Fixed #33506 -- Made QuerySet.bulk_update() perform atomic writes against write database.

The lack of _for_write = True assignment in bulk_update prior to
accessing self.db resulted in the db_for_read database being used to
wrap batched UPDATEs in a transaction.

Also tweaked the batch queryset creation to also ensure they are
executed against the same database as the opened transaction under all
circumstances.

Refs #23646, #33501.

django/db/models/query.py

@@ -725,6 +725,7 @@ class QuerySet:
             )
         # PK is used twice in the resulting update query, once in the filter
         # and once in the WHEN. Each field will also have one CAST.
+        self._for_write = True
         connection = connections[self.db]
         max_batch_size = connection.ops.bulk_batch_size(["pk", "pk"] + fields, objs)
         batch_size = min(batch_size, max_batch_size) if batch_size else max_batch_size
@@ -746,9 +747,10 @@ class QuerySet:
                 update_kwargs[field.attname] = case_statement
             updates.append(([obj.pk for obj in batch_objs], update_kwargs))
         rows_updated = 0
+        queryset = self.using(self.db)
         with transaction.atomic(using=self.db, savepoint=False):
             for pks, update_kwargs in updates:
-                rows_updated += self.filter(pk__in=pks).update(**update_kwargs)
+                rows_updated += queryset.filter(pk__in=pks).update(**update_kwargs)
         return rows_updated
 
     bulk_update.alters_data = True