[#111] Fix expiration epoch headers

Signed-off-by: Angira Kekteeva <kira@nspcc.ru>

downloader/download.go

@@ -153,6 +153,9 @@ func (r request) receiveFile(clnt pool.Object, objectAddress *object.Address) {
 		if !isValidToken(key) || !isValidValue(val) {
 			continue
 		}
+		if strings.HasPrefix(key, utils.SystemAttributePrefix) {
+			key = systemBackwardTranslator(key)
+		}
 		r.Response.Header.Set(utils.UserAttributeHeaderPrefix+key, val)
 		switch key {
 		case object.AttributeFileName:
@@ -190,6 +193,26 @@ func (r request) receiveFile(clnt pool.Object, objectAddress *object.Address) {
 	r.Response.Header.Set("Content-Disposition", dis+"; filename="+path.Base(filename))
 }
 
+// systemBackwardTranslator is used to convert headers looking like '__NEOFS__ATTR_NAME' to 'Neofs-Attr-Name'.
+func systemBackwardTranslator(key string) string {
+	// trim specified prefix '__NEOFS__'
+	key = strings.TrimPrefix(key, utils.SystemAttributePrefix)
+
+	var res strings.Builder
+	res.WriteString("Neofs-")
+
+	strs := strings.Split(key, "_")
+	for i, s := range strs {
+		s = strings.Title(strings.ToLower(s))
+		res.WriteString(s)
+		if i != len(strs)-1 {
+			res.WriteString("-")
+		}
+	}
+
+	return res.String()
+}
+
 func bearerOpts(ctx context.Context) pool.CallOption {
 	if tkn, err := tokens.LoadBearerToken(ctx); err == nil {
 		return pool.WithBearer(tkn)

downloader/download_test.go

@@ -0,0 +1,23 @@
+package downloader
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestSystemBackwardTranslator(t *testing.T) {
+	input := []string{
+		"__NEOFS__EXPIRATION_EPOCH",
+		"__NEOFS__RANDOM_ATTR",
+	}
+	expected := []string{
+		"Neofs-Expiration-Epoch",
+		"Neofs-Random-Attr",
+	}
+
+	for i, str := range input {
+		res := systemBackwardTranslator(str)
+		require.Equal(t, expected[i], res)
+	}
+}