Date: Mon, 11 Apr 2022 12:00:42 -0700
From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: anisimow.d@gmail.com, pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #17462: Invalid memory access in heapam_tuple_lock
Message-ID: <20220411190042.o5ebdhc5qsejleff@alap3.anarazel.de>
References: <17462-9c98a0f00df9bd36@postgresql.org>
 <286748.1649692519@sss.pgh.pa.us>
 <288544.1649693991@sss.pgh.pa.us>
 <20220411173417.ocwjn5xrkqqjiysl@alap3.anarazel.de>
 <352840.1649699498@sss.pgh.pa.us>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <352840.1649699498@sss.pgh.pa.us>
Archived-At: 
 <https://www.postgresql.org/message-id/20220411190042.o5ebdhc5qsejleff%40alap3.anarazel.de>
Precedence: bulk

Hi,

On 2022-04-11 13:51:38 -0400, Tom Lane wrote:
> Andres Freund <andres@anarazel.de> writes:
> > One way to address it in a way not requiring an API break would be to pass
> > SnapshotAny to heap_fetch and then do an explicit visibility check "ourselves"
> > in heapam_lock_tuple().
> 
> I'm not really interested in fixing this without an API break (going
> forward anyway), because as it stands heap_fetch is just an invitation
> to make this same mistake again.

My suggestion was about the back branch situation... But it seems viable going
forward as well, if we we reset tuple->t_data in the !valid case. As you say:

> It should never return a tuple pointer if we don't keep the pin on the
> associated buffer.

Agreed. If tuple->t_data were reset in the !valid case, not just the
!ItemIdIsNormal() case, bug would have been noticed immediately (isolation
tests do fail, I checked).


Another approach is to extend the SatisfiesDirty approach and store the tid of
the next tuple version in addition the xmin/xmax we already store. And have
heap_fetch() always set t_data to NULL if the snapshot check fails.

Greetings,

Andres Freund