Date: Mon, 14 Oct 2019 18:35:38 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Andres Freund <andres@anarazel.de>, buschmann@nidsa.net,
	pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #16045: vacuum_db crash and illegal memory alloc after
 pg_upgrade from PG11 to PG12
Message-ID: <20191014163538.cqn6j4cl7s2mk2yz@development>
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On Mon, Oct 14, 2019 at 10:16:40AM -0400, Tom Lane wrote:
>Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
>> On Sun, Oct 13, 2019 at 02:26:48PM -0400, Tom Lane wrote:
>>> Might be a good idea to exclude attisdropped columns in the part of the
>>> recursive query that's looking for sql_identifier columns of composite
>>> types.  I'm not sure if composites can have dropped columns today,
>
>[ I checked this, they can ]
>
>>> but even if they can't it seems like a wise bit of future-proofing.
>>> (We'll no doubt have occasion to use this logic again...)
>
>> Hmm? How could that be safe? Let's say we have a composite type with a
>> sql_identifier column, it's used in a table with data, and we drop the
>> column. We need the pg_type information to parse the existing, so how
>> could we skip attisdropped columns?
>
>It works exactly like it does for table rowtypes.
>
>regression=# create type cfoo as (f1 int, f2 int, f3 int);
>CREATE TYPE
>regression=# alter type cfoo drop attribute f2;
>ALTER TYPE
>regression=# select attname,atttypid,attisdropped,attlen,attalign from pg_attribute where attrelid = 'cfoo'::regclass;
>           attname            | atttypid | attisdropped | attlen | attalign
>------------------------------+----------+--------------+--------+----------
> f1                           |       23 | f            |      4 | i
> ........pg.dropped.2........ |        0 | t            |      4 | i
> f3                           |       23 | f            |      4 | i
>(3 rows)
>
>All we need to skip over the dead data is attlen/attalign, which are
>preserved in pg_attribute even if the pg_type row is gone.
>
>As this example shows, you don't really *have* to check attisdropped
>because atttypid will be set to zero.  But the latter is just a
>defense measure in case somebody forgets to check attisdropped;
>you're not supposed to forget that.
>

Aha! I forgot we copy the necessary stuff into pg_attribute. Thanks for
clarifying, I'll polish and push the fix shortly.

regards

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