Re: pgsql: Restrict the use of temporary namespace in two-phase transaction

From: Michael Paquier <michael(at)paquier(dot)xyz>
To: pgsql-committers(at)lists(dot)postgresql(dot)org
Subject: Re: pgsql: Restrict the use of temporary namespace in two-phase transaction
Date: 2019-01-18 00:59:49
Message-ID: 20190118005949.GD1883@paquier.xyz
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On Fri, Jan 18, 2019 at 12:22:52AM +0000, Michael Paquier wrote:
> Restrict the use of temporary namespace in two-phase transactions
>
> Attempting to use a temporary table within a two-phase transaction is
> forbidden for ages. However, there have been uncovered grounds for
> a couple of other object types and commands which work on temporary
> objects with two-phase commit. In short, trying to create, lock or drop
> an object on a temporary schema should not be authorized within a
> two-phase transaction, as it would cause its state to create
> dependencies with other sessions, causing all sorts of side effects with
> the existing session or other sessions spawned later on trying to use
> the same temporary schema name.

I have been monitoring the buildfarm and crake is complaining:
https://buildfarm.postgresql.org/cgi-bin/show_history.pl?nm=crake&br=HEAD

Here is the problem:
SET search_path TO 'pg_temp';
BEGIN;
SELECT current_schema() ~ 'pg_temp' AS is_temp_schema;
- is_temp_schema
-----------------
- t
-(1 row)
-
+ERROR: cannot create temporary tables during a parallel operation
PREPARE TRANSACTION 'twophase_search';
-ERROR: cannot PREPARE a transaction that has operated on temporary namespace

I am actually amazed to see the planner choose a parallel plan for
that, and the test can be fixed by enforcing those parameters I think:
SET max_parallel_workers = 0;
SET max_parallel_workers_per_gather = 0;
Could somebody confirm my assumption here by the way? This enforces a
non-parallel plan, right?

Anyway, it seems to me that this is pointing out to another issue:
current_schema() can trigger a namespace creation, hence shouldn't we
mark it as PARALLEL UNSAFE and make sure that we never run into this
problem?
--
Michael

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