extract: refactor hardlinks related code

prepare for a extract_helper context manager

(some changes may seem superfluous, but see the following changesets)

src/borg/archive.py

@@ -566,24 +566,25 @@ Utilization of max. archive size: {csize_max:.0%}
         if stat.S_ISREG(mode):
             with backup_io('makedirs'):
                 make_parent(path)
+            hardlink_set = False
             # Hard link?
             if 'source' in item:
                 source = os.path.join(dest, *item.source.split(os.sep)[stripped_components:])
-                with backup_io('link'):
-                    if item.source not in hardlink_masters:
-                        os.link(source, path)
-                        return
-                item.chunks, link_target = hardlink_masters[item.source]
+                chunks, link_target = hardlink_masters.get(item.source, (None, source))
                 if link_target:
                     # Hard link was extracted previously, just link
-                    with backup_io:
+                    with backup_io('link'):
                         os.link(link_target, path)
-                    return
-                # Extract chunks, since the item which had the chunks was not extracted
-            with backup_io('open'):
-                fd = open(path, 'wb')
+                        hardlink_set = True
+                elif chunks is not None:
+                    # assign chunks to this item, since the item which had the chunks was not extracted
+                    item.chunks = chunks
+            if hardlink_set:
+                return
             if sparse and self.zeros is None:
                 self.zeros = b'\0' * (1 << self.chunker_params[1])
+            with backup_io('open'):
+                fd = open(path, 'wb')
             with fd:
                 ids = [c.id for c in item.chunks]
                 for data in self.pipeline.fetch_many(ids, is_preloaded=True):
@@ -595,7 +596,7 @@ Utilization of max. archive size: {csize_max:.0%}
                             fd.seek(len(data), 1)
                         else:
                             fd.write(data)
-                with backup_io('truncate'):
+                with backup_io('truncate_and_attrs'):
                     pos = item_chunks_size = fd.tell()
                     fd.truncate(pos)
                     fd.flush()
@@ -608,7 +609,7 @@ Utilization of max. archive size: {csize_max:.0%}
             if has_damaged_chunks:
                 logger.warning('File %s has damaged (all-zero) chunks. Try running borg check --repair.' %
                                remove_surrogates(item.path))
-            if hardlink_masters:
+            if not hardlink_set and hardlink_masters:  # 2nd term, is it correct/needed?
                 # Update master entry with extracted file path, so that following hardlinks don't extract twice.
                 hardlink_masters[item.get('source') or original_path] = (None, path)
             return