[#142] Fix multipart-objects download

Signed-off-by: Nikita Zinkevich <n.zinkevich@yadro.com>

internal/handler/multipart.go

@@ -1,13 +1,17 @@
 package handler
 
 import (
+	"errors"
 	"io"
+	"strconv"
 
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/handler/multipart"
 	"git.frostfs.info/TrueCloudLab/frostfs-http-gw/internal/logs"
 	"go.uber.org/zap"
 )
 
+const attributeMultipartObjectSize = "S3-Multipart-Object-Size"
+
 // MultipartFile provides standard ReadCloser interface and also allows one to
 // get file name, it's used for multipart uploads.
 type MultipartFile interface {
@@ -45,3 +49,30 @@ func fetchMultipartFile(l *zap.Logger, r io.Reader, boundary string) (MultipartF
 		return part, nil
 	}
 }
+
+// getPayload returns initial payload if object is not multipart else composes new reader with parts data.
+func (h *Handler) getPayload(p getMultiobjectBodyParams) (io.ReadCloser, uint64, error) {
+	cid, ok := p.obj.Header.ContainerID()
+	if !ok {
+		return nil, 0, errors.New("no container id set")
+	}
+	oid, ok := p.obj.Header.ID()
+	if !ok {
+		return nil, 0, errors.New("no object id set")
+	}
+	size, err := strconv.ParseUint(p.strSize, 10, 64)
+	if err != nil {
+		return nil, 0, err
+	}
+	ctx := p.req.RequestCtx
+	params := PrmInitMultiObjectReader{
+		Addr:   newAddress(cid, oid),
+		Bearer: bearerToken(ctx),
+	}
+	payload, err := h.frostfs.InitMultiObjectReader(ctx, params)
+	if err != nil {
+		return nil, 0, err
+	}
+
+	return io.NopCloser(payload), size, nil
+}