Re: 7.2.3 vacuum bug

Is this a TODO?

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Tom Lane wrote:
> Rod Taylor <rbt(at)rbt(dot)ca> writes:
> > ERROR: RelationClearRelation: relation 11584078 deleted while still in
> > use
>
> > I've been unable to come up with a test case that will cause the
> > problem, seems to be timing related. The queries that are currently
> > running when these errors occur do a lot or work with temp tables that
> > are frequently truncated.
>
> Hm. vacuum.c tries to avoid this class of problem:
>
> /*
> * Race condition -- if the pg_class tuple has gone away since the
> * last time we saw it, we don't need to vacuum it.
> */
> if (!SearchSysCacheExists(RELOID,
> ObjectIdGetDatum(relid),
> 0, 0, 0))
> {
> CommitTransactionCommand(true);
> return true; /* okay 'cause no data there */
> }
>
> ...
>
> onerel = relation_open(relid, lmode);
>
> but on reflection it's clear that this doesn't really prevent a race
> condition. If the table is already exclusive-locked by a DROP TABLE
> that hasn't committed yet (eg, the implicit DROP that happens when temp
> tables are cleared out at backend exit), then the syscache lookup will
> go fine, but the relation_open() routine blocks waiting for lock and
> eventually fails.
>
> What would probably work better is to first lock the relation OID,
> then see if we can open the relation or not.
>
> Thinking further, it's really kinda bogus that LockRelation() works on
> an already-opened Relation; if possible we should acquire the lock
> before attempting to create a relcache entry. (We only need to know the
> OID and the relisshared status before we can make a locktag, so it'd be
> possible to acquire the lock using only the contents of the pg_class row.)
> Not sure how much code restructuring might be involved to make this
> happen, but it'd be worth thinking about for 7.4.
>
> regards, tom lane
>
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