posix.RenameFile(): Allow overwriting an empty directory (#5551)

Overwriting files is allowed, but since the introduction of
the object directory, we will aslo need to allow overwriting
an empty directory. Putting twice the same object directory
won't fail with 403 error anymore.

cmd/posix.go

@@ -932,14 +932,14 @@ func (s *posix) RenameFile(srcVolume, srcPath, dstVolume, dstPath string) (err e
 		return err
 	}
 	// Stat a volume entry.
-	_, err = os.Stat((srcVolumeDir))
+	_, err = os.Stat(srcVolumeDir)
 	if err != nil {
 		if os.IsNotExist(err) {
 			return errVolumeNotFound
 		}
 		return err
 	}
-	_, err = os.Stat((dstVolumeDir))
+	_, err = os.Stat(dstVolumeDir)
 	if err != nil {
 		if os.IsNotExist(err) {
 			return errVolumeNotFound
@@ -953,23 +953,24 @@ func (s *posix) RenameFile(srcVolume, srcPath, dstVolume, dstPath string) (err e
 		return errFileAccessDenied
 	}
 	srcFilePath := slashpath.Join(srcVolumeDir, srcPath)
-	if err = checkPathLength((srcFilePath)); err != nil {
+	if err = checkPathLength(srcFilePath); err != nil {
 		return err
 	}
 	dstFilePath := slashpath.Join(dstVolumeDir, dstPath)
-	if err = checkPathLength((dstFilePath)); err != nil {
+	if err = checkPathLength(dstFilePath); err != nil {
 		return err
 	}
 	if srcIsDir {
-		// If source is a directory we expect the destination to be non-existent always.
-		_, err = os.Stat((dstFilePath))
-		if err == nil {
+		// If source is a directory, we expect the destination to be non-existent but we
+		// we still need to allow overwriting an empty directory since it represents
+		// an object empty directory.
+		_, err = os.Stat(dstFilePath)
+		if err == nil && !isDirEmpty(dstFilePath) {
 			return errFileAccessDenied
 		}
 		if !os.IsNotExist(err) {
 			return err
 		}
-		// Destination does not exist, hence proceed with the rename.
 	}
 
 	if err = renameAll(srcFilePath, dstFilePath); err != nil {