)).Elem()
+
+func isScanner(dst interface{}) bool {
+	if _, ok := dst.(sql.Scanner); ok {
+		return true
+	}
+	if t := reflect.TypeOf(dst); t.Kind() == reflect.Ptr && t.Elem().Implements(scannerType) {
+		return true
+	}
+	return false
+}
+
 // PlanScan prepares a plan to scan a value into dst.
 func (ci *ConnInfo) PlanScan(oid uint32, formatCode int16, dst interface{}) ScanPlan {
 	switch formatCode {
@@ -825,13 +869,13 @@ func (ci *ConnInfo) PlanScan(oid uint32, formatCode int16, dst interface{}) Scan
 	}
 
 	if dt != nil {
-		if _, ok := dst.(sql.Scanner); ok {
+		if isScanner(dst) {
 			return (*scanPlanDataTypeSQLScanner)(dt)
 		}
 		return (*scanPlanDataTypeAssignTo)(dt)
 	}
 
-	if _, ok := dst.(sql.Scanner); ok {
+	if isScanner(dst) {
 		return scanPlanSQLScanner{}
 	}
 

pgtype_test.go

@@ -310,3 +310,44 @@ func BenchmarkScanPlanScanInt4IntoGoInt32(b *testing.B) {
 		}
 	}
 }
+
+type pgCustomInt int64
+
+func (ci *pgCustomInt) Scan(src interface{}) error {
+	*ci = pgCustomInt(src.(int64))
+	return nil
+}
+
+func TestScanPlanBinaryInt32ScanScanner(t *testing.T) {
+	ci := pgtype.NewConnInfo()
+	src := []byte{0, 42}
+	var v pgCustomInt
+
+	plan := ci.PlanScan(pgtype.Int2OID, pgtype.BinaryFormatCode, &v)
+	err := plan.Scan(ci, pgtype.Int2OID, pgtype.BinaryFormatCode, src, &v)
+	require.NoError(t, err)
+	require.EqualValues(t, 42, v)
+
+	ptr := new(pgCustomInt)
+	plan = ci.PlanScan(pgtype.Int2OID, pgtype.BinaryFormatCode, &ptr)
+	err = plan.Scan(ci, pgtype.Int2OID, pgtype.BinaryFormatCode, src, &ptr)
+	require.NoError(t, err)
+	require.EqualValues(t, 42, *ptr)
+
+	ptr = new(pgCustomInt)
+	err = plan.Scan(ci, pgtype.Int2OID, pgtype.BinaryFormatCode, nil, &ptr)
+	require.NoError(t, err)
+	assert.Nil(t, ptr)
+
+	ptr = nil
+	plan = ci.PlanScan(pgtype.Int2OID, pgtype.BinaryFormatCode, &ptr)
+	err = plan.Scan(ci, pgtype.Int2OID, pgtype.BinaryFormatCode, src, &ptr)
+	require.NoError(t, err)
+	require.EqualValues(t, 42, *ptr)
+
+	ptr = nil
+	plan = ci.PlanScan(pgtype.Int2OID, pgtype.BinaryFormatCode, &ptr)
+	err = plan.Scan(ci, pgtype.Int2OID, pgtype.BinaryFormatCode, nil, &ptr)
+	require.NoError(t, err)
+	assert.Nil(t, ptr)
+}