Fix decoding nil pointer to the previously used object (stellar#13)

This commit fixes a bug in `Decoder.decodePtr`. When decoding a struct
with a pointer field into previously used object (ex. in a loop with
object created outside of the loop) the previous non-`nil` pointer value
was not overwritten if the new value was `nil`. See
`TestDecodeNilPointerIntoExistingObjectWithNotNilPointer` for more
context.

xdr3/decode.go

@@ -941,6 +941,26 @@ func (d *Decoder) decode(ve reflect.Value, maxSize int) (int, error) {
 	return 0, err
 }
 
+func setPtrToNil(v *reflect.Value) error {
+	if v.Kind() != reflect.Ptr {
+		msg := fmt.Sprintf("value is not a pointer: '%v'",
+			v.Type().String())
+		err := unmarshalError("decodePtr", ErrBadArguments, msg,
+			nil, nil)
+		return err
+	}
+	if !v.CanSet() {
+		msg := fmt.Sprintf("pointer value cannot be changed for '%v'",
+			v.Type().String())
+		err := unmarshalError("decodePtr", ErrNotSettable, msg,
+			nil, nil)
+		return err
+	}
+
+	v.Set(reflect.Zero(v.Type()))
+	return nil
+}
+
 func allocPtrIfNil(v *reflect.Value) error {
 	if v.Kind() != reflect.Ptr {
 		msg := fmt.Sprintf("value is not a pointer: '%v'",
@@ -969,7 +989,12 @@ func (d *Decoder) decodePtr(v reflect.Value) (int, error) {
 
 	present, n, err := d.DecodeBool()
 
-	if err != nil || !present {
+	if err != nil {
+		return n, err
+	}
+
+	if !present {
+		err = setPtrToNil(&v)
 		return n, err
 	}
 

xdr3/decode_test.go

@@ -93,6 +93,10 @@ func (u aUnion) ArmForSwitch(sw int32) (string, bool) {
 	return "-", false
 }
 
+type structWithPointer struct {
+	Data *string
+}
+
 // testExpectedURet is a convenience method to test an expected number of bytes
 // read and error for an unmarshal.
 func testExpectedURet(t *testing.T, name string, n, wantN int, err, wantErr error) bool {
@@ -1054,6 +1058,38 @@ func TestPaddedReads(t *testing.T) {
 	}
 }
 
+func TestDecodeNilPointerIntoExistingObjectWithNotNilPointer(t *testing.T) {
+	var buf bytes.Buffer
+	data := "data"
+	_, err := Marshal(&buf, structWithPointer{Data: &data})
+	if err != nil {
+		t.Error("unexpected error")
+	}
+
+	var s structWithPointer
+	_, err = Unmarshal(&buf, &s)
+	if err != nil {
+		t.Error("unexpected error")
+	}
+
+	// Note:
+	// 1. structWithPointer.Data is nil.
+	// 2. We unmarshal into previously used object.
+	_, err = Marshal(&buf, structWithPointer{})
+	if err != nil {
+		t.Error("unexpected error")
+	}
+
+	_, err = Unmarshal(&buf, &s)
+	if err != nil {
+		t.Error("unexpected error")
+	}
+
+	if s.Data != nil {
+		t.Error("Data should be nil")
+	}
+}
+
 func TestDecodeUnionIntoExistingObject(t *testing.T) {
 	var buf bytes.Buffer
 	var idata int32 = 1