[#67] Added streaming to download

Signed-off-by: Denis Kirillov <denis@nspcc.ru>

downloader/reader_test.go

@@ -0,0 +1,43 @@
+package downloader
+
+import (
+	"fmt"
+	"io"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestReader(t *testing.T) {
+	data := []byte("test string")
+	err := fmt.Errorf("something wrong")
+
+	for _, tc := range []struct {
+		err  error
+		buff []byte
+	}{
+		{err: nil, buff: make([]byte, len(data)+1)},
+		{err: nil, buff: make([]byte, len(data))},
+		{err: nil, buff: make([]byte, len(data)-1)},
+		{err: err, buff: make([]byte, len(data)+1)},
+		{err: err, buff: make([]byte, len(data))},
+		{err: err, buff: make([]byte, len(data)-1)},
+	} {
+		var res []byte
+		var err error
+		var n int
+
+		r := newReader(data, tc.err)
+		for err == nil {
+			n, err = r.Read(tc.buff)
+			res = append(res, tc.buff[:n]...)
+		}
+
+		if tc.err == nil {
+			require.Equal(t, io.EOF, err)
+		} else {
+			require.Equal(t, tc.err, err)
+		}
+		require.Equal(t, data, res)
+	}
+}