do not overwritte files when Buffer is not loaded in memory

decoder.go

@@ -22,6 +22,7 @@ type ReadQuotas struct {
 
 // ReadResourceCallback defines a callback that will be called when an external resource should be loaded.
 // The string parameter is the URI of the resource.
+// If the reader and the error are nil the buffer data won't be loaded into memory.
 type ReadResourceCallback = func(string) (io.ReadCloser, error)
 
 // Open will open a glTF or GLB file specified by name and return the Document.
@@ -149,7 +150,7 @@ func (d *Decoder) decodeBuffer(buffer *Buffer) error {
 		buffer.Data, err = buffer.marshalData()
 	} else if err = validateBufferURI(buffer.URI); err == nil {
 		r, err = d.cb(buffer.URI)
-		if err == nil {
+		if r != nil && err == nil {
 			buffer.Data = make([]uint8, buffer.ByteLength)
 			_, err = r.Read(buffer.Data)
 			r.Close()

encode.go

@@ -75,6 +75,11 @@ func (e *Encoder) encodeBuffer(buffer *Buffer) error {
 	if buffer.IsEmbeddedResource() {
 		return nil
 	}
+
+	if len(buffer.Data) == 0 {
+		return nil
+	}
+
 	if err := validateBufferURI(buffer.URI); err != nil {
 		return err
 	}

encode_test.go

@@ -27,7 +27,11 @@ func saveMemory(doc *Document, asBinary bool) (*Decoder, error) {
 		return nil, err
 	}
 	rcb := func(uri string) (io.ReadCloser, error) {
-		return ioutil.NopCloser(chunks[uri]), nil
+		if chunk, ok := chunks[uri]; ok {
+			return ioutil.NopCloser(chunk), nil
+		} else {
+			return nil, nil
+		}
 	}
 	return NewDecoder(buff, rcb), nil
 }
@@ -72,6 +76,7 @@ func TestEncoder_Encode(t *testing.T) {
 			{Extras: 8.0, Name: "binary", ByteLength: 3, URI: "a.bin", Data: []uint8{1, 2, 3}},
 			{Extras: 8.0, Name: "embedded", ByteLength: 2, URI: "data:application/octet-stream;base64,YW55ICsgb2xkICYgZGF0YQ==", Data: []byte("any + old & data")},
 			{Extras: 8.0, Name: "external", ByteLength: 4, URI: "b.bin", Data: []uint8{4, 5, 6, 7}},
+			{Extras: 8.0, Name: "external", ByteLength: 4, URI: "a.drc"},
 		}}}, false},
 		{"withBufView", args{&Document{BufferViews: []BufferView{
 			{Extras: 8.0, Buffer: 0, ByteOffset: 1, ByteLength: 2, ByteStride: 5, Target: ArrayBuffer},

struct.go

@@ -97,6 +97,8 @@ type SparseIndices struct {
 }
 
 // A Buffer points to binary geometry, animation, or skins.
+// If Data length is 0 and the Buffer is an external resource the Data won't be flushed,
+// which can be useful when there is no need to load data in memory.
 type Buffer struct {
 	Extensions Extensions  `json:"extensions,omitempty"`
 	Extras     interface{} `json:"extras,omitempty"`
@@ -119,10 +121,14 @@ func (b *Buffer) EmbeddedResource() {
 // marshalData decode the buffer from the URI. If the buffer is not en embedded resource the returned array will be empty.
 func (b *Buffer) marshalData() ([]uint8, error) {
 	if !b.IsEmbeddedResource() {
-		return []uint8{}, nil
+		return nil, nil
 	}
 	startPos := len(mimetypeApplicationOctet) + 1
-	return base64.StdEncoding.DecodeString(b.URI[startPos:])
+	sl, err := base64.StdEncoding.DecodeString(b.URI[startPos:])
+	if len(sl) == 0 || err != nil {
+		return nil, err
+	}
+	return sl, nil
 }
 
 // BufferView is a view into a buffer generally representing a subset of the buffer.

struct_test.go

@@ -403,9 +403,9 @@ func TestBuffer_marshalData(t *testing.T) {
 		want    []uint8
 		wantErr bool
 	}{
-		{"error", &Buffer{URI: "data:application/octet-stream;base64,_"}, []uint8{}, true},
-		{"external", &Buffer{URI: "http://web.com"}, []uint8{}, false},
-		{"empty", &Buffer{URI: "data:application/octet-stream;base64,"}, []uint8{}, false},
+		{"error", &Buffer{URI: "data:application/octet-stream;base64,_"}, nil, true},
+		{"external", &Buffer{URI: "http://web.com"}, nil, false},
+		{"empty", &Buffer{URI: "data:application/octet-stream;base64,"}, nil, false},
 		{"test", &Buffer{URI: "data:application/octet-stream;base64,TEST"}, []uint8{76, 68, 147}, false},
 		{"complex", &Buffer{URI: "data:application/octet-stream;base64,YW55IGNhcm5hbCBwbGVhcw=="}, []uint8{97, 110, 121, 32, 99, 97, 114, 110, 97, 108, 32, 112, 108, 101, 97, 115}, false},
 	}