Date: Thu, 2 Jul 2026 04:06:01 +0000
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: Michael Paquier <michael@paquier.xyz>
Cc: Ewan Young <kdbase.hack@gmail.com>, pgsql-hackers@lists.postgresql.org
Subject: Re: Prevent crash when calling pgstat functions with unregistered
 stats kind
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Hi,

On Thu, Jul 02, 2026 at 12:43:43PM +0900, Michael Paquier wrote:
> On Thu, Jul 02, 2026 at 03:27:18AM +0000, Bertrand Drouvot wrote:
> > On Wed, Jul 01, 2026 at 04:20:39PM +0800, Ewan Young wrote:
> >> One small thing: in pgstat_snapshot_fixed(), the existing
> >> Assert(pgstat_is_kind_valid(kind)); becomes redundant after the new NULL
> >> check. A non-NULL kind_info already implies the kind is valid (that's the
> >> only way pgstat_get_kind_info() returns non-NULL), so the assert can never
> >> fire. Might as well drop it and keep just the fixed_amount one.
> > 
> > Yeah good catch, done in the attached.
> 
> I am not convinced that it is worth bothering in the core code about
> this class of failures; they are just not interesting, and impossible
> to miss.
> 
> It seems to me that this error is in the _PG_init() of the modules in
> modules/test_custom_stats/: we should not bypass the
> pgstat_register_kind() if not loading the library from
> shared_preload_libraries, but let the call happen and fail.

I agree that the responsibility should primarily be in the extension. However,
the issue is that the NULL dereference happens inside core code (pgstat_prep_pending_entry,
etc.), and the resulting segfault(s) cause the postmaster to terminate all
backends (not just the offending session).

Given that one misconfigured extension can crash all connections on the server,
a defensive check in core seems reasonable (kind of similar to 341e9a05e7b).

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com