From: Tom Lane <tgl@sss.pgh.pa.us>
To: Peter Geoghegan <pg@bowt.ie>
cc: Aleksandr Parfenov <a.parfenov@postgrespro.ru>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: PostgreSQL crashes with SIGSEGV
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Comments: In-reply-to Peter Geoghegan <pg@bowt.ie>
	message dated "Tue, 06 Feb 2018 14:27:47 -0800"
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Date: Wed, 07 Feb 2018 19:41:48 -0500
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Peter Geoghegan <pg@bowt.ie> writes:
> On Wed, Jan 17, 2018 at 2:23 PM, Peter Geoghegan <pg@bowt.ie> wrote:
>> A complicating factor for this fix of mine is that mode_final() seems
>> to have its own ideas about tuple memory lifetime, over and above what
>> tuplesort_getdatum() explicitly promises, as can be seen here:
>> /*
>> * Note: we *cannot* clean up the tuplesort object here, because the value
>> * to be returned is allocated inside its sortcontext.  We could use
>> * datumCopy to copy it out of there, but it doesn't seem worth the
>> * trouble, since the cleanup callback will clear the tuplesort later.
>> */

>> ISTM that either grouping sets or mode_final() must necessarily be
>> wrong, because each oversteps, and infers a different contract from
>> tuplesort tuple fetching routines (different assumptions about memory
>> contexts are made in each case). Only one can be right, unless it's
>> okay to have one rule for tuplesort_getdatum() and another for
>> tuplesort_gettupleslot() (which seems questionable to me). I still
>> think that grouping sets is right (and that mode_final() is wrong). Do
>> you?

> It would be nice to get an opinion on this mode_final() + tuplesort
> memory lifetime business from you, Tom.

I'm fairly sure that that bit in mode_final() was just a hack to make
it work.  If there's a better, more principled way, let's go for it.

> Note that you removed the quoted comment within be0ebb65, back in October.

There were multiple instances of basically that same comment before.
AFAICS I just consolidated them into one place, in the header comment for
ordered_set_shutdown.

			regards, tom lane