Re: Missing dependency tracking for TableFunc nodes

On Thu, Nov 14, 2019 at 05:35:06PM -0500, Tom Lane wrote:
>Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
>> On Thu, Nov 14, 2019 at 04:36:54PM -0500, Tom Lane wrote:
>>> 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);
>
>Ah, I see, we already have AEL on the stats object itself.  So that
>eliminates my concern about a race between two RemoveStatisticsById
>calls, but what we have instead is fear of deadlock.  A DROP STATISTICS
>command will acquire AEL on the stats object but then AEL on the table,
>the opposite of what will happen during DROP TABLE, so concurrent
>executions of those will deadlock.  That might be better than the
>failures Mark is seeing now, but not by much.
>

Hmmm, yeah :-(

>A correct fix I think is that the planner ought to acquire AccessShareLock
>on a stats object it's trying to use (and then recheck whether the object
>is still there).  That seems rather expensive, but there may be no other
>way.

Yes, so something like for indexes, although we don't need the recheck
in that case. I think the attached patch does that (but it's 1AM here).

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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