Re: Missing dependency tracking for TableFunc nodes

On Thu, Nov 14, 2019 at 04:36:54PM -0500, Tom Lane wrote:
>Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
>> On Wed, Nov 13, 2019 at 08:37:59PM -0500, Tom Lane wrote:
>>> I concur with Tomas' suspicion that this must be a race condition
>>> during DROP STATISTICS. If we're going to allow people to do that
>>> separately from dropping the table(s), there has to be some kind of
>>> locking around it, and it sounds like there's not :-(
>
>> I think the right thing to do is simply acquire AE lock on the relation
>> in RemoveStatisticsById, per the attached patch. It's possible we'll
>> need to do something more complicated once join stats are added, but
>> for now this should be enough (and backpatchable).
>
>Hm. No, it's not enough, unless you add more logic to deal with the
>possibility that the stats object is gone by the time you have the
>table lock. Consider e.g. two concurrent DROP STATISTICS commands,
>or a statistics drop that's cascading from something like a drop
>of a relevant function and so has no earlier table lock.
>

Isn't that solved by RemoveObjects() doing this?

/* Get an ObjectAddress for the object. */
address = get_object_address(stmt->removeType,
object,
&relation,
AccessExclusiveLock,
stmt->missing_ok);

I've actually done some debugging before sending the patch, and I think
this prevent the issue you describe.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services