migrate: Remove obsolete act_on_message_ranges.

This has a cool structure, but it's written against the long-dead
South API, and we can always pull it out of the Git history if we want
to use this approach in the future.

tools/test-backend

@@ -75,7 +75,6 @@ not_yet_fully_covered = {
     'zerver/lib/html_diff.py',
     'zerver/lib/import_realm.py',
     'zerver/lib/logging_util.py',
-    'zerver/lib/migrate.py',
     'zerver/lib/outgoing_webhook.py',
     'zerver/lib/parallel.py',
     'zerver/lib/profile.py',

zerver/lib/migrate.py

@@ -25,43 +25,3 @@ def create_index_if_not_exist(index_name: str, table_name: str, column_string: s
         END$$;
         ''' % (index_name, index_name, table_name, column_string, where_clause)
     return stmt
-
-def act_on_message_ranges(db: Any,
-                          orm: Dict[str, Any],
-                          tasks: List[Tuple[Callable[[QuerySet], QuerySet], Callable[[QuerySet], None]]],
-                          batch_size: int=5000,
-                          sleep: float=0.5) -> None:
-    # tasks should be an array of (filterer, action) tuples
-    # where filterer is a function that returns a filtered QuerySet
-    # and action is a function that acts on a QuerySet
-
-    all_objects = orm['zerver.Message'].objects
-
-    try:
-        min_id = all_objects.all().order_by('id')[0].id
-    except IndexError:
-        print('There is no work to do')
-        return
-
-    max_id = all_objects.all().order_by('-id')[0].id
-    print("max_id = %d" % (max_id,))
-    overhead = int((max_id + 1 - min_id) / batch_size * sleep / 60)
-    print("Expect this to take at least %d minutes, just due to sleeps alone." % (overhead,))
-
-    while min_id <= max_id:
-        lower = min_id
-        upper = min_id + batch_size - 1
-        if upper > max_id:
-            upper = max_id
-
-        print('%s about to update range %s to %s' % (time.asctime(), lower, upper))
-
-        db.start_transaction()
-        for filterer, action in tasks:
-            objects = all_objects.filter(id__range=(lower, upper))
-            targets = filterer(objects)
-            action(targets)
-        db.commit_transaction()
-
-        min_id = upper + 1
-        time.sleep(sleep)