Re: simplehash: tb->sizemask = 0

From: Andres Freund <andres(at)anarazel(dot)de>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: simplehash: tb->sizemask = 0
Date: 2017-11-29 18:49:54
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On 2017-11-27 22:53:41 -0500, Tom Lane wrote:
> Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
> > I'm a bit puzzled by this code in SH_COMPUTE_PARAMETERS:
> > if (tb->size == SH_MAX_SIZE)
> > tb->sizemask = 0;
> > else
> > tb->sizemask = tb->size - 1;
> > Doesn't that mean that with SH_MAX_SIZE we end up with sizemask being 0
> > (i.e. no bits set)?
> Yeah, which is very obviously broken: for one thing, the Asserts
> in SH_NEXT/SH_PREV would surely go off.

That's obviously wrong. Not sure how that happened. I might have had it
as a shift at first?

> (Why are those assertions, anyway, and not test-and-elog?
> I do not think an assertion failure is a suitable way to
> report "hash table full".)

There's a test and elog during insert. Adding actual branches into
SH_NEXT/SH_PREV seems like a bad idea.

Will test a fix.


Andres Freund

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