Re: 9.4 failure on skink in _bt_newroot/XLogCheckBuffer

From: Andres Freund <andres(at)anarazel(dot)de>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: 9.4 failure on skink in _bt_newroot/XLogCheckBuffer
Date: 2016-05-23 00:32:22
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Hi tom,

On 2016-05-21 17:18:14 -0400, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > The valgrind animal just reported a large object related failure on 9.4:
> The proximate cause seems to be that _bt_newroot isn't bothering to
> fill the buffer_std field here:
> /* Make a full-page image of the left child if needed */
> rdata[2].data = NULL;
> rdata[2].len = 0;
> rdata[2].buffer = lbuf;
> rdata[2].next = NULL;
> which is indeed an actual bug, but the only consequence would be poor
> compression of the full-page image (if the value chanced to be zero),
> so it's not much of a problem.

Thanks for fixing that one!

> What remains unclear is how come this only fails once in a blue moon.
> Seems like any valgrind run of the regression tests should have caught it.

Looks like a timing issue. The relevant access to the uninitialized
buffer_std field only happens when
if (*lsn <= RedoRecPtr)
which presumably is not that likely to be hit. Even under valgrind the
individual tests are likely to finish below a checkpoint timeout.


Andres Freund

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