[#313] client/object: Always return number of bytes read from Get stream (#316)

Fix failure to comply with a requirement of stdlib `io.Reader` docs: `When
Read encounters an error or end-of-file condition after successfully reading
n > 0 bytes, it returns the number of bytes read.`

Prepare a platform for unit tests and test the affected case.

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

pkg/client/object.go

@@ -496,31 +496,35 @@ func (p *GetObjectParams) WithPayloadReaderHandler(f ReaderHandler) *GetObjectPa
 
 // wrapper over the Object Get stream that provides io.Reader.
 type objectPayloadReader struct {
-	stream *rpcapi.GetResponseReader
+	stream interface {
+		Read(*v2object.GetResponse) error
+	}
 
 	resp v2object.GetResponse
 
 	tail []byte
 }
 
-func (x *objectPayloadReader) Read(p []byte) (int, error) {
+func (x *objectPayloadReader) Read(p []byte) (read int, err error) {
 	// read remaining tail
-	read := copy(p, x.tail)
+	read = copy(p, x.tail)
 
 	x.tail = x.tail[read:]
 
 	if len(p)-read == 0 {
-		return read, nil
+		return
 	}
 
 	// receive message from server stream
-	err := x.stream.Read(&x.resp)
+	err = x.stream.Read(&x.resp)
 	if err != nil {
 		if errors.Is(err, io.EOF) {
-			return 0, io.EOF
+			err = io.EOF
+			return
 		}
 
-		return 0, fmt.Errorf("reading the response failed: %w", err)
+		err = fmt.Errorf("reading the response failed: %w", err)
+		return
 	}
 
 	// get chunk part message
@@ -528,12 +532,14 @@ func (x *objectPayloadReader) Read(p []byte) (int, error) {
 
 	chunkPart, ok := part.(*v2object.GetObjectPartChunk)
 	if !ok {
-		return 0, errWrongMessageSeq
+		err = errWrongMessageSeq
+		return
 	}
 
 	// verify response structure
-	if err := signature.VerifyServiceMessage(&x.resp); err != nil {
-		return 0, fmt.Errorf("response verification failed: %w", err)
+	if err = signature.VerifyServiceMessage(&x.resp); err != nil {
+		err = fmt.Errorf("response verification failed: %w", err)
+		return
 	}
 
 	// read new chunk
@@ -546,7 +552,7 @@ func (x *objectPayloadReader) Read(p []byte) (int, error) {
 	// save the tail
 	x.tail = append(x.tail, chunk[tailOffset:]...)
 
-	return read, nil
+	return
 }
 
 var errWrongMessageSeq = errors.New("incorrect message sequence")

pkg/client/object_test.go

@@ -0,0 +1,95 @@
+package client
+
+import (
+	"io"
+	"testing"
+
+	"github.com/nspcc-dev/neofs-api-go/v2/object"
+	"github.com/nspcc-dev/neofs-api-go/v2/signature"
+	"github.com/nspcc-dev/neofs-crypto/test"
+	"github.com/stretchr/testify/require"
+)
+
+type singleResponseStream struct {
+	called bool
+	resp   object.GetResponse
+}
+
+func (x *singleResponseStream) Read(r *object.GetResponse) error {
+	if x.called {
+		return io.EOF
+	}
+
+	x.called = true
+
+	*r = x.resp
+
+	return nil
+}
+
+var key = test.DecodeKey(0)
+
+func chunkResponse(c []byte) (r object.GetResponse) {
+	chunkPart := new(object.GetObjectPartChunk)
+	chunkPart.SetChunk(c)
+
+	body := new(object.GetResponseBody)
+	body.SetObjectPart(chunkPart)
+
+	r.SetBody(body)
+
+	if err := signature.SignServiceMessage(key, &r); err != nil {
+		panic(err)
+	}
+
+	return
+}
+
+func data(sz int) []byte {
+	data := make([]byte, sz)
+
+	for i := range data {
+		data[i] = byte(i) % ^byte(0)
+	}
+
+	return data
+}
+
+func checkFullRead(t *testing.T, r io.Reader, buf, payload []byte) {
+	var (
+		restored []byte
+		read     int
+	)
+
+	for {
+		n, err := r.Read(buf)
+
+		read += n
+		restored = append(restored, buf[:n]...)
+
+		if err != nil {
+			require.Equal(t, err, io.EOF)
+			break
+
+		}
+	}
+
+	require.Equal(t, payload, restored)
+	require.EqualValues(t, len(payload), read)
+}
+
+func TestObjectPayloadReader_Read(t *testing.T) {
+	t.Run("read with tail", func(t *testing.T) {
+		payload := data(10)
+
+		buf := make([]byte, len(payload)-1)
+
+		var r io.Reader = &objectPayloadReader{
+			stream: &singleResponseStream{
+				resp: chunkResponse(payload),
+			},
+		}
+
+		checkFullRead(t, r, buf, payload)
+	})
+}