[#313] client/object: Add payload-handling parameter to GetObjectParams

Define `ReaderHandler` type. Add `GetObjectParams.WithPayloadReaderHandler`
method which accepts `ReaderHandler`. The handler is called right after
header receipt in `Client.GetObject`. After the handler is called,
`GetObject` returns object header immediately.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/client/object.go

@@ -75,6 +75,8 @@ type GetObjectParams struct {
 	raw bool
 
 	w io.Writer
+
+	readerHandler ReaderHandler
 }
 
 type ObjectHeaderParams struct {
@@ -475,6 +477,78 @@ func (p *GetObjectParams) RawFlag() bool {
 	return false
 }
 
+// ReaderHandler is a function over io.Reader.
+type ReaderHandler func(io.Reader)
+
+// WithPayloadReaderHandler sets handler of the payload reader.
+//
+// If provided, payload reader is composed after receiving the header.
+// In this case payload writer set via WithPayloadWriter is ignored.
+//
+// Handler should not be nil.
+func (p *GetObjectParams) WithPayloadReaderHandler(f ReaderHandler) *GetObjectParams {
+	if p != nil {
+		p.readerHandler = f
+	}
+
+	return p
+}
+
+// wrapper over the Object Get stream that provides io.Reader.
+type objectPayloadReader struct {
+	stream *rpcapi.GetResponseReader
+
+	resp v2object.GetResponse
+
+	tail []byte
+}
+
+func (x *objectPayloadReader) Read(p []byte) (int, error) {
+	// read remaining tail
+	read := copy(p, x.tail)
+
+	x.tail = x.tail[read:]
+
+	if len(p)-read == 0 {
+		return read, nil
+	}
+
+	// receive message from server stream
+	err := x.stream.Read(&x.resp)
+	if err != nil {
+		if errors.Is(err, io.EOF) {
+			return 0, io.EOF
+		}
+
+		return 0, fmt.Errorf("reading the response failed: %w", err)
+	}
+
+	// get chunk part message
+	part := x.resp.GetBody().GetObjectPart()
+
+	chunkPart, ok := part.(*v2object.GetObjectPartChunk)
+	if !ok {
+		return 0, errWrongMessageSeq
+	}
+
+	// verify response structure
+	if err := signature.VerifyServiceMessage(&x.resp); err != nil {
+		return 0, fmt.Errorf("response verification failed: %w", err)
+	}
+
+	// read new chunk
+	chunk := chunkPart.GetChunk()
+
+	tailOffset := copy(p[read:], chunk)
+
+	read += tailOffset
+
+	// save the tail
+	x.tail = append(x.tail, chunk[tailOffset:]...)
+
+	return read, nil
+}
+
 var errWrongMessageSeq = errors.New("incorrect message sequence")
 
 func (c *clientImpl) GetObject(ctx context.Context, p *GetObjectParams, opts ...CallOption) (*object.Object, error) {
@@ -527,6 +601,7 @@ func (c *clientImpl) GetObject(ctx context.Context, p *GetObjectParams, opts ...
 		resp    = new(v2object.GetResponse)
 	)
 
+loop:
 	for {
 		// receive message from server stream
 		err := stream.Read(resp)
@@ -563,6 +638,14 @@ func (c *clientImpl) GetObject(ctx context.Context, p *GetObjectParams, opts ...
 			hdr := v.GetHeader()
 			obj.SetHeader(hdr)
 
+			if p.readerHandler != nil {
+				p.readerHandler(&objectPayloadReader{
+					stream: stream,
+				})
+
+				break loop
+			}
+
 			if p.w == nil {
 				payload = make([]byte, 0, hdr.GetPayloadLength())
 			}