ngclient: simplify storing a downloaded file

Replace the usage of securesystemslib.util.persist_temp_file() with
shutil.copyfileobj() as file system abstraction is not used in the
client.
This way we prevent securesystemslib.exception.StorageError from
leaking through client API calls.

Note: with those changes we are no longer do fsync.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

tuf/ngclient/updater.py

@@ -35,12 +35,11 @@
 
 import logging
 import os
+import shutil
 import tempfile
 from typing import Optional, Set
 from urllib import parse
 
-from securesystemslib import util as sslib_util
-
 from tuf.api import exceptions
 from tuf.api.metadata import (
     Metadata,
@@ -218,8 +217,7 @@ def download_target(
             ValueError: Invalid arguments
             DownloadError: Download of the target file failed in some way
             RepositoryError: Downloaded target failed to be verified in some way
-            exceptions.StorageError: Downloaded target could not be written
-                to disk
+            OSError: Failed to write target to file
 
         Returns:
             Local path to downloaded file
@@ -252,7 +250,9 @@ def download_target(
         ) as target_file:
             targetinfo.verify_length_and_hashes(target_file)
 
-            sslib_util.persist_temp_file(target_file, filepath)
+            target_file.seek(0)
+            with open(filepath, "wb") as destination_file:
+                shutil.copyfileobj(target_file, destination_file)
 
         logger.info("Downloaded target %s", targetinfo.path)
         return filepath