Subject: Re: BUG #14932: SELECT DISTINCT val FROM table gets stuck in an
 infinite loop
To: Thomas Munro <thomas.munro@enterprisedb.com>,
 Andres Freund <andres@anarazel.de>
Cc: "Todd A. Cook" <tcook@blackducksoftware.com>,
 PostgreSQL Bugs <pgsql-bugs@postgresql.org>
References: <20171127185700.1470.20362@wrigleys.postgresql.org>
 <fd924fd9-8f27-d945-04a9-57b503896107@2ndquadrant.com>
 <7a22f496-92b5-d199-984e-183965918304@blackducksoftware.com>
 <20171127221041.mezzysc4zcfsqkyc@alap3.anarazel.de>
 <CAEepm=3SC9zYLKhvtZbsnwf9vEpi96WCGz1rdGLofuzBau=fqA@mail.gmail.com>
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <ef6e817b-4a60-cb70-5375-eaf71a82372c@2ndquadrant.com>
Date: Tue, 28 Nov 2017 14:08:10 +0100
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On 11/28/2017 03:20 AM, Thomas Munro wrote:
> On Tue, Nov 28, 2017 at 11:10 AM, Andres Freund <andres@anarazel.de> wrote:
>> On 2017-11-27 15:59:37 -0500, Todd A. Cook wrote:
>>> COPY reproducer (val) FROM stdin;
>>> 2976219712004784288
>>> -6429122065899879392
>>> -7471109962990387136
>>> -7471109962990387136
>>> -2895470491222113184
>>> -4083509061952565472
>>> 1019481548263425664
>>> 4639248884787347648
>>> -6999443831165647744
>>> -4199917803455020480
>>> -4110530183001439680
>>
>> How are these values generated? They awfully look like hash values
>> (~same lenght, full numerical range)...
> 
> When SH_INSERT tries to insert that final extra value, insertdist
> keeps exceeding SH_GROW_MAX_DIB (25) no matter how many times we
> double the size (at least until my computer gives up, somewhere around
> 11 doublings and 75GB of virtual memory).  If you set SH_GROW_MAX_DIB
> to 26 then it succeeds, but I guess some other attack could be crafted
> for that.  What is the theory behind this parameter?
> 

Yeah, I came to the same hypothesis yesterday, but I see I failed to
keep pgsql-bugs on CC :-(

FWIW I believe the last doubling (from 2147483648 to 2*2147483648) is
doomed to fail due to the sizemask=0 bug. I mean, if oldsize=2147483648,
then newsize=2*2147483648, which triggers this:

    if (tb->size == SH_MAX_SIZE)
        tb->sizemask = 0;

which would explain why the last grow did not complete even after 40
minutes (while the 1073741824->2147483648 took 15 seconds). Because with
sizemask=0 the SH_NEXT/SH_PREV/.. can only ever returns 0.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services