#26522: Non-determinstic crash in django.db.models.sql.Query.combine()
-------------------------------------+-------------------------------------
Reporter: Ole Laursen | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 1.9
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 1 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Changes (by Bo Marchman):
* has_patch: 0 => 1
Comment:
PR at https://github.com/django/django/pull/8139
I ran into this bug yesterday and spent some time trying to figure out
what's going on. It seems like the circular reference arises when
`alias_map` is iterated in `Query.join` in order to find the next alias to
reuse:
{{{
reuse = [a for a, j in self.alias_map.items()
if (reuse is None or a in reuse) and j == join]
if reuse:
self.ref_alias(reuse[0])
return reuse[0]
}}}
Because `alias_map` is a dict, the alias to reuse is chosen non-
deterministically. Using an `OrderedDict` for `alias_map` solves this
problem.
It can also by solved by changing the reuse code to
{{{
reuse = [a for a, j in self.alias_map.items()
if (reuse is None or a in reuse) and j == join]
if reuse:
if join.table_alias in reuse:
to_use = join.table_alias
else:
to_use = reuse[0]
self.ref_alias(to_use)
return to_use
}}}
But the `OrderedDict` seems cleaner to me.
I don't understand much of the join internals here so I'd love to hear if
someone has a better solution.
--
Ticket URL: <https://code.djangoproject.com/ticket/26522#comment:6>
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