Date: Wed, 12 May 2021 09:13:54 -0700
From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>,
	Robert Haas <robertmhaas@gmail.com>, a.wicht@gmail.com,
	pgsql-bugs@lists.postgresql.org,
	Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Subject: Re: BUG #15990: PROCEDURE throws "SQL Error [XX000]: ERROR: no known
 snapshots" with PostGIS geometries
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Hi,

On 2021-05-12 12:00:10 -0400, Tom Lane wrote:
> Yeah, on further thought, the real question to be asking here is
> "what's protecting that in-flight datum from becoming a dangling
> pointer?".  AFAICT, the answer right now is "nothing".  Therefore,
> it is *never* okay for plpgsql to be executing without a registered
> transaction snapshot; and it seems quite unlikely that it'd be any
> safer for any other PL.

A bit independently of this concrete issue, I wonder if we could make it
easier to detect such bugs. If we somehow™ associated an xmin horizon
with toasted datums, we could then assert before dereferencing that we
still guarantee that such a reference is still valid. The "somehow" part
obviously isn't entirely trivial, but it seems we could hack it by
encoding it as a different vartag?

Greetings,

Andres Freund