Re: pgsql: Avoid improbable PANIC during heap_update.

On Thu, Sep 29, 2022 at 02:55:40AM -0500, Jaime Casanova wrote:
> On Tue, Apr 13, 2021 at 04:17:39PM +0000, Tom Lane wrote:
> > Avoid improbable PANIC during heap_update.
> > 
> > heap_update needs to clear any existing "all visible" flag on
> > the old tuple's page (and on the new page too, if different).
> > Per coding rules, to do this it must acquire pin on the appropriate
> > visibility-map page while not holding exclusive buffer lock;
> > which creates a race condition since someone else could set the
> > flag whenever we're not holding the buffer lock.  The code is
> > supposed to handle that by re-checking the flag after acquiring
> > buffer lock and retrying if it became set.  However, one code
> > path through heap_update itself, as well as one in its subroutine
> > RelationGetBufferForTuple, failed to do this.  The end result,
> > in the unlikely event that a concurrent VACUUM did set the flag
> > while we're transiently not holding lock, is a non-recurring
> > "PANIC: wrong buffer passed to visibilitymap_clear" failure.
> > 
> 
> Hi,
> 
> This doesn't look as improbable because I saw it at least 3 times with
> v15beta4.
> 
> The first time I thought it was my fault, then I tried with a commit on
> september 25 (didn't remember which exactly but that doesn't seems too
> relevant).
> Finally I saw it again in a build with TRACE_VISIBILITYMAP defined (the
> same commit).
> 
> But I haven't see it anymore on rc1. Anyway I'm attaching the backtrace
> (this is from the build with TRACE_VISIBILITYMAP), the query that was 
> running at the time was (no changes were made to quad_poly_tbl table 
> nor any indexes were added to this table):
> 
> """
> update public.quad_poly_tbl set
>   id = public.quad_poly_tbl.id
> returning
>   public.quad_poly_tbl.id as c0
> """
> 

Just to confirm I saw this on RC1

-- 
Jaime Casanova
Director de Servicios Profesionales
SystemGuards - Consultores de PostgreSQL