borg export-tar: fix memory leak with ssh: remote repository, fixes #5568

also: added a comment how to avoid this kind of memory leak.

src/borg/archive.py

@@ -523,6 +523,8 @@ Utilization of max. archive size: {csize_max:.0%}
         return filter(item) if filter else True
 
     def iter_items(self, filter=None, partial_extract=False, preload=False, hardlink_masters=None):
+        # note: when calling this with preload=True, later fetch_many() must be called with
+        # is_preloaded=True or the RemoteRepository code will leak memory!
         assert not (filter and partial_extract and preload) or hardlink_masters is not None
         for item in self.pipeline.unpack_many(self.metadata.items, partial_extract=partial_extract,
                                               preload=preload, hardlink_masters=hardlink_masters,

src/borg/archiver.py

@@ -1031,7 +1031,8 @@ class Archiver:
             """
             Return a file-like object that reads from the chunks of *item*.
             """
-            chunk_iterator = archive.pipeline.fetch_many([chunk_id for chunk_id, _, _ in item.chunks])
+            chunk_iterator = archive.pipeline.fetch_many([chunk_id for chunk_id, _, _ in item.chunks],
+                                                         is_preloaded=True)
             if pi:
                 info = [remove_surrogates(item.path)]
                 return ChunkIteratorFileWrapper(chunk_iterator,