[#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_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)
+	})
+}