Re: [HACKERS] Re: PANIC: invalid index offnum: 186 when processingBRIN indexes in VACUUM

On Tue, Oct 31, 2017 at 4:56 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
>> Tom Lane wrote:
>>> So: I put the blame on the fact that summarize_range() thinks that
>>> the tuple offset it has for the placeholder tuple is guaranteed to
>>> hold good, even across possibly-long intervals where it's holding
>>> no lock on the containing buffer.
>
>> Yeah, I think this is a pretty reasonable explanation for the problem.
>> I don't understand why it doesn't fail in 9.6.
>
> Yeah, we're still missing an understanding of why we didn't see it
> before; the inadequate locking was surely there before.  I'm guessing
> that somehow the previous behavior of PageIndexDeleteNoCompact managed
> to mask the problem (perhaps only by not throwing an error, which doesn't
> imply that the index state was good afterwards).  But I don't see quite
> how it did that.

Because 24992c6d has added a check on the offset number by using
PageIndexTupleDeleteNoCompact() in brin_doupdate() making checks
tighter, no? I have not tested, and I lack knowledge about the brin
code, but it seems to me that if we had a similar check then things
could likely blow up.
-- 
Michael


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